IVOA SED Data Model
Version 0.93 (rev b)

In section 4 we elaborate these concepts in detail, including some complications that we explicitly do not attempt to handle in this version.

A moderately complicated SED with five segments a through e. segments a and b are photometry points with associated errors (vertical bars) and bandpass widths (horizontal bars). segment c is a time series with four measurements at different times with the same instrumental configuration. segment d is an upper limit measurement. segment e is a simple spectrum, with point-to-point statistical errors indicated by vertical bars and an overall (correlated) systematic error indicated by the dashed lines.

In section 4, the data model fields and possible values are listed. We distinguish between optional and required fields, and where appropriate we list those values of the physical units which interoperable implementations are required to recognize.

UML class diagram for the SED data model.

The Frame, Coverage, Curation, DataID classes are 
shown in detail below in Section 4, 
both in diagram form and text descriptions. The text descriptions
are definitive.


The minimal required content is:

 Target name
 For each segment: Coverage.Location and Coverage.Extent descriptions
of the location and extent of the data in the RA, Dec, time and spectral domains
 For each segment: the Curation.Publisher field
 For each segment: the descriptions of the spectral coordinate and flux
fields including UCD and units
 For each point: the values of the spectral coordinate and flux.

3.2 Units

Briefly, units are given in the form

10**(-14) erg/cm**2/s/Hz, 10**3 Jy Hz

This format is mostly consistent with the AAS standards for online tables in journals (http://grumpy.as.arizona.edu/~gschwarz/unitstandards.html) except for the use of space rather than "." for multiplication and the fact that we do not require the use of SI units.

SI prefixes for units are to be recognized; for instance, the listing of "m" as a known unit for wavelength implies that "cm", "nm", and "um" (with "u" the OGIP convention for rendering "micro") are also acceptable.

Until IVOA generic unit conversion software is mature and widely deployed, it is helpful to interoperable applications to include a representation of the units in "base SI form", including only the base units kg, m, s (and possibly A, sr) with a numeric prefix. Pedro Osuna and Jesus Salgado have proposed a representation in the spirit of dimensional analysis, using the symbols M, L, T to signify kg, m, s respectively and omitting the ** for powers, so that

10**3 Jy Hz

10**-23 kg s**-2

10-23MT-2

This alternate representation is supported for the main model fields (time, spectral coordinate and flux) only and is supported in the Frame field of the model.

Although the SED model is flexible enough to permit different units for each field, as a matter of style we strongly recommend that whenever possible the same units should be used for compatible fields (e.g. flux and error on flux).

3.3 UCDs

UCDs or Uniform Content Descriptors are the IVOA's standardized vocabulary for astronomical concepts. In this document we use UCDs as field attributes (for example, element attributes in XML) to distinguish alternate physics within the same data model roles - for example, to distinguish frequency versus wavelength on the spectral coordinate `X-axis'.

UCDs are currently going through the IVOA formal process. The list we are using is from http://cdsweb.u-strasbg.fr/UCD/ucd1p-words.txt with syntax defined in the UCD recommendation http://www.ivoa.net/Documents/latest/UCD.html

4. Spectral data model Measurement objects

4.1 Spectrum and Time Series

A simple Spectrum is an SED object for which the spectral coordinate varies but the time coordinate is fixed, and for which the associated metadata are constant (i.e. a single segment with a fixed time.)

A simple Time Series is an SED object for which the spectral coordinate is constant but the time coordinate varies, and for which the associated metadata are constant (i.e. one set of metadata for the whole time series; a single segment at a fixed spectral coordinate).

4.2 Spectral Coordinate

Astronomers use a number of different spectral coordinates to label the electromagnetic spectrum. The cases enumerated by Greisen et al. (2003) are listed below with their UCDs.

REQUIRED: Exactly one SpectralCoord.Value field should be present, with units and one of the UCD values listed below. We distinguish between the VO data model field name (which might be used for VOTABLE UTYPE), the FITS WCS name (provided for comparison only), and the UCD1+ names. For UCD1+, we propose UCDs in italics for those not currently covered by the document http://www.ivoa.net/Documents/latest/UCD.html

Note 1: For this version, only the first four entries, Wavelength, Frequency, Energy, and spectral channel, should be used for interoperable transmission of data - implementations are not required to understand (convert) the other UCD values.

Note 2: For the velocity cases, we propose an em.veloc tree, rather than a src.veloc tree, because the velocity here is really a labelling of a spectral coordinate, and the link to the physical radial velocity of the different emission sources contributing to the spectrum is rather indirect.

4.3 Flux (Spectral Intensity) Object

Two alternate representations of the flux are supported: Flux (the background-subtracted net flux) and TotalFlux (the source+background), possibly with the BackgroundModel (see below). Since there are no current UCDs to distinguish between these cases, we use data model field names.

REQUIRED: There should be exactly one Flux.Value or TotalFlux.Value field present. It is not currently permitted to have both Flux and TotalFlux in the same Spectrum segment. The field must include units and UCD.

For each of these cases, there are many slightly different physical quantities covered by the general concept of Flux; we distinguish them by their UCD.

Note in particular the distinction between the unit count (an instrumental value) and the unit photon (used in the photon number flux, i.e. the number of photons incident; photon number flux = energy flux divided by photon energy).

4.4 BackgroundModel Object

The BackgroundModel array is required to have the same UCD and units as the Flux array. It represents a model for the expected flux values if the Target had zero flux.

OPTIONAL: There may optionally be at most one BackgroundModel.Value field present. It must have the same UCD as the Flux or TotalFlux.

4.5 Time coordinate

The time unit is specified by a string, and the only valid values for this unit are 's' (seconds) and 'd' (days).

OPTIONAL: The Time.Value field is optional and defaults to 0.0 (relative to the reference time, which defaults to the Coverage.Location.Time as described in the Frame section).

4.6 Position coordinate

4.7 Accuracy Fields

We express the bandpass for each spectral bin as a low and high value for the spectral coordinate, or as a width. The same is done for photometry points, which amounts to approximating a filter by a rectangular bandpass. Time bins are also given as low and high values or as a width.

In addition to the binning, we allow the model to express uncertainties (which may be larger than the bin width), both statistical and systematic. We allow two-sided statistical errors but only one-sided systematic errors, and for position we have a single statistical error - eventually we may want a full error ellipse but this is too complicated for the present model.

We also use a very simple error model for the fluxes: we include plus and minus flux errors, and a quality flag. The errors are understood as 1 sigma gaussian errors which are uncorrelated for different points in the spectrum. If the data provider has only upper limit information, it should be represented by setting the flux value and the lower error value equal to the limit, and the upper error value equal to zero (e.g. 5 (+0,-5)). In general applications may choose to render measurements as upper limits if the flux value is less than some multiple (e.g. 3) of the lower error. We also allow a systematic error value, assumed constant across a given spectrum and fully correlated (so that, e.g. it does not enter into estimating spectral slopes). We also include a trivial resolution model: a single number nominally representing a FWHM spectral or time resolution expressed in the same units as the spectral or time coordinate. The spatial (sky) resolution may be useful to know if it exceeds the aperture size.

CLARIFICATION: the ErrorLow and ErrorHigh are the plus/minus ERRORS, not the (value+error, value-error). In other words, if Value = 10 and there is a symmetric uncertainty of 3, the ErrorLow and ErrorHigh are both +3.0, and NOT 7.0, 13.0. This is different from the sampling description BinLow and BinHigh, which give the VALUES at the low and high end of the bin. Thus if the central wavelength of the bin is 4200.0, and the bin size is 10, then the BinLow and BinHigh values are 4195.0, 4205.0 and NOT 10.0, 10.0. Note that because of this, 0.0 is NOT an acceptable default for BinLow and BinHigh, while it IS acceptable (albeit unlikely) for ErrorLow and ErrorHigh.

We add a further measure of accuracy, the Confidence, which expresses a probability between 0 and 1 that the quoted errors do apply. This measure is used in the Sloan spectral service to provide a way of describing the estimated probability that the redshift is completely in error because the lines have been misidentified - in general, such a Confidence could be useful for any measurement where the error probability distribution has multiple peaks in parameter space. Its default value is 1.0. This field is illustrated in the Target model (section 6.2).

The Quality model represents quality by an integer, with the following meanings: 0 is good data, 1 is data which is bad for an unspecified reason (e.g., no data in the sample interval), and other positive integers greater than 1 may be used to flag data which is bad or dubious for specific reasons. The data provider may define scalar string-valued metadata fields Quality.2, Quality.3... to define specific quality flags on a per-segment basis. Bitmasks, used in some archives such as SDSS, should be remapped to such independent Quality fields. All the Accuracy fields are optional (both in the table fields and in the Coverage instances).

OPTIONAL: (SpectralCoord, Time) Only one of BinSize, or both BinLow and BinHigh, must be present (possibly as a header parameter implying a constant value for each flux point). If there is only one coordinate value (e.g. a time series with only one spectral coordinate value) the bin limits should be the entire range given in Coverage.Extent given in the Coverage model, else the bin limits are assumed to be halfway between the spectral coordinate values.

OPTIONAL: (SpectralCoord, Time) Resolution: - default is BinSize.

OPTIONAL: Sky.Resolution - default is the size of the aperture, given in Coverage.Extent.

OPTIONAL: StatErrLow, StatErrHigh, SysError fields for SpectralCoord, Time, Sky and Flux; omitting these fields indicates that the errors are unknown. Data providers are STRONGLY encouraged to provide explicit error measures whenever possible.

OPTIONAL: Flux.Quality field , default is 0 (good data).

5. Associated Metadata Fields

5.1 Coverage Fields

We give a value giving the effective exposure time (useful for selecting among multiple spectra from the same instrument). The aperture is important to determine what part of an extended object is contributing to the spectrum; we allow a simple aperture description consisting of a single number representing the aperture diameter in decimal degrees; it is anticipated that a full region description based on the CoordArea object will be supported in later versions.

REQUIRED: Coverage.Location.Sky, Coverage.Location.Time, Coverage.Extent.Sky, Coverage.Extent.Time .

REQUIRED: Coverage.Location.Spectral; Coverage.Extent.Spectral. These are required to be present at the level of the abstract model. However, in the serializations of the model we break the symmetry between the spectral and other axes because we'll often have a segment with just one spectral point, so it's easier to derive these values directly from the data.

OPTIONAL: Coverage.Region (defaults to full region of Coverage.Extent centered on Coverage.Location)

OPTIONAL: Coverage.Fill

Diagram for Coverage object

5.2 Frame Fields

For all the numeric fields, units must be supplied. In addition, we add an SIDim field for each axis giving the SI units of the values in the Osuna-Salgado dimensional format.

For the spectral coordinate, position and time we need to add further metadata to define the frames used. Here we try and match existing FITS conventions (TIMESYS is a convention used in the X-ray community).

One standard reference time in astronomy is the origin of Julian Day Number on the TT (Terrestrial Time) timescale, BC 4713 Nov 24 at 11:59:27.81 (Gregorian). Using TT is preferable to UTC because it does not contain leap seconds, so the elapsed time in days is just equal to the difference in JD values.

The ISO-8601 calendar format standard does not support dates before AD 1, so cannot express this reference time. Therefore, it is not a suitable format for internal representations of such reference times. However, non-default choices of reference time may be specified in external serializations by a date in ISO-8601 format, e.g. "2004-11-30T11:59:00.01".

In this version of the model we require use of MJD as the time type for absolute times. (ISO dates and JD are other possibilities covered by the STC document).

(Note that in the FITS serialization, the MJDREF keyword allows definition of reference times in decimal days relative to MJD 0.0 = JD 2400000.5.)

Other frame information needed for velocity spectral coordinates include the observation-fixed spectral frame, the observatory location, the source redshift, and the velocity zero point (in Greisen et al, SSYSOBS, OBSGEO, VELOSYS, RESTFRQ/RESTWAV).

For Flux, the only entry is the SIDim field; we may later add a Type field to make finer distinctions in the type of flux scale than is possible using the Flux.Value's UCD.

Notes on compatibility with, and differences from, STC 1.0:

• We include the types (e.g. MJD, Cartesian) with the Frame (STC's CoordFrame), while STC puts them with the individual Coords instances. Implicitly, this requires that within a segment, all coordinate values for a particular coordinate be of the same type.
• Frame.Sky (STC SpaceFrame): we restrict the allowed expression of coordinates to a few simple cases.
  • the allowed values of Frame.Sky.Type (STC SpaceRefFrame) are ICRS or FK5. STC 1.0 allows additional cases.
  • For ICRS the equinox may be omitted; if present it should be J2000.0 (ICRS is not parameterized by equinox, but it does happen to coincide with the J2000 equator). For FK5 the equinox is required to be J2000.0.
  • the RefPos is not included and is assumed to be BARYCENTER (the solar system barycenter). This only matters for objects near enough to have measurable parallax; we do not support description of the position of solar system objects in this document, to make implementation simpler.
  • the Flavor is not included. It is implicitly required to be UNITSPHERE.
• TimeFrame: if you have a light curve whose times are corrected to the solar system barycenter, the results depend on the assumed source direction. RefPos = BARYCENTER and a RefDir with the assumed RA and Dec allow you to describe this. STC1.0 gives many possibilities for RefPos, but we support only TOPOCENTER, BARYCENTER, HELIOCENTER, GEOCENTER.

  The Frame.Time.Zero element defines the reference time (zero point) for all elapsed times in the segment, i.e. those in Coverage.Location.Time, Coverage.Extent.Time, Coverage.Region.Time, Point.Time.Value.

• SpectralFrame: the Frame.SpectralCoord.RefPos indicates the frame of the spectral coordinates, as in STC SpectralFrame ReferencePosition.
• RedshiftFrame: we don't include this - in this edition of the document, velocity axes (as opposed to spectral axes) are deprecated and their description is not supported.

OPTIONAL: All Frame values are optional, but data providers should take special care to check whether or not the defaults are appropriate for their data. The implications of the defaults are:

• Positions are given in ICRS RA,Dec in degrees and are heliocentric values (i.e. corrected for annual parallax and aberration, as normally found in source catalogs).
• Times are given in MJD days and represent times of photon arrival at the telescope.
• Spectral coordinates are as observed at the telescope, and not corrected for redshift, the motion of the Earth, etc.
• Note: we are considering making the Frame values required, particularly the SIDIM entries.

Diagram for Frame object

5.3 Derived Data Fields

OPTIONAL: DerivedData (all fields)

5.4 Curation model

Diagram for Curation, DataID, Derived objects

REQUIRED: Curation.Publisher. As with the VO Resource Metadata document, this is the only required field in the Curation object. All other fields are optional.

5.5 Data Identification model

The DataID.DatasetID is the IVOA/ADEC/ADS dataset identifier. By agreement between the AAS journals, the ADS and the ADEC (NASA data centers), described in http://vo.ads.harvard.edu/dv/, these dataset identifiers will be used to link journal articles back to the archival datasets containing the relevant observational data.

OPTIONAL: All DataId fields are optional.

• Archival, indicating that it is one of a collection of datasets (in this case SEDs) generated in a systematic, homogeneous way and stored statically (or at least versioned). It will be possible to regenerate this dataset at a later date.
• Dynamic, indicating that the dataset was created `on-the-fly', possibly by assembling the latest available data and applying a possibly changeable processing algorithm. A future attempt to generate this dataset in the same way may give different results.
• Custom, indicating that the dataset was created by manual analysis and processing, whether by data center staff or as part of a scientist's research project. Traceability of the processing may be incomplete.

The dataset is associated with one or more Collections (instrument name, survey name. etc.) indicating some degree of compatibility with other datasets sharing the same Collection properties. Examples of possible Collection values are: "WFC", "Sloan", "BFS Spectrograph", "MSX Galactic Plane Survey".

5.6 Segment type and size

This attribute is OPTIONAL and defaults to Spectrum.

6.0 Global metadata

6.1 SED attributes

We introduce an SED.Type field with values observed or simulated or composite (default = observed). The values indicate whether the data consists of actual measurements versus theoretical or empirical calculations, or is a composite of several observed datasets.

OPTIONAL: The SED attributes are all optional. The NSegments and SpectralMin/MaxWavelength can be derived from the metadata in the segments. Note that the latter fields, if present, are required to be wavelength in meters and are consistent with the Coverage.SpectralMinimumWavelength/SpectralMaximumWavelength in the IVOA Resource Metadata document.

6.2 Target model

Diagram for SED, Target objects

REQUIRED: The Target.Name field is required and is used to refer to the SED.

OPTIONAL: All other Target fields are optional.

At the moment there is no international standard list of valid values for Target class and spectral class. Nevertheless an initial deployment of the VO would gain some benefit from using archive-specific classes, and provide a framework for converging on a standard list.

6.3 Packaging model

These packaging values will be part of the SSA protocol response, and are implicit in the individual serializations.

• (1) FITS (standard BINTABLE for SED, to be defined)
• (2) VOTABLE
• (3) XML (native XML for web services and XML tools)
• (4) text (simple text table with columns of data and no markup)
• (5) text/html
• (6) graphics; a JPG, GIF etc. representation of the data
• (7) metadata; only the XML metadata.

7.0 Relationship to general VO data models

8.0 Serializations

8.1 FITS serialization

Here we give the mapping of data model fields to FITS columns and keywords. For each column, the standard keywords TTYPEn, TUNITn, TFORMn should be provided. Order of keywords and columns is not significant, except that it is strongly recommended that RA and Dec be in adjacent columns or keywords.

We adopt the convention that columns which are constant (same value for all rows) may if desired be omitted and the value given as a keyword instead. (e.g. the column TTYPEn='INSTRUME' is replaced by a keyword INSTRUME = 'value'). This is a small overhead in the FITS reading interface.

In particular, the SPCO_UCD and FLUX_UCD columns give the spectral coordinate and flux UCDs for each segment. If all the segments use the same flux and spectral coordinate UCDs, as will often be the case, these can be keywords instead.

We add a new keyword VOCLASS to describe the VO object represented by the FITS table.

If each segment has the same spectral coordinate UCD, the spectral coordinate may also be identified by optional 1Sn_1 and 1CTYPn keywords as per WCS Paper 3. Table 9 of that paper implies that each data column which is a function of the spectral coord needs a pair of such keywords. Applications which implement the SED data model may ignore the WCS keys and interpret the file by recognizing 'by spec' that SPCO is the spectral coordinate and that FLUX, etc. are functions of it, but the WCS keys give a general FITS application a chance at making sense of the file.

The following fields are scalar columns. These columns may be replaced by keywords if they are constant for the whole table. The Position.Type field is not explicitly serialized: the names of the two columns used for Position are used to infer the type.

The following fields are array columns. In each row, all of these arrays have the same length - the number of points in the segment. Since different segments may have different lengths, FITS' variable-length array mechanism may be used for these columns to save space. However, fixed-length arrays may also be used; in this case the SIZE column noted above must be used to specify the number of points in each segment.

Note: The ENERG_LO, ENERG_HI columns are already used in the X-ray community.

We summarize this with a full sample FITS extension header.

XTENSION= 'BINTABLE'           / binary table extension
BITPIX  =                    8 / 8-bit bytes
NAXIS   =                    2 / 2-dimensional binary table
NAXIS1  =                   80 / width of table in bytes
NAXIS2  =                 2048 / number of rows in table
PCOUNT  =                  208 / size of special data area
GCOUNT  =                    1 / one data group (required keyword)
TFIELDS =                   15 / number of fields in each row
EXTNAME = 'SPECTRUM '           / name of this binary table extension
VOCLASS = 'Spectrum V0.93'      / VO Data Model
VOCREATE= 'MMT Archive'       / VO Creator
VOCRID  = 'ivoa://cfa.harvard.edu' / VO Publisher ID URI
DATE    = '2004-08-30T14:18:17' / Date and time of file creation
DATE-OBS= '2004-06-03T21:18:17' / Date and time of observation
BANDPASS= ' '    / SED.Bandpass
RA_NOM       =     233.72789197     / [deg] Nominal RA    
DEC_NOM      =      23.49792615     / [deg] Nominal Dec   
OBJECT  = 'ARP 220 '           / Source name
SRCCLASS= 'Galaxy'             /
SPECTYPE= 'ULIRG'              /
RA_TARG      =     233.73791700     / Observer's specified target RA
DEC_TARG     =      23.50333300     / Observer's specified target Dec
REDSHIFT=              0.01812  / Emission redshift
DS_IDENT= 'cfa://whatever'   /
TITLE   = 'Observations of Merging Galaxies' /
VERSION =  2                     / Reprocessed 2004 Aug
VOPUB   = 'CfA Archive'        / VO Publisher authority
VOLOGO  = 'http://cfa.harvard.edu/vo/cfalogo.jpg' / VO Creator logo
SPCO_UCD= 'em.wl'                    /
FLUX_UCD= 'phot.fluxDens;em.wl'     /
EQUINOX =  2.0000000000000E+03 / default
RADECSYS= 'ICRS    '           / default
TIMESYS = 'TT     '           / Time system
TIMEUNIT= 's'                / Time unit                
SPECSYS = 'TOPOCENT'            / Wavelengths are as observed
TIMESDIM= 'T'                  / Time SIDim 
SPECSDIM= '10-10 L'             / Spectral SIDim
FLUXSDIM= '10+7 ML-1T-3'       / Flux SDim
MJDREF  =  0.0                 / MJD zero point for times
SPEC_RES=              5.0  / [Angstrom] Spectral resolution
SKY_RES =              1.0  / [arcsec] Spatial.Resolution

TELESCOP= 'MMT '           / Telescope
INSTRUME= 'BCS    '           / Instrument
FILTER  = 'G220    '           / Grating
COMMENT  ---------------------------
COMMENT  WCS Paper 3 Keywords
1S11_1   = 'SPCO'            / Column name with spectral coord
1CTYP11  = 'WAVE-TAB'        / Spectral coord is WAVE 
1S12_1   = 'SPCO'            / Column name with spectral coord
1CTYP12  = 'WAVE-TAB'        / Spectral coord is WAVE 
1S13_1   = 'SPCO'            / Column name with spectral coord
1CTYP13  = 'WAVE-TAB'        / Spectral coord is WAVE 
1S14_1   = 'SPCO'            / Column name with spectral coord
1CTYP14  = 'WAVE-TAB'        / Spectral coord is WAVE 
COMMENT  ---------------------------
TTYPE1  = 'INSTRUME  '           / Instrument ID
TFORM1  = '8A      '           / format of field
TTYPE2  = 'FILTER  '           / Filter ID
TFORM2  = '8A      '           / Format
TTYPE3  = 'RA '                / Position RA of aperture center
TFORM3  = '1D      '           /
TUNIT3  = 'deg     '           /
TTYPE4  = 'DEC     '                / Position Dec of aperture center
TFORM4  = '1D      '           /
TUNIT4  = 'deg     '           /
TTYPE5  = 'APERTURE'           / Aperture diameter (physical or extraction)
TFORM5  = '1E       ' /
TUNIT5 =  'arcsec '   /
TTYPE6 = 'TIME'
TFORM6 = '1D '
TUNIT6 =  'd'  /  MJD days
TTYPE7 = 'EXPOSURE' /  Effective exposure time
TFORM7 = '1E'
TUNIT7 = 's' 
TTYPE8 = 'SYS_ERR' / Fractional systematic error
TFORM8 = '1E'
TUNIT8 = 's'
TTYPE9 = 'SPCO'   / Wavelength
TFORM9 = '1PE'
TUNIT9 = 'Angstrom'
TTYPE10= 'SPCO_LO' /
TFORM10= '1PE'
TUNIT10= 'Angstrom'
TTYPE11= 'SPCO_HI' /
TFORM11= '1PE'
TUNIT11= 'Angstrom'
TTYPE12= 'FLUX' /
TFORM12= '1PE'
TUNIT12= 'erg cm**(-2) s**(-1) Angstrom**(-1)'
TTYPE13= 'ERR_LO' /
TFORM13 = '1PE'
TUNIT13 ='erg cm**(-2) s**(-1) Angstrom**(-1)'
TTYPE14= 'ERR_HI' /
TFORM14 = '1PE'
TUNIT14 ='erg cm**(-2) s**(-1) Angstrom**(-1)'
TTYPE15= 'QUALITY' /
TFORM15 = '1PI'

The data would look like

MMT/BCS  G300  233.73791  23.50333 2.0 52984.301203 1500.0 0.15  3200.0 3195.0 3205.0 1.48E-12 2.0E-14 2.0E-14   0
                                                                 3210.0 3205.0 3215.0 1.52E-12 3.0E-14 3.0E-14   0
                                                                 3220.0 3215.0 3225.0 0.38E-12 0.38E-12 0.0      0
                                                                 3230.0 3225.0 3235.0 1.62E-12 3.0E-14 3.0E-14   0
MMT/BCS  G300  233.73792  23.50334 2.0 52102.103211 1480.0 0.15  3200.0 3195.0 3205.0 3.48E-12 2.0E-14 2.0E-14   0
                                                                 3210.0 3205.0 3215.0 2.52E-12 3.0E-14 3.0E-14   0
                                                                 3220.0 3215.0 3225.0 1.38E-12 0.38E-13 0.38E-13 1
                                                                 3230.0 3225.0 3235.0 1.62E-12 3.0E-14 3.0E-14   0
FLWO/4S  B     233.73791  23.50333 4.5 48776.001234  300.0 0.05  4400.0 4200.0 4600.0 1.82E-12 1.2E-14 3.1E-14   0
FLWO/4S  V     233.73791  23.50333 4.5 48776.012012  300.0 0.05  5400.0 5200.0 5600.0 3.82E-12 1.3E-14 1.5E-14   0
FLWO/4S  R     233.73791  23.50333 4.5 48776.019013  240.0 0.05  7000.0 6200.0 7500.0 5.82E-12 1.3E-13 2.1E-13   0
FLWO/4S  I     233.73791  23.50333 4.5 48776.024988  240.0 0.08  9000.0 8200.0 9900.0 8.12E-12 3.3E-13 3.4E-13   0

8.2 Alternate FITS serialization

8.3 VOTable serialization

The VOTable version of Spectrum will represent an SED by a series of tables. First there is a table with no fields containing the overall SED and target information. Then there is one table for each individual spectrum segment. The data model fields described above as arrays map to VOTable FIELDs, while the remaining fields map to PARAM.

We use nested GROUP constructs to delimit data model objects within the main object, and PARAM and FIELD tags for attributes. Names of fields and parameters are left to the data provider. The utype and ucd attributes are used to denote data model and UCD tags. The schema and namespace for the utypes is the XML schema given in section 8.4. I have made up arbitrary NAME attributes for the PARAM and these are not to be considered standard; the name fields are free to be whatever the data provider wants, allowing compatibility with local archive nomenclature. The NAME attributes for the FIELD elements are also not standardized (of course they must be the same as in the matching FIELDrefs); it is the utype attribute which is standardized.

The one departure from the XML schema below is that the `ArrayOfPoints' element and the individual `Point' elements are implicitly represented by the table structure itself. Perhaps a UTYPE attribute to the TABLEDATA element could be used to make this explicit.

The example below has a single RESOURCE element containing three TABLE elements: the first TABLE element contains the top level SED object and the second and third TABLE elements contain SED segements.

If there is only one segment in the SED, the top level SED fields may be included in the same TABLE as the segment data (so that there is only one TABLE element in all.)

<?xml version="1.0" encoding="UTF-8"?>
<VOTABLE version="1.1"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="http://www.ivoa.net/xml/VOTable/v1.1"
  xmlns:sed="http://www.ivoa.net/xml/SedModel/v0.93"
  xmlns="http://www.ivoa.net/xml/VOTable/v1.1">
<RESOURCE utype="sed:SED">

<TABLE utype="sed:SED">   
<PARAM name="Date" utype="sed:Date" datatype="char" arraysize="*" value="2004-05-10"/>
<PARAM name="Nseg" utype="sed:NSegments" ucd="meta.number" datatype="int" value="1"/>
<PARAM name="Type" utype="sed:Type" datatype="char" arraysize="*" value="observed"/>
<GROUP utype="sed:Target">
 <PARAM name="Target" utype="sed:Target.Name" datatype="char" arraysize="*" value="Arp 220"/>
 <PARAM name="TargetPos" utype="sed:Target.pos" unit="deg" datatype="double" arraysize="2" value="233.737917 23.503330"/>
 <PARAM name="z" utype="sed:Target.redshift" datatype="float" value="0.0018"/>
</GROUP>
</TABLE>

<TABLE utype="sed:Segment">   
<!-- SegmentType can be Photometry, TimeSeries or Spectrum -->
<PARAM name="Segtype" utype="sed:Segment.SegmentType" datatype="char" arraysize="*" value="Photometry" ucd="meta.code"/>
<GROUP name="Frame" utype="sed:Segment.Frame">
 <GROUP utype="sed:Segment.Frame.Sky">
   <PARAM name="System" utype="sed:Segment.Frame.Sky.Type" ucd="frame.pos.system" datatype="char" arraysize="*" value="ICRS"/>
   <PARAM name="Equinox" utype="sed:Segment.Frame.Sky.Equinox" ucd="time.equinox;pos.eq" datatype="float" value="2000.0" />
 </GROUP>
 <GROUP utype="sed:Segment.Frame.Time">
  <PARAM name="TimeFrame" utype="sed:Segment.Frame.Time.Type" ucd="frame.time.scale" datatype="char" arraysize="*" value="UTC"/>
  <PARAM name="TimeZero" utype="sed:Segment.Frame.Time.Zero" ucd="frame.time.zero" datatype="double" value="0.0"/>
 </GROUP>
 <GROUP utype="sed:Segment.Frame.SpectralCoord">
  <PARAM name="SpectralFrame" utype="sed:Segment.Frame.SpectralCoord.RefPos" ucd="frame.em.system" datatype="char" arraysize="*" value="Barycent"/>
 </GROUP>
</GROUP>
<GROUP utype="sed:Segment.Coverage">
 <GROUP utype="sed:Segment.Coverage.Location">
  <PARAM name="SkyPos" utype="sed:Segment.Coverage.Location.Sky.Value" ucd="pos.eq" unit="deg" datatype="double" arraysize="2" value="132.4210 12.1232"/>
  <PARAM name="TimeObs" utype="sed:Segment.Coverage.Location.Time.Value" ucd="time.obs" datatype="double" value="52148.3252"/>
 </GROUP>
 <GROUP utype="sed:Segment.Coverage.Extent">
  <PARAM name="SkyExtent" utype="sed:Segment.Coverage.Extent.Sky" ucd="pos.region.diameter" datatype="double" unit="arcsec" value="20"/>
  <PARAM name="TimeExtent" utype="sed:Segment.Coverage.Extent.Time" ucd="time.expo;phot.spectrum" unit="s" datatype="double" value="1500.0" />
  <PARAM name="SpectralExtent" utype="sed:Segment.Coverage.Extent.Spectral" ucd="instr.bandwidth" unit="Angstrom" datatype="double" value="3000.0"/>
 </GROUP>

 <GROUP utype="sed:Segment.Coverage.Region">
  <GROUP utype="sed:Segment.Coverage.Region.Time">
   <PARAM name="TimeStart" utype="sed:Segment.Coverage.Region.Time.Start" ucd="time" unit="s" datatype="double" value="52100.000" />
   <PARAM name="TimeStop" utype="sed:Segment.Coverage.Region.Time.Stop" ucd="time" unit="s" datatype="double" value="52300.000" />
  </GROUP>
 </GROUP>


</GROUP>

<GROUP utype="sed:Segment.Curation">
 <PARAM name="Publisher" utype="sed:Segment.Curation.Publisher" ucd="meta.organization;meta.curation" datatype="char" arraysize="*" value="SAO"/>
 <PARAM name="PubID" utype="sed:Segment.Curation.PubID" ucd="meta.curation.pubid" datatype="char" arraysize="*" value="ivoa://cfa.harvard.edu"/>
 <PARAM name="Logo" utype="sed:Segment.Curation.Logo" ucd="meta.curation.logo" datatype="char" arraysize="*" value="http://cfa-www.harvard.edu/nvo/cfalogo.jpg"/>
 <PARAM name="Contact" utype="sed:Segment.Curation.ContactName" ucd="meta.human;meta.curation" datatype="char" arraysize="*" value="Jonathan McDowell"/>
 <PARAM name="email" utype="sed:Segment.Curation.ContactEmail" ucd="meta.email" datatype="char" arraysize="*" value="jcm@cfa.harvard.edu"/>
</GROUP>

<GROUP utype="sed:Segment.DataID">
 <PARAM name="Title" utype="sed:Segment.DataID.Title" datatype="char" arraysize="*" value="Arp 220 SED"/>
 <PARAM name="Creator" utype="sed:Segmenet.DataID.Creator" ucd="meta.curation.creator" datatype="char" arraysize="*" value="SAO/FLWO"/>
 <PARAM name="DateDate" utype="sed:Segment.DataID.Date" ucd="time;soft.dataset;meta.curation" datatype="char" arraysize="*" value="2003-12-31T14:00:02Z"/>
 <PARAM name="Version" utype="sed:Segment.DataID.Version" ucd="soft.dataset.version;meta.curation" datatype="char" arraysize="*" value="1"/>
 <PARAM name="Instrument" utype="sed:Segment.DataID.Instrument" ucd="inst.id" datatype="char" arraysize="*" value="BCS"/>
 <PARAM name="Filter" utype="sed:Segment.DataID.Collection" ucd="inst.filter.id" datatype="char" arraysize="*" value="G300"/>
 <PARAM name="CreationType" utype="sed:Segment.DataID.CreationType" datatype="char" arraysize="*" value="Archival"/>
</GROUP>

<GROUP utype="sed:Segment.Derived">
 <PARAM name="SNR" utype="sed:Segment.Derived.SNR" datatype="float" value="3.0"/>
</GROUP>

<GROUP utype="sed:Segment.Points.SpectralCoord">
 <FIELDref ref="Coord"/>

 <GROUP utype="sed:Segment.Points.SpectralCoord.Accuracy">
  <FIELDref ref="BinLow"/>
  <FIELDref ref="BinHigh"/>
 </GROUP>
<!-- In this case Resolution is demoted from Field to Param since it is constant -->
 <PARAM name="Resolution" utype="sed:Segment.Points.SpectralCoord.Accuracy.Resolution" unit="Angstrom" datatype="float" value="14.2"/>
</GROUP>

<GROUP utype="sed:Segment.Points.Flux">
 <FIELDref ref="Flux1"/>
 <GROUP utype="sed:Segment.Points.Flux.Accuracy">
  <FIELDref ref="ErrorLow"/>
  <FIELDref ref="ErrorHigh"/>
  <PARAM name="SysErr" utype="sed:Segment.Points.Flux.SysErr" unit="" datatype="float" value="0.05"/>
 </GROUP>
 <FIELDref ref="Quality"/>
</GROUP>

<FIELD name="Coord" ID="Coord" utype="sed:Segment.Points.SpectralCoord.Value" ucd="em.wavelength" datatype="double" unit="Angstrom"/>
<FIELD name="BinLow" ID="BinLow" utype="sed:Segment.Points.SpectralCoord.BinLow" ucd="stat.min;em.wavelength" datatype="double" unit="Angstrom"/>
<FIELD name="BinHigh" ID="BinHigh" utype="sed:Segment.Points.SpectralCoord.BinHigh" ucd="stat.max;em.wavelength" datatype="double" unit="Angstrom"/>
<FIELD name="Flux" ID="Flux1" utype="sed:Segment.Points.Flux.Value" ucd="phot.flux;em.wavelength" datatype="double" unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
<FIELD name="ErrorLow" ID="ErrorLow" utype="sed:Segment.Points.Flux.Accuracy.StatErrLow" datatype="double" unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
<FIELD name="ErrorHigh" ID="ErrorHigh" utype="sed:Segment.Points.Flux.Accuracy.StatErrHigh" datatype="double" unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
<FIELD name="Quality" ID="Quality" datatype="int" utype="sed:Segment.Points.Flux.Quality"/>

<DATA>
<TABLEDATA>
<!-- Note slightly nonlinear wavelength solution -->
<!-- Second row is upper limit -->
<!-- Third row has quality mask set -->
<TR><TD>3200.0</TD><TD>3195.0</TD><TD>3205.0</TD><TD>1.38E-12</TD><TD>5.2E-14</TD><TD>6.2E-14</TD><TD>0</TD></TR>
<TR><TD>3210.5</TD><TD>3205.0</TD><TD>3216.0</TD><TD>1.12E-12</TD><TD>1.12E-12</TD><TD>0</TD><TD>0</TD></TR>
<TR><TD>3222.0</TD><TD>3216.0</TD><TD>3228.0</TD><TD>1.42E-12</TD><TD>1.3E-14</TD><TD>0.2E-14</TD><TD>3</TD></TR>
</TABLEDATA>
</DATA>
</TABLE>

<!-- The second table below gives a radio light curve, and uses more defaults. -->
<TABLE utype="sed:Segment">   

<PARAM name="Segtype" utype="sed:Segment.SegmentType" datatype="char" arraysize="*" value="TimeSeries" ucd="meta.code"/>
<GROUP name="Frame" utype="sed:Segment.Frame">
 <GROUP utype="sed:Segment.Frame.Sky">
   <PARAM name="System" utype="sed:Segment.Frame.Sky.Type" ucd="frame.pos.system" datatype="char" arraysize="*" value="FK5"/>
   <PARAM name="Equinox" utype="sed:Segment.Frame.Sky.Equinox" ucd="time.equinox;pos.eq" datatype="double" value="2000.0"/>
 </GROUP>
 <GROUP utype="sed:Segment.Frame.Time">
  <PARAM name="TimeType" utype="sed:Segment.Frame.Time.Type" datatype="char" arraysize="*" value="MJD"/>
  <PARAM name="TimeZero" utype="sed:Segment.Frame.Time.Zero" ucd="frame.time.zero" datatype="double" value="0.0"/>
  <PARAM name="TimeSIDim" utype="sed:Segment.Frame.Time.SIDim" datatype="char" arraysize="*" value="T"/>
  <PARAM name="TimeSystem"  utype="sed:Segment.Frame.Time.System" ucd="frame.time.scale" datatype="char" arraysize="*" value="TT"/>
  <PARAM name="TimeRefPos"  utype="sed:Segment.Frame.Time.RefPos" ucd="pos.eq.ra" datatype="char" arraysize="*" value="TOPOCENTER"/>
 </GROUP>
 <GROUP utype="sed:Segment.Frame.SpectralCoord">
  <PARAM name="SpectralRefPos" utype="sed:Segment.Frame.SpectralCoord.RefPos" ucd="frame.em.system" datatype="char" arraysize="*" value="Barycent"/>
  <PARAM name="SpecSIDim"  utype="sed:Segment.Frame.SpectralCoord.SIDim" datatype="char" arraysize="*" value="10-10 L"/>
 </GROUP>
 <GROUP utype="sed:Segment.Frame.Flux">
  <PARAM name="FluxSIDim"  utype="sed:Segment.Frame.Flux.SIDim" datatype="char" arraysize="*" value="10+7 ML-1T-3"/>
 </GROUP>
</GROUP>
<GROUP utype="sed:Segment.Coverage">
 <GROUP utype="sed:Segment.Coverage.Location">
  <PARAM name="SkyPos" utype="sed:Segment.Coverage.Location.Sky.Value" ucd="pos.eq" unit="deg" datatype="double" arraysize="2" value="132.4210 12.1231"/>
  <PARAM name="TimeObs" utype="sed:Segment.Coverage.Location.Time.Value" ucd="time.obs" datatype="double" value="0.0"/>
  <PARAM name="SpectralValue" utype="sed:Segment.Coverage.Location.SpectralCoord.Value" ucd="em.freq" unit="MHz" datatype="double" value="1660.0"/>
 </GROUP>
 <GROUP utype="sed:Segment.Coverage.Extent">
  <PARAM name="SkyExtent" utype="sed:Segment.Coverage.Extent.Sky" ucd="pos.region.diameter" unit="arcsec" datatype="double" value="2.5"/>
  <PARAM name="TimeExtent" utype="sed:Segment.Coverage.Extent.Time" ucd="time" unit="d" datatype="double" value="1095.0"/>
  <PARAM name="SpectralExtent" utype="sed:Segment.Coverage.Extent.Spectral" ucd="instr.bandwidth" unit="MHz" datatype="double" value="400"/>
 </GROUP>
</GROUP>

<GROUP utype="sed:Curation">
 <PARAM name="Publisher" utype="sed:Segment.Curation.Publisher" ucd="meta.organization;meta.curation" datatype="double" arraysize="*" value="NRAO"/>
</GROUP>

<GROUP utype="sed:DataID">
 <PARAM name="Title" utype="sed:Segment.DataID.Title" datatype="char" arraysize="*" value="Arp 220 1660 MHz Monitoring"/>
 <PARAM name="Creator" utype="sed:Segment.DataID.Creator" ucd="meta.curation.creator" datatype="char" arraysize="*" value="NRAO"/>
 <PARAM name="Instrument" utype="sed:Segment.DataID.Instrument" ucd="inst.id" datatype="char" arraysize="*" value="VLA"/>
</GROUP>

<GROUP utype="sed:Segment.Points.Time">
 <FIELDref ref="Time"/>
</GROUP>

<GROUP utype="sed:Segment.Points.SpectralCoord">
 <PARAM name="Coord" utype="sed:Segment.Points.SpectralCoord.Value" ucd="em.freq" unit="MHz" datatype="double" value="1660.0"/>
</GROUP>

<GROUP utype="sed:Segment.Points.Flux">
 <FIELDref ref="Flux2"/>
 <GROUP utype="sed:Segment.Points.Flux.Accuracy">
  <PARAM name="ErrorLow" utype="sed:Segment.Points.Flux.Accuracy.StatErrLow" unit="mJy" datatype="double" value="0.15"/>
  <PARAM name="ErrorHigh" utype="sed:Segment.Points.Flux.Accuracy.StatErrHigh" unit="mJy" datatype="double" value="0.15"/>
  <PARAM name="SysErr" utype="sed:Segment.Points.Flux.Accuracy.SysErr" datatype="double" unit="" value="0.10"/>
 </GROUP>
</GROUP>

<FIELD name="Time" ID="Time" utype="sed:Segment.Points.Time.Value" ucd="time" datatype="float" unit="d"/>
<FIELD name="Flux" ID="Flux2" utype="sed:Segment.Points.Flux.Value" ucd="phot.flux;em.freq" datatype="double" unit="mJy"/>

<DATA>
<TABLEDATA>
<TR><TD>46066.5</TD><TD>5.18</TD></TR>
<TR><TD>46246.5</TD><TD>6.21</TD></TR>
<TR><TD>46431.0</TD><TD>11.32</TD></TR>
<TR><TD>46796.0</TD><TD>15.00</TD></TR>
<TR><TD>46975.3</TD><TD>7.32</TD></TR>
<TR><TD>47161.5</TD><TD>3.10</TD></TR>
</TABLEDATA>
</DATA>
</TABLE>

</RESOURCE>
</VOTABLE>

8.4 XML schema serialization

<?xml version="1.0" encoding="utf-8"?>
<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <!-- The top level element: an SED with one target and many segments -->
  <xs:element name="SED" nillable="true" type="SEDType" />
  <xs:complexType name="SEDType">
    <xs:sequence>
      <xs:element minOccurs="0" maxOccurs="1" name="Date" type="TimeParam" />
      <xs:element minOccurs="0" maxOccurs="1" name="Target" type="Target" />
      <xs:element minOccurs="0" maxOccurs="1" name="CustomParams" type="ArrayOfParam" />
      <xs:element minOccurs="0" maxOccurs="1" name="Type" type="TextParam" />
      <xs:element minOccurs="0" maxOccurs="1" name="NSegments" type="IntParam" />
      <xs:element minOccurs="0" maxOccurs="unbounded" name="Segment" type="Spectrum" />
    </xs:sequence>
  </xs:complexType>
  <!-- A single segment corresponding to a spectrum or single point -->
  <xs:complexType name="Spectrum">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
         <xs:element minOccurs="0" maxOccurs="1" name="SegmentType" type="TextParam" />
         <xs:element minOccurs="0" maxOccurs="1" name="Frame" type="Frame" />
         <xs:element minOccurs="0" maxOccurs="1" name="Coverage" type="Coverage" />
         <xs:element minOccurs="0" maxOccurs="1" name="Curation" type="Curation" />
         <xs:element minOccurs="0" maxOccurs="1" name="CustomParams" type="ArrayOfParam" />
         <xs:element minOccurs="0" maxOccurs="1" name="DataID" type="DataID" />
         <xs:element minOccurs="0" maxOccurs="1" name="Derived" type="DerivedData" />
         <xs:element minOccurs="0" maxOccurs="1" name="Fields" type="Point" />
         <xs:element minOccurs="0" maxOccurs="1" name="Points" type="ArrayOfGenPoint" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <!-- Define the UCDs etc for the SED coordinate and the flux coordinate,
      and include a global to specify accuracy etc which happens to be 
      constant for the entire segment (note that in SEDCoord,
      value has minOccurs=0 so it can be omitted)  -->
      

  <!-- A single SEDCoord (time or spectral coord) value. -->

  <xs:complexType name="SEDCoord">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Value" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Accuracy" type="Accuracy" />
          <xs:element minOccurs="0" maxOccurs="1" name="Resolution" type="DoubleParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="Flux">
    <xs:complexContent mixed="false">
      <xs:extension base="SEDQuantity">
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>


  <xs:complexType name="SEDQuantity">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Value" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Accuracy" type="Accuracy" />
          <xs:element minOccurs="0" maxOccurs="1" name="Quality" type="IntParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>


  <!-- The error model. Bin entries will usually be omitted for the flux coordinate -->

  <xs:complexType name="Accuracy">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="BinLow" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="BinHigh" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="BinSize" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="StatErrLow" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="StatErrHigh" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="SysErr" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Confidence" type="DoubleParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <!-- A set of useful types to add UCDs and units to base types; like BasicQuantity -->
  <xs:complexType name="Group" />
  <xs:complexType name="TextParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param" />
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="Param">
    <xs:attribute name="name" type="xs:string" />
    <xs:attribute name="ucd" type="xs:string" />
  </xs:complexType>
  <xs:complexType name="DateParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param" />
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="PositionParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param">
        <xs:attribute name="unit" type="xs:string" />
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="DoubleParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param">
        <xs:attribute name="unit" type="xs:string" />
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="TimeParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param">
        <xs:attribute name="unit" type="xs:string" />
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="IntParam" mixed="true">
    <xs:complexContent mixed="false">
      <xs:extension base="Param">
        <xs:attribute name="unit" type="xs:string" />
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <!-- Define the space-time frame; eventually replace with STC -->

  <xs:complexType name="Frame">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Sky" type="SkyFrame" />
          <xs:element minOccurs="0" maxOccurs="1" name="Time" type="TimeFrame" />
          <xs:element minOccurs="0" maxOccurs="1" name="Spectral" type="SpectralFrame" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="SpectralFrame">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="SpectralRefPos" type="TextParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="TimeFrame">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="TimeType" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="TimeZero" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="TimeSystem" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="TimeRefPos" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="TimeRefDir" type="PositionParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="SkyFrame">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Type" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Equinox" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="System" type="TextParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <!-- The Field type allows us to define what our axes are -->

  <xs:complexType name="ArrayOfField">
    <xs:sequence>
      <xs:element minOccurs="0" maxOccurs="unbounded" name="Field" nillable="true" type="Field" />
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="Field">
    <xs:attribute name="Name" type="xs:string" />
    <xs:attribute name="Unit" type="xs:string" />
    <xs:attribute name="Ucd" type="xs:string" />
  </xs:complexType>

  <!-- The Point type groups a single set of time, spectral and flux values -->

  <xs:complexType name="ArrayOfGenPoint"/>

  <xs:complexType name="ArrayOfPoint">
   <xs:complexContent mixed="false">
    <xs:extension base="ArrayOfGenPoint">
     <xs:sequence>
      <xs:element minOccurs="0" maxOccurs="unbounded" name="Point" nillable="true" type="Point" />
     </xs:sequence>
    </xs:extension>
   </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="Point">
   <xs:sequence>
    <xs:element name="Time" minOccurs="0" maxOccurs="1" type="SEDCoord" />
    <xs:element name="SpectralCoord" minOccurs="0" maxOccurs="1" type="SEDCoord" />
    <xs:element name="Flux" minOccurs="0" maxOccurs="1"  type="Flux" />
   </xs:sequence>
  </xs:complexType>

  <xs:complexType name="ArrayOfFlatPoint">
   <xs:complexContent mixed="false">
    <xs:extension base="ArrayOfGenPoint">
     <xs:sequence>
      <xs:element minOccurs="0" maxOccurs="unbounded" name="Point" nillable="true" type="FlatPoint" />
     </xs:sequence>
    </xs:extension>
   </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="FlatPoint">
          <xs:attribute name="T" type="xs:double"/>
          <xs:attribute name="T_BinL" type="xs:double" />
          <xs:attribute name="T_BinH" type="xs:double" />
          <xs:attribute name="T_Size" type="xs:double" />
          <xs:attribute name="T_Res" type="xs:double" />
          <xs:attribute name="SP" type="xs:double" />
          <xs:attribute name="SP_BinL" type="xs:double" />
          <xs:attribute name="SP_BinH" type="xs:double" />
          <xs:attribute name="SP_Size" type="xs:double" />
          <xs:attribute name="SP_Res" type="xs:double" />
          <xs:attribute name="F" type="xs:double" />
          <xs:attribute name="F_ErrL" type="xs:double" />
          <xs:attribute name="F_ErrH" type="xs:double" />
          <xs:attribute name="F_Sys" type="xs:double" />
          <xs:attribute name="F_Qual" type="xs:int" />
  </xs:complexType>

  <!-- A special Position type including support for RA/Dec and HTM -->

  <xs:complexType name="Position">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Value" type="PositionParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="HTMID" type="IntParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>


  <!-- Now we define the higher level metadata -->

  <xs:complexType name="Curation">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Publisher" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="PubID" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Reference" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Creator" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Logo" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Date" type="DateParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Version" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Contact" type="Contact" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="Contact">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Name" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Email" type="TextParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="Coverage">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Location" type="CoverageLocation" />
          <xs:element minOccurs="0" maxOccurs="1" name="Extent" type="CoverageExtent" />
          <xs:element minOccurs="0" maxOccurs="1" name="Region" type="CoverageRegion" />
          <xs:element minOccurs="0" maxOccurs="1" name="Fill" type="CoverageFill" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="CoverageLocation">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Sky" type="Position" />
          <xs:element minOccurs="0" maxOccurs="1" name="Time" type="TimeParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Spectral" type="TextParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="CoverageExtent">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Sky" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Time" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Spectral" type="DoubleParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="CoverageFill">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Sky" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Time" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Spectral" type="DoubleParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>



  <xs:complexType name="CoverageRegion">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Sky" type="SkyRegion" />
           <xs:element minOccurs="0" maxOccurs="1" name="Time" type="Interval" />
          <xs:element minOccurs="0" maxOccurs="1" name="Spectral" type="Interval" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="Interval">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Min" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Max" type="DoubleParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="SkyRegion">
    <xs:complexContent mixed="false">
      <xs:extension base="TextParam"/>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="DerivedData">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="SNR" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="VarAmpl" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="redshift" type="SEDQuantity" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="DataID">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Title" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Creator" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Collection" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="DatasetID" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Date" type="TimeParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Version" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Format" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Instrument" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="CreationType" type="TextParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="Target">
    <xs:complexContent mixed="false">
      <xs:extension base="Group">
        <xs:sequence>
          <xs:element minOccurs="0" maxOccurs="1" name="Name" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="Description" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="TargetClass" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="spectralClass" type="TextParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="redshift" type="SEDQuantity" />
          <xs:element minOccurs="0" maxOccurs="1" name="pos" type="PositionParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="VarAmpl" type="DoubleParam" />
          <xs:element minOccurs="0" maxOccurs="1" name="CustomParams" type="ArrayOfParam" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="ArrayOfParam">
    <xs:sequence>
      <xs:element minOccurs="0" maxOccurs="unbounded" name="Param" nillable="true" type="Param" />
    </xs:sequence>
  </xs:complexType>
</xs:schema>

<?xml version="1.0" encoding="UTF-8"?>
<SED xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns="http://ivoa.net/xml/SED/SED-0.93.xsd">
<!-- xml instance example -->
 <Date>2004-05-10</Date>
 <NSegments ucd="meta.number">1</NSegments>
 <Target>
  <name>Arp 220 </name>
  <pos>233.737917 23.503330</pos>
  <redshift>0.0018</redshift>  
 </Target>
 <Segment>
  <SegmentType ucd="meta.code">Photometry</SegmentType>
  <Frame>
   <Sky>
    <Equinox ucd="time.equinox;pos.eq">2000.0</Equinox>
    <System ucd="frame.pos.system">ICRS</System>
   </Sky>
   <Time>
     <TimeType ucd="frame.time.system?">MJD</TimeType> 
     <TimeZero ucd="frame.time.zero">0.0 </TimeZero>
     <TimeSystem ucd="frame.time.scale">UTC</TimeFrame> 
     <TimeRefPos ucd="pos;frame.time">TOPOCENTER</TimeZero>
   </Time>
   <SpectralCoord>
     <SpectralRefPos ucd="frame.em.system">Barycent</SpectralRefPos>
   </SpectralCoord>
  </Frame>
  <Coverage>
  <Location>
    <Sky ucd="pos.eq" unit="deg">132.4210 12.1232</Sky>
    <Time ucd="time.obs" unit="d">52148.3252</Time>
   </Location>
   <Extent>
     <Sky ucd="pos.region.diameter" unit="arcsec">20</Sky>
     <Time ucd="time.expo;phot.spectrum" unit="s">1500.0</Time>
     <Spectral ucd="instr.bandwidth" unit="Angstrom">3000.0</Spectral>
   </Extent>
  </Coverage>
  <Curation>
   <Publisher ucd="meta.organization;meta.curation">SAO</Publisher>
   <PubID ucd="meta.curation.pubid">ivoa://cfa.harvard.edu</PubID>
   <Logo ucd=meta.curation.logo">http://cfa-www.harvard.edu/nvo/cfalogo.jpg</Logo>
   <Contact>
     <Name ucd="meta.human;meta.curation">Jonathan McDowell</Name>
     <Email ucd="meta.email">jcm@cfa.harvard.edu</Email>
   </Contact>
  </Curation>
  <DataID>
    <Title>Arp 220 SED</Title>
    <Creator ucd="meta.curation.creator">SAO/FLWO</Creator>
    <Date ucd="time;soft.dataset;meta.curation">2003-12-31T14:00:02Z</Date>
    <Version ucd="soft.dataset.version;meta.curation">1</Version>
    <Instrument ucd="inst.id">BCS</Instrument>
    <Collection>Archival</Collection>
  </DataID>
  <Derived>
    <SNR>3.0</SNR>
  </Derived>
  <!-- Define table structure -->
  <Fields>
    <SpectralCoord>
     <Value ucd="em.wavelength" unit="Angstrom"/>    
     <Accuracy>
      <BinLow ucd="stat.min;em.wavelength" unit="Angstrom"/> 
      <BinHigh ucd="stat.max;em.wavelength" unit="Angstrom"/>
     </Accuracy>
   </SpectralCoord>
   <Flux>
     <Value ucd="phot.flux;em.wavelength" unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
     <Accuracy>
       <StatErrLow  unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
       <StatErrHigh unit="erg cm**(-2) s**(-1) Angstrom**(-1)"/>
       <SysErr>0.05</SysErr>
     </Accuracy>
     </Quality>
   </Flux>
  </Fields>
  <!-- Use table structure -->
  <Points>
   <Point> 
    <SpectralCoord>
     <Value>3200.0</Value>
     <Accuracy><BinLow>3195.0</BinLow><BinHigh>3205.0</BinHigh></Accuracy>
    </SpectralCoord>
    <Flux>
      <Value>1.38E-12</Value>
      <Accuracy><StatErrLow>5.2E-14</StatErrLow><StatErrHigh>6.2E-14</StatErrHigh></Accuracy>
      <Quality>0</Quality>
    </Flux>
   </Point>

   <Point> 
    <SpectralCoord>
     <Value>3210.5</Value>
     <Accuracy><BinLow>3205.0</BinLow><BinHigh>3216.0</BinHigh></Accuracy>
    </SpectralCoord>
    <Flux>
      <Value>1.12E-12</Value>
      <Accuracy><StatErrLow>1.12E-12</StatErrLow><StatErrHigh>0</StatErrHigh></Accuracy>
      <Quality>0</Quality>
    </Flux>
   </Point>

   <Point> 
    <SpectralCoord>
     <Value>3222.0</Value>
     <Accuracy><BinLow>3216.0</BinLow><BinHigh>3228.0</BinHigh></Accuracy>
    </SpectralCoord>
    <Flux>
      <Value>1.42E-12</Value>
      <Accuracy><StatErrLow>1.3E-14</StatErrLow><StatErrHigh>0.2E-14</StatErrHigh></Accuracy>
      <Quality>3</Quality>
    </Flux>
   </Point>

  </Points>
 </Segment>



 <Segment>
  <SegmentType ucd="meta.code">TimeSeries</SegmentType>
  <Frame>
   <Sky>
    <Equinox ucd="time.equinox;pos.eq">2000.0</Equinox>
    <System ucd="frame.pos.system">FK5</System>
   </Sky>
   <Time>
     <TimeType ucd="frame.time.type">MJD</TimeType> 
     <TimeFrame ucd="frame.time.scale">TT</TimeFrame> 
     <TimeZero ucd="frame.time.zero">0.0 </TimeZero>
   </Time>
   <SpectralCoord>
     <SpectralRefPos ucd="frame.em.system">Barycent</SpectralRefPos>
   </SpectralCoord>
  </Frame>

  <Coverage>
   <Location>
    <Sky ucd="pos.eq" unit="deg">132.4210 12.1231</Sky>
    <Time ucd="time.obs" unit="d">0</Time>
   </Location>
   <Extent>
     <Sky ucd="pos.region.diameter" unit="arcsec">2.5</Sky>
     <Time ucd="time.expo;phot.spectrum" unit="s">1095.0</Time>
     <Spectral ucd="instr.bandwidth" unit="Angstrom">400.0</Spectral>
   </Extent>
  </Coverage>

  <Curation>
   <Publisher ucd="meta.organization;meta.curation">NRAO</Publisher>
  </Curation>

  <DataID>
    <Title>Arp 220 1660 MHz Monitoring</Title>
    <Creator ucd="meta.curation.creator">NRAO</Creator>
    <Instrument ucd="inst.id">VLA</Instrument>
  </DataID>

  <Fields>
   <Time>
     <Value ucd="time" unit="d"/>    
   </Time>
   <SpectralCoord>
     <Value ucd="em.freq" unit="MHz"> 1660.0   </Value>    
   </SpectralCoord>
   <Flux>
     <Value ucd="phot.flux;em.freq" unit="mJy"/>
     <Accuracy>
       <StatErrLow  unit="mJy">0.15 </StatErrLow>
       <StatErrHigh unit="mJy">0.15 </StatErrHigh>
       <SysErr>0.10</SysErr>
     </Accuracy>
   </Flux>
  </Fields>

  <Points>

   <Point> 
    <Time>
     <Value>46066.5</Value>
    </Time>
    <Flux>
      <Value>5.18</Value>
    </Flux>
   </Point>

   <Point> 
    <Time>
     <Value>46246.5</Value>
    </Time>
    <Flux>
      <Value>6.21</Value>
    </Flux>
   </Point>
  <!-- etc. -->
  </Points>
</Segment>





</SED>