Database Contents
Plate Selection
Plates are referenced by a plate series which usually identifies individual telescopes and a plate number, which usually increases monotonically for every plate exposed at the telescope.Observatory Location
Telescopes did not necessary remain at one location but were moved between observatories worldwidePlate Class
The plate class serves to identify the kind of exposure that was taken and is useful in filtering out plates that may require special techniques for analysis or for selecting plates taken with particular filters or gratings.Plate Jacket Photographs
Prior to scanning, a photograph was taken of every plate jacket to capture standard plate information and additional notations that may have been made by observers or researchersPlate Annotation Photographs
Many researchers made ink annotations on the rear of plates to identify objects. If such annotations were found prior to scanning, they were photographed and then the annotations were cleaned from the plate. The emulsion side of the plates was never cleaned.Exposure Information
Many of the plates have multiple exposures to increase the dynamic range of the emulsion, include calibration fields, or look for short-period brightness changes. For each exposure, the Right Ascension and Declination are calculated for the center of the plate.Geocentric and Heliocentric Dates
The time of the exposure is expressed as a geocentric date. The corresponding heliocentric date is calculated with reference to the right ascension and declination of the center of the plate. Since some plate series contain plates which cover wide areas of the sky, the heliocentric date may vary significantly from the value at the center of the plate. If the user specifies a specific location or object in the search page, then the Julian Date result will show the heliocentric date for that object in the search page results table.Plate Center Source and Date Source
The initial digitization effort involved typing in the card catalog for the stacks. Approximately 129,000 catalog entries were digitized prior to the availability of initial results from the plate scanner. At this point, the scanner results showed that the Julian Date information available in the card catalog was not of sufficient accuracy for the discovery of objects such as eclipsing binaries. The decision was then made to photograph and digitize the logbooks which were the source of the card catalog data. Information from the card catalog is still designated asCatalogand this information will be eventually replaced by information from the logbooks which are designated by
Logbook. Because the logbook plate centers have a median error of about 0.5 degrees, these logbook plate centers will eventually be replaced with the results of wcs fitting to star catalogs which are designated by
imWCS.
Exposure Notes
The digitized card catalog information contains a remarks field which is preserved in the database.Time Accuracy
There are a number of automated checks possible on the digitized logbook data. The results of these checks is a time accuracy estimate to give researchers an indication of the reliability of the quoted time. Time accuracy is not available for catalog entries.Logbook Information
The database contains images of 663 bound logbook volumes and approximately 500 loose leaf logbooks. Some of the data has been digitized at Harvard and in India, but currently the data is being digitized by volunteers at the American Museum of Natural History in New York City. We appreciate the assistance of Dr Michael Shara, Curator, Dept. of Astrophysics, Holly Klug, Supervisor of Explainer Programs, Dept. of Volunteer Services and their team of volunteers. All of these photographs are available for browsing.Exposure Start and Stop Times
Exposure start and stop times were generally recorded in Local Sideral Time, but in some cases Eastern Standard Time or Universal Time was used. The time type in use can usually be determined from the plate center right ascension and the hour angle. The final reported exposure time in the plate exposure information table is the average of the start and stop times. These times are given to a varying number of significant digits and are used to initialze the time accuracy entry.Hour Angle
The hour angle was reported not only in the standard sense of increasing time West of the meridian, but sometimes the opposite. Occasionally errors in the sign of the angle can be detected from the context of the rest of the observation.Logbook Remark
Because comments entered into the logbooks have no defined formats, the typists were instructed to enter eitherYor
N. If this field is blank, the logbook entry has not yet been processed.
Logbook Notes
Entries are usually entered at Harvard for certain targets of opportunity. These notes contain the reason for transcription.Errors Detected Automated Checks
A number of automatic checks of the logbook data is possible. If one of these checks fail, then the reported time accuracy is reduced accordingly or simply set to 1 day if the actual error magnitude is not obvious. The following definition list shows the error check flags that appear in the database should one of these tests fail. Theversionsreferred to below are used to differentiate new logbook data being entered into the database from previously existing logbook data.
- Discrepancy:
- Discrepancy between India and Cambridge transcriptions. (Deprecated because of exposure number uncertainty for multiple exposures)
- E vs W Fixed:
- E Hour angle mistaken for W or vice-versa. Automatically corrected
- Bad Date Fixed:
- Bad date fixed from nearby data (Not yet implemented)
- Bad Date:
- Bad date which has not been corrected
- 12H Fixed:
- 12 Hour LST error Fixed
- Exposure Times Disagree:
- Exposure Times Disagree
- Non-Numeric Exposure:
- Non-numeric exposure time
- Daylight Exposure:
- Exposure while sun is up
- Bad Volume Number:
- Volume numbers disagree
- HA not LST minus RA:
- Error in Hour Angle
- Field Length Exceeded:
- Field Length Exceeded
- Bad Remark:
- Remark field not "Y" or "N", corrected to "N"
- Bad Multiple (OLD):
- Multiple exposures on different pages - compared with old version
- Bad Multiple (NEW):
- Multiple exposures on different pages - compared with current version
- Decreasing plateNumber:
- Plate number not monotonically increasing
- Unknown Time Type:
- Unable to determine if time is LST, UT, or EST
- Below Horizon:
- The telescope is apparently pointing below the horizon
- Bad Page:
- Error decoding the logbook volume and page
Scan and Mosaic Information
Scan Number If a plate is scanned multiple times, the scan number keeps track of individual scans.Mosaic Number
If a second mosaic must be created from a single scan using a new mosaic algorithm, the mosaic number identifies the algorithm. If available, a new mosaic number is assigned with the corresponding scan number. If not available, new mosaic numbers are assigned from unused scan and mosaic numbers. Afterwards, a new scan number will not be assigned an existing scan or mosaic number.Scan Date
The scan date is important for identifying early scans which are likely to be of lower quality while the scanner was being tested. Because of the mutually exclusive means of assigning scan and mosaic numbers, the scan date is a better way to determine the order of scanning.Average Tile ADU
Tiles are individual exposures of the scanner camera which are later combined into a mosaic. Since the scanner camera has a 12 bit CCD readout, the average tile ADU will range from the bias level of 50 ADU to the maximum level of 4095 ADU.Average and Maximum Tile Saturation
In order to improve the dynamic range for dark plates, the scanner exposure time is automatically adjusted. However, pixels imaging film defects and other areas of missing emulsion will saturate. The saturation count is the number of pixels at 4095 ADU on a per tile basis. Since there are 16M pixels per exposure, this is the upper limit of saturation.Scan Comment
If the average and maximum tile saturation levels reach preset amounts, the scanner software will automatically enterSaturation Errorin this field. The scanner operator is free to add any additional comments describing the state of the plate or the reason for the scan.
Mosaic Comment
If a special issue arises with the creation of the mosaic, a comment will be entered here. Currently, the only comment for some mosaics isLinearity Bugwhich means that the mosaic should be rejected because of a bug in the early version of the mosaic algorithm.
Mosaic Binning
Normally, the mosaic algorithm generates two mosaics simultaneously. These are the full size mosaics with1in this field and a 16 x 16 binned mosaic with
16in this field. Some other binning factors for early mosaics are also present, but the binning factor must be a power of two.