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Coordinate Systems

The IRAF environment provides three coordinates systems:

The PROS units for sky coordinates can be: decimal degrees, radians, $\prime$, $\prime\prime$, hours:minutes:seconds (hms), or degrees:arcmin:arcsec (dms). The default epoch is that of the reference image (i.e., B1950 for Einstein images, and J2000 for ROSAT images). However, the sky system may be changed by explicitly declaring the new system, as in the examples below.

The following syntax, (for the ROSAT test data target, AR Lac) can be used to specify source positions in the imcontour task (cf. ``help imcontour''):

Logical, physical and world coordinate systems are all supported in the PROS software, and regions can be specified with any combination of them. Sky coordinates are identified by the input format (i.e., :, d, r, $^{\prime}$, $^{\prime\prime}$). Coordinates in pixels are logical by default (this is also the IRAF default). To use physical pixel coordinates, you need to declare them as physical, as in the examples below (unique abbreviations are accepted).

For example, to specify a circular region at the center of xdata$rp110590.qp [block=30] at the peak position :

N.B.: When a qpoe file is displayed and no blocking or subsectioning has been applied, the physical and logical units are the same. However, if the input file is, for example, xdata$rp110590.qp[6675:7187,9135:9647] (selects a region from the central portion of the file only, reducing the file size to 512$\times$512 while preserving full resolution), the values in the ``physical'' example above should be:

In an example using the Einstein data, xdata$snr.qp[block=2], the values would be:

N.B.: Input can be in any of the above syntaxes, in physical, logical and world coordinates. If SAOimage is used to make a region file the output is in logical pixels, regardless of the input units. There is a task, fixsaoreg, which will convert these logical coordinates to physical coordinates, which free the region definition from the particular section used in SAOimage. Then a PROS task like imcnts can use the converted region file with any image or section derived from the original source image.

For more information see `help regions', `help regcoords', and `help coords'. Also, see fixsaoreg, skypix, and wcscoords for information regarding coordinate conversion.

next up previous contents
Next: Treatment of Errors Up: Features of the PROS Previous: Features of the PROS