Figure 1: SAOimage displays world coordinates at the cursor position, in this case FK4/B1950 right ascension and declination, above the image pixel coordinates.
Figure 1: PS 971 Kb
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Douglas J. Mink
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
The availability over the Internet of many large-scale astronomical surveys, such as the IRAS Infrared Sky Survey (Van Buren et al. 1995), and the HST Digitized Sky Survey (Morrison 1995), as well as the Hubble Space Telescope archives (Travisano 1995) was discussed at the 1994 Conference on Astronomical Data Analysis Software and Systems. In addition to such World Wide Web interfaces to specific data sets, SkyView (McGlynn et al. 1994) provides a versatile interface to data from many surveys.
Information linking spatial coordinates (world coordinate system or WCS) to image pixel coordinates is inserted into the headers of the FITS images derived from these surveys. Descriptions of standard projective systems are now being standardized (Greisen & Calabretta 1995a, 1995b). The Digitized Sky Survey, scanned from photographic plates rather than created from digital detectors, provides plate solutions with each extracted image. All that was needed was an easy-to-use viewing program which could display WCS information.
SAOimage was developed in the late 1980's at the Smithsonian Astrophysical Observatory to display astronomical images (Van Hilst 1990). Its ease of use, simplicity, and portability have made it popular throughout the astronomical community.
When the wealth of available WCS-containing FITS-format information became apparent, steps were taken to add world coordinate access to SAOimage. Both cursor tracking in sky coordinates (see Figure 1) and the ability to export cursor positions as sky coordinates were added to the program. Additional capabilities have been added as users requested them and new data formats became available.
Figure 1: SAOimage displays world coordinates at the cursor position,
in this case FK4/B1950 right ascension and declination, above the image
pixel coordinates.
Figure 1: PS 971 Kb
SAOimage was first modified in version 1.09 to extract WCS parameters from FITS image headers. A WCS data structure was developed, and the worldpos C subroutines by Bill Cotton and Don Wells, which implement the eight projections from classic AIPS (see Table 1), were modified to work with it. Subroutines to initialize the data structure and to convert WCS cursor positions into character strings completed a self-contained library. Digitized Sky Survey plate solution algorithms were added in version 1.12. As an example of its portability, this library is also being used in a new image display program, SAOtng (Mandel & Tody 1995).
Table 1: World coordinate systems supported by SAOimage
The option of displaying or exporting coordinates in systems other than that used in which the image header was added in version 1.14. Subroutines derived from Patrick Wallace's SLALIB positional astronomy library (Wallace 1995) were used to translate between FK4, FK5, and galactic coordinate systems. Version 1.17 extended WCS access to IRAF OIF images, by internally translating IRAF headers to FITS headers and extracting parameters using the code already used for FITS files.
The ``c'' command was added in SAOimage version 1.09 to print the current cursor position in world and image pixel coordinates, along with the value of the closest pixel. Right ascension and declination are printed as hh:mm:ss.sss dd:mm:ss.ss; galactic latitude and longitude are printed as dd.ddddd dd.ddddd. If WCS information is not available in the header, only pixel coordinates and value are returned.
In version 1.14, commands were added to SAOimage to print the world coordinates of the current cursor position in any of three coordinate systems. Coordinates will be converted if this is not the same system as that in the image header. b prints the current cursor coordinates in the B1950 (FK4) system as hh:mm:ss.sss dd:mm:ss.ss B1950. j prints the cursor coordinates in the J2000 (FK5) system as hh:mm:ss.sss dd:mm:ss.ss J2000. g prints the cursor position in galactic coordinates as dd.ddddd dd.ddddd galactic. If SAOimage is being used to look at a local image, output will be to the window from which SAOimage was invoked. If SAOimage is being used with Mosaic, output is sent to the console window. Netscape sends output from helper applications to a new window, but not until several lines have been sent.
Additional command line flags set the format in which the world coordinates of the current cursor position will be displayed. The same position string that is displayed above the pixel coordinate in the SAOimage window is sent to standard output when the ``c'' command is used and to an external program when the ``w'' cursor command is issued.
Adding the switch -fk4 or -b1950 to the command line causes the cursor to track in FK4/B1950 right ascension and declination. The coordinates in hours, minutes, and seconds of right ascension and degrees, minutes, and seconds of declination will put into a string of the format hh:mm:ss.sss dd:mm:ss.ss B1950.
The switch -fk5 or -j2000 makes the cursor track in FK5/J2000 right ascension and declination, displaying hh:mm:ss.sss dd:mm:ss.ss J2000.
The command line switch -galactic (which may be shortened to -gal) causes the cursor to track in galactic longitude and latitude. The coordinates in decimal degrees will be displayed to five decimal places in the format dd.ddddd dd.ddddd galactic. Leading zeroes are dropped.
To set the external command to be executed, add -wcscom followed by a space-free string containing a UNIX command string to the command line. The world coordinate string will be substituted for %s in the UNIX command string, as in C formatted output. Underscores should be used instead of spaces. The ``w'' cursor command executes the command specified on the command line using -wcscom, passing the string corresponding to the current cursor position.
To search the Hubble Space Telescope Guide Star Catalog in a
30-arcsecond-square region centered on the current cursor position using
the RGSC program, add
-wcscom rgsc_%s_-box_0:00:15 to the saoimage command.
When the cursor is centered on the star in the upper right corner of
Figure 1, the ``w'' command prints the following:
HST Guide Stars B1950
HSTGSC at 13:27:32.976 47:29:10.88 30" x 30" (B1950)
3460.0534 13:27:32.285 47:29:04.27 14.98 0 1 9.87
SAOimage can be used as a ``helper'' application to a World Wide Web
browser such as Netscape or Mosaic.
To make sure SAOimage is used whenever a FITS image is encountered,
this line should be added to your .mime.types file:
image/x-fits fit fits fts FIT FITS
and this line to your .mailcap file
image/x-fits; saoimage -fits %s
To serve FITS files from a http server so that SAOimage will
be invoked to browse them, add this line to the httpd srm.conf file:
AddType image/x-fits .fit .fits .fts .FIT .FITS
and make sure that any FITS files on your server have one of those extensions.
Greisen, E., & Calabretta, M. 1995a, in Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., Vol. 77, eds. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco, ASP), p. 233
Greisen, E., & Calabretta, M. 1995b, submitted to A&A
Mandel, E., & Tody, D. 1995, in Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., Vol. 77, eds. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco, ASP), p. 125
McGlynn, T., White, N., & Scollick, K. 1994, presented at Astronomical Data Analysis Software and Systems IV, Baltimore, MD
Morrison, J. 1995, in Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., Vol. 77, eds. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco, ASP), p. 179
Travisano, J. 1995, in Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., Vol. 77, eds. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco, ASP), p. 80
Van Buren, D., Ebert, R., & Egret, D. 1995, in Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., Vol. 77, eds. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco, ASP), p. 84
Van Hilst, M. 1990, BAAS, 22, 935
Wallace, P. 1995, SLALIB---Positional Astronomy Library, Starlink User Note 67.3, CCLRC/Rutherford Appleton Laboratory