A sky coordinate grid can be overlayed on the image using grid, and a slice in the x and y planes made using the slice command. The skymap command is used to read in a file of star positions and overlay them on the current image. Circular and box regions can be defined and written to an SAOIMAGE type region file using the circle and box commands. The HEASARC database can be directly queried using the skymap/db=, where /db= is used to specify the name of a HEASARC database.
To draw a skygrid type grid. The default grid spacing is usually optimal, but can be changed by giving the degree steps e.g. grid 0.5 0.5. If the grid is not optimal use display to redisplay, and then try a new grid. The grid will be drawn using the current equinox. The /gal qualifier will draw the grid in galactic coordinates.
There are two commands to convert pixel coordinates to sky positions and vice-a-versa. These are pix[el_to_ra_dec] and ra[_dec_to_pixel]. They can, as with all ximage commands, be abbreviated to make them unique e.g. pix and ra. Use pix/cur to input the pixel coordinates using the cursor.
The default equinox used by ximage will be that of the original image. All coordinates will use this equinox, unless it is changed using the cey year command, where year is the equinox e.g. 1950. Once the equinox is changed all coordinates will be given referenced to that year. The equinox year can be changed many time during a session.
To draw a slice along the x or y axis then use slice/x or slice/y. The cursor is used to select the range of y or x to sum the image over to make the slice (the opposite axis to which the slice is to be made). For am x slice then click the left button to choose two points along the y axis, then click the right hand button to draw the slice. The middle button can be used to undo a selection before the roght button is clicked. The /log qualifier will draw the slice on a log scale.
The skymap command reads in a list of star positions and overlays them on the current image. The default will read an ascii file with the first line containing the equinox year (e.g. 1950) and on each following line the RA and DEC in decimal degrees. The /label qualifier will also overlay the names of the objects and the /class will give the classification.
A powerful feature in ximage 2.50 and greater is to directly make overlays of catalogs in the HEASARC database. This is done as follows:
[3]XIMAGE> skymap/db=rc3/label
This will open a connection to the HEASARC database system at GSFC and will download all objects within the image that are found in the rc3 database into a local file. This is then automatically overlayed on the image. The /label qualifier will cause the objects to be writen. The /class qualifier will write the object class as well, if it is known. Useful master databases such as OPTICAL, XRAY, RADIO and ROSID give an overview of the contents of the many individual databases. See the HEASARC online services users guide for further details (to obtain a copy send an e-mail to request@legacy.gsfc.nasa.gov).
The SIMBAD source lists provided in FITS format by the US ROSAT data center can be read in using the /RFITS_SIMBAD qualifier.
It is possible to access the HEASARC skyview facility directly from XIMAGE using the skyview command. This will take the coordinates and image size from the current image, and request the image from the specified survey. This is useful for making overlays and identifying X-ray sources. A limited number of surveys are hardwired into XIMAGE and can be accessed using qualifiers (use skyview/? to see the qualifiers). Other surveys may have been added to skyview and the full list can be accessed using skyview/list and the chosen survey accessed by giving the full name as a parameter e.g. skyview digitized sky survey. After the image is retrieved it must be read back in, and displayed in the usual way.