Your astrometric reduction software can be one of the popular image-processing packages or a more specialized software program. As with photometry, the popular packages (which you may already own) are quite adequate for most projects; but the specialized packages have features that may simplify your data analysis or provide improved accuracy.
All star charting or planetarium programs contain an internal database of stars. The database entry for each star has its position (RA and Dec), proper motion (maybe), magnitude (usually), and color (sometimes). Most popular programs use the Hubble Space Telescope Guide Star Catalog (GSC) as their faint-star database. This catalog provides accurate positions for all of its stars, and can be used for most amateur astrometric projects. In general, any planetarium or CCD image-processing program that is capable of performing astrometric reductions will automatically select the astrometric stars to use, without bothering you with the details. Examples include CCDSoft + TheSky, MaximDL, and AstroArt.
Some programs can use other (and more accurate) stellar catalogs, such as the USNO SA-2 or the UCAC2 catalogs. If your first few astrometric projects infect you with the "astrometry bug", then you will most likely want to upgrade to an astro-metric star catalog, and the software that can take advantage of it (examples include MPO Canopus and IRAF). Use the software that you have for your first couple of astrometry projects. Then, if you decide that you enjoy those projects, make the modest investment in specialized software.
The key functions of the astrometric software are to (a) determine the position [x, y] of each star in the image to a fraction of a pixel, (b) find the function that relates pixel position to RA-Dec across the image (the so-called "plate constants"), and finally (c) use the plate constants to determine the RA-Dec position of your target. It is also a good thing if the software provides you with a table of the "residuals" between actual vs. calculated reference star positions. The residuals give you an indication of how good your solution was, and an estimate of the accuracy of the astrometry of your target.
If you would like to know a little bit about what your astrometric software is doing "behind the scenes'', refer to Appendix B. There, you'll find some background information on the celestial coordinate system and a description of the modern astrometric reference catalogs.
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