Doing the four steps (select aperture size, measure "star + sky'', measure "sky-only", and subtract to get "star-only") for two or three stars on each image, and then repeating for several images, is a tedious and error-prone process if you do it manually. Happily, all of the most popular astronomical image-processing software programs include routines that automate most of these steps. CCDSoft (Software Bisque), AstroArt (MSB Software), MaximDL (Diffraction-Limited), MPO Canopus (BDW Publishing), IRAF (distributed free by National Optical Astronomy Observatories) all provide powerful photometry routines that automate the four steps, and most of the other functions that are discussed in this chapter. The user's manual for the software that you use for image acquisition and processing will explain the capabilities and procedures for using its photometry tools. Some packages will enable you to define the target and comp stars, and bang, out comes the target star delta-magnitude. Most will allow you to define two or more "comp stars'' (e.g., a "comp star'' and a "check star''). The "check star'' is used to confirm that the comp star isn't varying in brightness. Some of these programs can be instructed to analyze a whole series of images, and create a light curve almost unattended.
Hence, you probably won't go through the meticulous manual detail of the four steps for your projects (except perhaps once or twice, to confirm that you understand what your software is doing behind the scenes). It is useful to understand the fundamental principles, however, so that you'll understand the reasons behind several of the image-processing and calibration steps, as well as the pitfalls that can come your way when you're doing a photometric research project.
Some software packages will link your image to a star-chart database and attempt to give you the actual magnitude of the target (instead of its delta-instrumental magnitude relative to the comp star). Unfortunately, all of the popular star catalogs used in the planetarium programs have serious photometric problems . The Guide Star Catalog, which forms the basis for most planetarium programs' stellar database, cannot be relied upon for photometry. Errors of ±0.5 magnitude are not unusual in the GSC. The V-magnitudes reported in the Tycho Catalog (also a popular basis for planetarium programs) are better, but still are good to only about ±0.1 magnitude. (There is a pretty simple formula that will transform Tycho magnitudes to the standard V-band with much better accuracy [3, 4], but that isn't included in the popular planetarium programs.) The UCAC2 Catalog—the recommended catalog for astrometry—is also not a very good photometric reference (±0.2 magnitude). This isn't a failing of the planetarium programs, nor of their star catalogs. These catalogs were developed specifically for astrometry, and they are very, very good at that. They were never intended to be used for accurate photometry.
Therefore, for most photometric projects, you will refer to a well-determined photometric database (such as the AAVSO star charts) for the "true" magnitude of your comp stars. You will then determine the brightness of your target from the delta-magnitude between your target and the comp stars.
If for some reason you don't have well-characterized comp stars in the FOV of your target, it is still possible to determine accurate photometry on the standard B, V, R color system. I'll describe how in Section 4.7. The procedure is well within the capability of a patient amateur astronomer, but it does entail some additional work compared with differential photometry. Happily, it is not necessary for most amateur photometric projects.
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