What is the difference between "differential" and other photometry?

Arne Henden

Originally posted to CCD-astrometry-photometry on Nov 6, 2003.

There are several approaches to doing CCD photometry.

  1. purely instumental differential. Pick a comparison star(s) in the same field of view as the program object, and just calculate the difference in magnitude between the comp star(s) and the program object. Good points: highest _precision_, especially if several comp stars are used; simple. Bad points: not on standard system; others have to use the same comp stars to get close to the same values; if you pick the comp star yourself, watch out for it being variable. To guard against the latter point, most observers use at least two comparison stars (the second one is often called a "check" or K star), and then form the difference (K-C) as well as (V-C).

  2. differential using standardized comp stars. Here you know the magnitude/color of the comp stars, so you can calculate magnitudes/colors of the program object and be close to the standard system. Example:
              Vstandard = Vinst - Cinst + C_standard
    Good points: close to standard system if you are using reasonable filters, still simple. Bad points: you have to have magnitudes/colors for the comp stars.

  3. differential using standardized comp stars, transformed onto the standard system. Here you apply your transformation coefficients to the instrumental differences and get standardized magnitudes and colors. Good points: you are now on the standard system. Bad points: you have to have magnitudes/colors for the comp stars and your transformation coefficients; more complex than 1) or 2).

  4. All-sky. Here you don't bother with local standards, but instead determine the nightly extinction coefficients and zeropoints and apply the full transformation equations to your observations. Bad points: requires photometric night, and even then, you will be lucky to get better than 0.01-0.02mag accuracy.

Most people use 1), 2) or 3); these differential methods, since program object and comparison star are in the same field of view, work well for most sky conditions, even an overcast sky. The hard parts with 2) and 3) are obtaining standardized magnitudes/colors for the comp stars, which is what I understood the original question to be asking. If you cannot find reasonable photometry, then you either have to calibrate the stars yourself or else just report which star(s) you used so that others can modify your results when calibration is available.

Option 4) is most often used when you are initially calibrating comparison stars around your eventual program objects. Performing all-sky calibration on a handful of good nights means that you now have calibrated comparison stars for the other, less photometric, nights. I have also seen option 4) used for photoelectric photometry from good sites. Since in PEP work you only look at one object at a time, doing all-sky work means you increase your efficiency by at least 2x (not doing the comp stars). For CCD work, where your comp star is in the FOV, this advantage goes away (except for those rare cases where you can't get a decent comparison star on the CCD at the same time as your program object).

The AAVSO is making considerable headway in providing good sequences around most of their program stars, usually in multiple colors. Like I have said many, many times, it is better, especially if you are a beginner, to pick easy projects, or at least projects that are within the capability of your equipment, site and observing ability. Working with a field that has good calibration is usually easier than going it on your own.