Absolute vs differential photometry

The discussion above implicitly assumed that the "comp star'' was in the same image as the target star, and that we were happy to monitor the brightness of the target relative to the comp star. In some cases, we might not know the "true" magnitude of the comp star, but that's OK since the variation of the target is completely described by its light curve relative to the comparison. Since we determined the magnitude difference between target and comp stars, this method is called "differential photometry".

Differential photometry is a surprisingly robust method of monitoring the brightness of a target. The delta-magnitude that you determine for the target is unaffected by changing the exposure duration, and almost unaffected by changing atmospheric conditions. For example, if your comp star is in the same FOV as the target, and your image FOV is pretty small (a fraction of a degree is typical), then you can be pretty confident that any haze or wispy cloud that affects the target star will also affect the comp star. Similarly, if your imager's shutter isn't precisely accurate, so that one image may have a slightly longer or shorter exposure than the next image, that doesn't matter because both target and comp stars were exposed for the same duration on any one image, and (ADUtarget/ADUcomp) isn't affected by changing exposure durations. This means that the delta-mag is unaffected by modest amounts of haze or changing atmospheric extinction. (There are some important caveats to this, discussed in Section 4.6.2.)

For this reason, almost all amateur research projects rely on differential photometry. This method is quite effective for studying variable stars, determining asteroid rotation periods, and detecting extra-solar planets.

"Absolute photometry"—to determine the brightness of a target on the standard stellar magnitude scale, without having well-characterized comp stars in the FOV—is quite a bit more complicated because of the many additional factors that must be considered. If you need to image the target star in one frame, and then move the telescope through 20 or 30 degrees to get to the "standard" comp star, you need to be sure that the imager's exposure is exactly the same on both images,* and you need to determine the atmospheric extinction in both directions so that it can be accounted for, and you need a way to be confident that the atmospheric conditions didn't change between one image and the next, etc. These can all be handled, and absolute photometry is well within the capability of the advanced amateur astronomer, but beware that it does require a quantum step in effort compared with differential photometry. The basics of "absolute photometry" will be described in Section 4.7.

For most variable stars, your differential photometry can be based on comparison stars whose photometry has already been determined. For example, the AAVSO star charts include comparison stars whose V-magnitude and color indices have been accurately determined. They are, in effect, secondary standards. By using a V-filter when you make your images, and using the AAVSO-recommended comp stars, you are closely anchored to the standard system, even though you are doing differential photometry.

All of this background is probably more than you really need to know in order to conduct the most common amateur CCD photometry project—variable star measurements—but hopefully it will help you understand the rationale behind the procedures that you will use for variable stars, and other projects.

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  • karen
    How do I do differential photometry?
    12 months ago

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