Unless I made a mistake in the math.
On the surface the math looks ok, for simple absolutes, and would work wondefully doing all-sky photometry under photometric conditions. But that's not what amateurs typically do. What we end up doing is differential photometry under less than perfect skies. So expect more noise in the calcs overall, some from the cameras themselves, some from the sky conditions (seeing, transparency, etc), and the measurement is one noisy source against another (target vs reference).
I have not tried things with current generation cmos sensors, but I do remember a few years ago after I did an exoplanet transit measurement for the first time using an SXV-H9 camera in a C8. At that point, my wife wanted to try the same kind of stuff, she was using a dslr, and had just read an article about folks doing a transit measurement with a dslr. That particular one was a very bright candidate, ie one of the easier measurements to do, so we decided to set up a test run on a fast variable with a significant change over a few hours, just to see the difference.
The end result was as shown in the graphs below. Top one, the ccd in the C8 with reducer, bottom one is a dslr in a Williams 110. They were set up on our back deck about 10 feet apart pointed at the same variable and both running exposure lengths to bring the target star up to about 50% saturation, typical of how we do differential photometry. Suffice it to say, after doing this side by side experiment, the next day I went shopping for another ccd. the plots below use the same reference stars against the same target star, just different camera setup, admittedly a poor camera for this kind of work. Camera in question is 12 bits with a bayer color matrix measured against a low end monochrome ccd with photometric filters, in this case if memory serves correctly, we had the V filter in for this run.
If one now assumes that the modern cmos sensors do give good linear results, then look more detailed at the differential photometry problem. For this exercise, there is no reason to set the gain at anything other than unity. Exposure time would be defined by whatever time it takes to get your desired count on the target star, so using the above numbers, expose till the target is at 2900 or so to achieve a 0.02 differential measurement. Any bright stars in the field will be saturated, and you need suitable reference stars in field that come up to roughtly 2500 as a minimum, and 3500 as a maximum. If there is any vignetting in the field, you need to choose reference stars that are not in the vignetted portion of the field. Choices of reference stars will be limited on a small sensor with any amount of vignetting in the field.
But in general, it really depends on your goals and objectives. If the goal is to measure variables with significant changes over time, the 12 bits wont be a huge hinderance. If your goal is to get serious on exoplanet transit measurements, many of them are somewhat less than 0.02 magnitude dips. I remember when the first keppler data set was released, a bunch of us jumped on what looked like the easiest of them to snag, and we had a bit of a competition going here on CN to see who would get one first. After the first night with runs on one of them (dont remember which) the conclusion was, gosh these are hard. We had to go deep enough that binning was required to get measurable numbers on 10 minute exposures, and none of us were able to produce a light curve that showed any recognizeable dip.
If one already has the camera setup, it's easy to see what it's capable of. The AAVSO variable star database has lots of suitable candidates for generating short term light curves, just search the database for something with a period of less than 0.1 days (2.4 hours) and variability of half a mag or so, that's a great way to wet your feet and see what kind of light curve you can produce, the change is large enough to be clearly visible thru any system noise, and fast enough you can get a couple cycles in one session. This is how I found the star used for the curves above.
On the flip side, if you are thinking of buying a camera specifically for the purpose, you have to start by defining the purpose, there are many different applications of photometry, and they have different requirements. In our case, I want to be able to take measurements of virtually all the stars of a magnitude range in a given field, so we set up a system specifically targetting that application. It's by no means a starter system, 16803 sensor in a 12 inch RC capable of illuminating the entire field with no vignetting. What got me hooked, was the first time I did a run on a transitting exoplanet, this is the curve we got from WASP-10b when set up at a star party many years ago. When we saw this result, it completely changed our approach to the astronomy hobby, and when we started choosing equipment for our 'retirement project' observatory, being able to do this kind of stuff at higher precision was the primary consideration. This curve was generated from data taken with an SXV-H9 in a C8 with 0.63 reducer, ie a relatively modest system. We bought all this equipment from the used marketplace, so it was not a huge outlay to set it up.
It's now the better part of a decade later, we have moved, now live under a dark sky and the observatory build is on hold due to snow, but that should clear up soon. I have a significant list of projects for this setup over time, it'll measure exoplanet transits, do some asteroid light curves, and likely start some longer term projects studying specific objects which have not yet been defined.
The point of this long ramble, photometry means different things to different people, and choosing the camera to use really depends on what it is you are trying to measure. There will be projects well within the reach of the cmos cameras, but, there will be many projects that are outside the reach of those sensors. It really depends on what you are trying to measure. The analogy is, a measuring tape marked out in 1/16" increments will do a fine job measuring a lot of things, but, it'll fall flat trying to measure something where you are trying to get accurate measurements of a half millimeter, for that you probably want a micrometer.
So to go back to a proper answer for the question in the original post, is a cmos camera suitable for doing photomtery for the professor in the astronomy club. To answer that question, one has to refine the question. What is it that you want to measure ?
Edited by groz, 27 December 2016 - 01:31 PM.