# need data-point: actual counts per second with APD (avalanche photodiode) detector

I'm designing a sensor system to perform specialized [astronomy and space-sciences] experiments, and need a "reality check" to support or adjust my theoretical calculations.

What I need is the "counts per second" produced by any modern APD (avalanche photo-diode) sensor through a telescope of any specific "aperture" of a star of any "visual magnitude". I also need the number of "counts per second" of "nothing" (the "dark count") to subtract that from the "counts per second" when illuminated by the star (to determine the "counts per second" generated by the star alone).

This "reality check" will help me assure various "inefficiency allowances" I made are realistic. Examples:

#1: overall detector QE over relevant visual [and near IR] wavelengths.
#2: loss of light in atmosphere before entering telescope.
#3: loss of light in telescope optics.
#4: loss of light in fiber (if any).
#5: anything/everything else.


As implied, I am only interested in the APD operating in "photon counting mode" (not analog).

I've read about 5 dozen articles that discuss APDs for astronomy, but none give a straightforward value. The closest I found was a vague statement that the limited magnitude was 22nd magnitude on a 6-meter telescope based upon observations of the crab nebula pulsar. But this is not specific and the object is highly variable (on a short time frame). They did not say, for example, whether they consider their "limiting magnitude" is where the count rate rises from 200 per second (dark count) to 220 per second (measurement), or 200cps to 400cps, or over what time period, or any other indication of their definition.

All I need is ONE clear statement of cps for any aperture and visual magnitude star. You'd think I could find that in dozens if not hundreds of articles, but... no. Probably a clear statement like I need exists in some article somewhere, but I haven't seen one. Have you? Or better yet, have you made such an observation yourself?

The following detail is not very important (but just to be complete), my primary applications perform fairly high time-resolution measures on fairly bright stars. In other words, the experiments generally need to measure in the range of "counts per microsecond" to "counts per millisecond". Typically APDs max out at around 15 to 50 million counts per second, and most of my experiments will be working at 10K to 10M counts per second to observe the short time-period phenomenon I need to measure.

-
I am not an expert of optical-IR astronomy, so I can't directly address you. Try to take a look at the "yellow book" of telescopes that could be similar to the one you are simulating: they contain all relevant information about the science and the detector. –  Py-ser Mar 16 at 3:43