| MadSci Network: Astronomy |
It's funny I would get sent this question. Part of my job is to figure out exactly this number-- for an object of a given brightness, how many photons do we see in our camera aboard Hubble?
The number of photons detected from an object depends on many things, including the brightness of the object, the size of the telescope and the sensitivity of the detector. A spectrum will tell you how bright an object is at a given wavelength. This in turn will tell you just how many photons you get at that wavelength. The sensitivity of the detector itself depends on lots of factors, such as the wavelength range (color range, if you will) at which you are looking, filters used, etc. Those numbers can be found by testing the equipment. We have tested the our camera thoroughly before launch, so we know those numbers pretty well. It allows us to predict how the camera will perform after launch.
An object like a quasar does not produce very many photons. Even over the area of Hubble's 2.4 meter mirror, only a handful of photons are counted every second. A typical quasar may produce less than 10 total photons per second detected! (This is for a typical quasar in the near ultra-violet range using numbers for STIS, one of the cameras on Hubble). That is why we need to take long exposures; a 1000 second exposure will give us 10,000 photon counts, which is not a bad number. Compare this to an image of Alpha Centauri, a bright star, which may produce as much as 30 billion photons every second! We actually need to block most of the light from bright stars to make sure we don't hurt our detectors!
For more info about Hubble and its detectors, go to the Space Telescope Science Institute web site. They have details about the telescope, its detectors and lots of images taken over the past few years.