Saturation limits for a satellite mission

Michael Richmond
Oct 18, 2008

For some particular satellite mission, how bright a star can we observe without saturating the detectors? In this document, I investigate the question for one particular generic satellite with the following parameters:

One can compare the importance of these several factors to the overall system throughput:

We end up with overall system sensitivity as a function of wavelength within each passband:

I compute stellar fluxes using Vega as a standard source. I'll assume that the magnitude of Vega in any filter is zero; thus, my calculations are on a "Vega-based" system, not an "AB" system. I used a spectrum of Vega supplied by Ralph Bohlin.

I also computed fluxes for two other stars: a G2V star, similar to the Sun, and a K7V star, much cooler than the Sun. These spectra -- not flux calibrated -- were taken from Pickles (1998) . In order to set these stars on the same flux scale as Vega, I did the following:

The result is that the scaled versions of these other stars' spectra will yield the same number of photons as Vega will, when each is observed through a Bessell V-band filter. In other words, I am matching stars of different spectral types only in the V-band; all my scaled spectra will yield magnitudes "V=0".

I then convolved light from these scaled spectra through the overall transmission curves. The result is the number of DETECTED ELECTRONS, per square cm per second, for a star of magnitude V=0. Using the collecting area of the telescope (area = 2.14 sq. meters), I then computed the total number of detected electrons in a single second.

What fraction of these electrons will lie within the central pixel? That depends on the location of the star's center with respect to the pixel grid on the detector. For circular gaussian PSFs on square detectors, we can compute the best case (star falls at corner of four pixels) and worst case (star falls at center of pixel):

How large will the PSF be? I assumed that the optics were diffraction limited, in which case the size of the PSF depends on the wavelength. Physics states that the half-width of a diffraction-limited spot is roughly, in angular units,

       theta  =  1.22  ----------------        radians

and we can then convert into meters on the focal plane by computing

       half_width  ~   (theta) * (focal_length)       meters

I then made the further assumption that this diffraction pattern was approximately a gaussian, and set the FWHM of the gaussian to (2 * half_width). We end up with

lambda           400 nm         800 nm         1500 nm
 FWHM            0.98 pix       1.96 pix       2.14 pix

Over this range, the fraction of light falling in the central pixel is approximately linear with FWHM. Using a simple linear approximation, I calculated the fraction of light falling into the central pixel for all passbands.

So, given the total number of detected electrons per second of exposure, and the fraction of those which lie within the central pixel, we can then compute the number of electrons which lie in the central pixel after a 1-second exposure, for a magnitude V=0 star. We can compare that number to the full-well depth of the detector to determine

Below is a table showing the minimum V-band magnitude required not to saturate the detector in a 1-second exposure.

#  spectral_type      filter   lambda      min_mag
#                               (nm)
   vega_spectrum.dat    8B0     426       11.278 
   vega_spectrum.dat    8B1     495       11.949 
   vega_spectrum.dat    8B2     593       11.904 
   vega_spectrum.dat    8B3     712       11.646 
   vega_spectrum.dat    8B4     845       11.011 

   ukg2v_mag0.dat       8B0     426       10.574 
   ukg2v_mag0.dat       8B1     495       11.659 
   ukg2v_mag0.dat       8B2     593       12.065 
   ukg2v_mag0.dat       8B3     712       12.191 
   ukg2v_mag0.dat       8B4     845       11.807 

   ukk7v_mag0.dat       8B0     426        9.741 
   ukk7v_mag0.dat       8B1     495       11.323 
   ukk7v_mag0.dat       8B2     593       12.298 
   ukk7v_mag0.dat       8B3     712       12.830 
   ukk7v_mag0.dat       8B4     845       12.721