Re: TASS - PSFs

[aah@nofs.navy.mil]

Tom wrote:
>BTW, now that I have three cameras all looking at the same spot, I can 
>compare stars found.  It is roughly what I would expect.  The I camera
>finds more stars.  The I filter has roughly twice the bandwidth of the
>V and R.  But it also sees more background light.  This is a tough game.
  The nominal center wavelengths and widths of the VRI filters are:
     filt  center  width  (nm)
       V    545      88
       R    638     138
       I    797     149
So the R filter is not that much narrower than I.  Also, if you look at
the QE response of the KAF0400, it peaks between 600-800nm, so both R & I
should have roughly the same throughput.  I think you go deeper with the
I filter due to: (1) less atmospheric and galactic extinction; (2) most
faint stars are red and so have more flux at I; (3) scattered light tends
to be blue, so any light pollution decreases at I as well.  With the
thinned, backside illuminated chips here at the observatory, I find exposures
that give the same number of stars to be in the ratios B:V:R:I = 8:5:3:3.

>  With the sky background limit, twice the light only gets you Sqrt(2)
>more S/N.  This holds true for larger diameter lenses.  S/N goes up 
>with the diameter, so it is just not worth the effort to go to more
>expensive lenses.
  This is true with all 3 filters since all TASS images are sky background
limited.  While the cost of larger lenses goes up much quicker than a linear
function of the diameter, what you mostly gain is higher quality images
(smaller fwhm, less astigmatism and coma when used wide-open).

Arne
--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
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