[ATM] Banishing the diagonal

[David Whysong <dwhysong@gmail.com>]

> So, in order to reduce diffraction you put the ccd in the light path and then
> run cooling tubes to it?   Sounds like we have come back around to the
> original problem. ;-)

Secondary diffraction is rarely an issue for CCD imaging because the
exposures are long enough that seeing dominates the PSF. (Spikes
from a spider do show up, of course.) As for thermal issues, you simply
insulate the camera.

If I had the money, I'd build a 20" f/4-ish Newtonian with a large CCD
and paracorr at the focus. The only reason to have a diagonal is that
the SBIG research series cameras have a large and asymmetric shape,
which might cause a "lumpy" PSF in good seeing conditions. That could
affect astrometric measurements.

--
David
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