Subject: Re: A comparison of different missions

From: Gary Bernstein

Submitted: 11 Jun 2004 09:06:48 -0400

Message number: 231 (previous: 230, next: 232 up: Index)

Hi Michael -
	I can tell you are very excited to have this document completed!

In the table, you have 0.34 deg^2 for the SNAP FOV, which is only the
CCD area.  There's an additional 0.34 deg^2 of NIR detector area which
should be counted.  The pixel scale is 0.18" on the NIR side.

The Image Quality for SNAP is worse than the diffraction-limited 0.09"
you report, because of aberrations, charge diffusion, and pixelization. 
A number more like 0.15 would be appropriate at 700 nm, as far as
calculating a point-source figure of merit.

The figure of merit given is the speed for background-limited point
sources.  As such it should have the sky brightness in the denominator. 
For the 3 ground-based observatories, this can be omitted because
they're all the same, but for SNAP there is an additional gain of factor
of 7 at R band in dark moon, growing very rapidly toward the IR.  I
think these correction make the FOM=420 for SNAP if we take the R-band
sky brightness factor.

The Figure of Merit might also include a duty cycle (3x better on
orbit), but this is mentioned in your table Notes, which is probably a
simpler way to present things.

I believe that DEC will not be competing for CTIO time, but will rather
have a grant of some fraction of the telescope time.

In the p4 text, I'd say
"SNAP..., a space-based mission to measure Type Ia Supernovae to
redshift 1.7,...."

In the Table, a couple of typos:
"Tescope" -> "Telescope"

You probably want "Pixel Scale" instead of "Plate Scale"

Image Quality might say "(arcsec FWHM)"


-Gary



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