Subject: Re: A comparison of different missions

From: Michael Strauss

Submitted: Fri, 11 Jun 2004 09:14:49 -0400

Message number: 232 (previous: 231, next: 233 up: Index)

Gary Bernstein writes:
 > 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.
Right; the optical and NIR observations are carried out
simultaneously, so you are correct; I will double the area. 

 > 
 > 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.
OK; I was hoping you'd give me the more appropriate number!  Thanks!

 > 
 > 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.
Rather than complicate things in the table, I will include a note
about this in the table caption. 

 > 
 > 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.
Yup. 

 > 
 > I believe that DEC will not be competing for CTIO time, but will rather
 > have a grant of some fraction of the telescope time.
OK, I'll reword to clarify that it is not a dedicated facility... 

 > 
 > 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)"
 > 
I'll make all those changes. 

 > 
 > -Gary
 > 
 > 

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