Subject: Tony Tyson, on figures of merit

From: Michael Strauss

Submitted: Fri, 11 Jun 2004 11:49:00 -0400

Message number: 234 (previous: 233, next: 235 up: Index)

(Because of the spam filtering at Princeton, all of Tony's messages
bounce; sorry for the confusion...  -M)


I agree with all of Gary's points regarding the new parameters table.
But it addresses only one category of science an LSST could undertake.
Some science drivers don't need IR, some are more sensitive to surface
brightness, and others are very sensitive to the exposure time.  I
suggest modifying SNAP either into two columns with Optical and IR
explicitly listed or at least say for example 0.34 + 0.34 etc if one
column.  We should be listing three figures of merit, and explain them
as we now do in the caption: etendue, etendue/arcsec^2 (as now), and
etendue/(arcsec^2 * exposure time).  The exposure time could be either
what is currently planned or the time to reach the sky limit.
Finally, we should be using effective aperture (including secondary
obscuration) for each facility.

Tony

Michael Strauss wrote:

>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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