Subject: Plate scale on DSC

From: Stupendous Man

Submitted: Tue, 17 Oct 95 06:54:43 EDT

Message number: 10 (previous: 9, next: 11 up: Index)

  Looking at two frames near NGC 2419 that I took last night
with the DSC + SDSS i'-band filter, I can find 8 stars
which appear in the Hubble Guide Star Catalog.  
The stars are spread over an area about 7.5 x 2.5 arcmin.
Solving for the plate scale with these 8 stars, I get

         plate scale = 0.281 +/- 0.001 arcsec/pixel

  I also note that the "quick look" routines in 'shiver',
the ones which act when on runs 'rview', clicks on Cursor
and then points to a star and presses Right Button, don't
seem to give the correct value for FWHM.  Last night, I 
consistently got values of "3.2" or "3.3" when I used
the 'rview' routine; taking that printed value as the 
FWHM in pixels, I thought the seeing was about 0.9 arcsec!
But in the morning, I looked at one frame and carefully
measured FWHM ~ 6.9 pixels = 1.8 arcsec.  

  Maybe I misunderstand the number that is printed by
'rview'.  Perhaps that's the Half-Width at Half-Max,
in pixels.  If so, then it _does_ yield a pretty good answer.
In that case, my apologies for casting aspersions on it :-)

                     Michael Richmond
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