THIS REPLACES FIG 2 IN THE 3/10/98 DRAFT OF "GENERAL REQUIREMENTS FOR
SOAR INSTRUMENTS".


There are 3 figures on the web site in both jpeg and postscript formats:

skynoise.jpg  shotnoise.jpg readnoise.jpg  
skynoise.ps   shotnoise.ps  readnoise.ps   

Caption:

Fractions of total variance for SOAR that arise from (left) sky,
(middle) shot, and (right) readout noise at 0.65 microns, various
spectral resolutions, and in 0.45 arcsec seeing. Sky brightness is for
CTIO dark time, mean extinctions at an airmass of 1.2, total
telescope+instrument efficiency of 23\%, slit matched to seeing FWHM,
detector scale 0.1 arcsec/pixel, readout noise of 1.5e- rms, dark noise
1.2e-4 e-/pixel/s, binned 1:1 in dispersion diection and 2:1 along
slit, 1-hour exposure and 100k e- full well. The three regions show
where the dominate noise source is the sky, readout noise or shot
noise. The preferred telescope for use in each region is also shown:
although Gemini is 4x faster than SOAR in the sky and shot regions, it
is 16x faster in the read-noise dominated region. SOAR is 4x faster
than Blanco in the sky-dominated region, but the two 4m telescopes in
principal will have the same efficiency in the read-noise dominated
region.