Maximum Exposure Times for the Echelle Spectrograph

The instrumental flexure is small enough to allow exposure times up to a few hours. A similar limit is set by the cosmic ray detection rate:


			COSMIC RAY (c.r.) RATES

	CCD         c.r./min    c.r./pix/min    c.r./pix/2-hrs

	Tek2K          100          2.e-5           .003
	Tek 1K          20          2.e-5           .002
	Loral 3K         8          3.e-6           .0003

The table shows that well under 1 percent of the CCD surface is knocked out by cosmic rays even for 2 hour exposures. For objects faint enough that you are read noise limited (V ~ 17-18), long exposures give the best overall s:n, provided that several exposures are being combined together. Baldwin, Williger, Carswell and collaborators have successfully been able to reduce two hour exposures. The catch is that the IRAF routines don't work well with the resulting combination of faint signal, many cosmic rays, and (in our case) crowded orders with short deckers... you have to write something on your own or try to figure out how to use ours (see " Reduction Procedures").