My experience with machined masks has not been good, because of nonuniformities along the edges. It is necessary that the slit width be very accurately constant and smooth for good sky subtraction. If these work well, fine, but be careful...you need a very good machine with a high spindle speed, very slow cuts, and VERY sharp tools to have a prayer--unless, of course, your objects are bright. I think that chem milling or laser cutting or photography is likely to be better. --jim APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 82 in the apo35-dis archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-dis/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-dis@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO