The reason Michael was measuring 1/2 the true FWHM is that the "rview" command is squashing the frame by a factor 2 in each dimension. This is done to speed up the transfer time. If you click on "cursor", center the cursor on a star and press the left button, a full-size piece of the original frame is displayed in another window. If you repeat the FWHM measurement on this frame, the correct value should be computed. Steve Kent APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 11 in the apo35-dsc archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-dsc/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-dsc@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO