Some time has been spent recently looking into the repeatability of the DIS grating tilts. Obviously, if the gratings are not moved, then the question of their repeatability is moot. It is with this fact in mind that we wish to make the community aware of a situation in which the gratings will move even though the observer has not intended to move them. Let's say you move DIS to low res and set the blue and red grating tilts to 4400 and 7700 Angstroms. Now, 4400 and 7700 are not at integer positions on the stepper motor, so instead it puts the gratings at 4391 and 7705, which corresponds to steps 534 and 496. Fine, no problem. The problem is this: If you now move to imaging to focus and then back to low res and again use 4400 and 7700 (which will be the defaults on your screen) instead of 4391 and 7705 (or steps 534 and 496), the gratings WILL move because Remark thinks that you have changed the tilt even though 4391 and 4400 correspond to the same stepper motor position. The simplest way to solve this problem is to always use step positions rather than central wavelengths to reference the grating tilts. This is because DIS determines the tilts based on steps rather than central wavelength, since a step is the quantity that is physically meaningful to the camera. An alternative would be to hit "sync" after configuring your setup; however, this assumes that only one grating configuration is required. Thereafter, when returning to spectroscopic mode, Remark will have the correct information, both steps and the corresponding wavelengths to ensure that the grating position does not change. Additionally, one could reference the reported wavelength instead of the input wavelength. It is our feeling that this problem probably affects a large fraction of the observers using DIS to do spectroscopy and we hope this helps fix any problems that people may be having. Our apologies if everyone but us already knows this, but we were unaware of the problem ourselves and are concerned that others were unaware as well. Gordon Richards Charles Corson APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 244 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/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-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO