Hello folks, This message is primarily of interest to GRIM users, but also to any observers who plan to use observing scripts or low-level offset commands. After the shutdown for emergency servicing of the azimuth drives last December, I noticed that "uncomputed" offsets frequently caused unpleasant-looking behavior of the telescope motors, including accelerations and velocities higher than we like to see for the sake of drive longevity. At that time, Craig took steps to make certain that all offset commands sent from Remark would be "computed," and I altered the most popular GRIM dither scripts to make sure that all of their offsets were also computed. More recently I discovered that there are additional advantages to using computed over uncomputed offsets, so I have gone through all dither scripts in the visitor1 home directory tree to eliminate commands that would result in uncomputed offsets. With this message, I am warning users to expect a change in the behavior of their dither scripts, and I am also requesting that users not restore the "nocompute" request to their scripts. If any users keep GRIM dither scripts somewhere other than tycho:/home/visitor1/ or its subdirectories, I ask that you go through your script and eliminate the word "nocompute" wherever it appears. A more complete explanation follows to motivate you to comply with these requests. A computed offset is essentially a re-slew of the telescope, including new calculations of refraction and aberration, and conforming to strict limits on acceleration, maximum velocity, and deceleration. An uncomputed offset is more what we expect from the term "offset"; it simply moves the drives a specified amount without extensive recalculation, and using a more generous set of restrictions on motion. For users of dither scripts, the advantage of using uncomputed offsets is that they are considerably faster, taking only 1-2 seconds instead of the up-to-ten seconds required for a computed offset. In December, I believed that the only advantage to using computed offsets in dither scripts would be the long-term effect of decreasing wear on the drives. I have since discovered that there are other short-term science advantages as well. Computed offsets are more accurate in terms of positioning stars, and they also have a known time of completion. Uncomputed offsets return a message of completion immediately, and in some instances the next exposure may begin while the drives are still settling from the sudden offset. After some recent experience where I paid very close attention to timing, I believe this is one explanation for the doubled images some users have reported while using GRIM. So I expect that using computed offsets will result in a much lower incidence of unusable exposures for each script. Because computed offsets require more time, GRIM users might want to re-order their scripts to take more exposures at a single position and minimize the number of offsets made. However, I believe that the benefits (both short-term and long-term) of computed offsets will outweigh the small loss of time. Please feel free to contact me if you have questions about your dither scripts or about offsets in general. Russet. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 668 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