Hello, folks, Here are some pointers to help observers avoid a problem we've been seeing too often lately. A lot of observers have been slewing the telescope to high altitudes, which is not good for the primary mirror. Although the telescope is mechanically capable of tracking objects up to 88 or 89 degrees, the rapid azimuth motion sets up oscillations in the primary mirror which can be unhealthy for the glass, as well as degrading image quality significantly. Anyone who tries to track an object this high should expect the observing specialist to move the telescope to a lower altitude immediately. The current high-altitude limit is approximately 80 degrees. This is approximate because it's all right for the telescope to be a little higher as long as it's tracking downward rather than moving rapidly in azimuth -- that means it's okay to be a little above 80 degrees in the west, but NOT okay to be higher than 80 degrees for an object transiting in the south or north. How can you know before slewing if your object will be too high? Remark can tell you, or you can use a simple rule of thumb for choosing targets. 1. When you enter slew coordinates in Remark, once you've finished typing in the coordinates but BEFORE you press the Slew button, click on empty gray space somewhere in Remark. The top window should show you where you're about to slew. The right-hand column will be labeled "source" and it will give the El(evation) of the target you just typed in. Don't slew if this target is higher than 83 degrees; ask the observing specialist for help if the target is between 80 and 83 degrees. [Note: if you entered the target coordinates a while ago, the elevation won't update unless you make a change. Change one of the coordinates a little, then change it back and click on empty gray space to update the elevation display.] 2. The simple rule of thumb is this: the zenith is at a Right Ascension equal to the current sideral time (displayed in your top Remark window as LST), and a declination equal to +33 degrees. For any target with a declination between about 24 to 42 degrees, avoid observing the target when it is within one hour of transit. That means you should wait until the LST is more than an hour different from the target's right ascension for targets in this range of declinations. What if the target is just a little above 80 degrees? Ask the observing specialist for advice. If the target is going through transit exactly now at an altitude of 80 degrees, waiting just fifteen minutes will reduce the azimuth tracking rate without increasing the airmass of your observations. If the target is in the east at 80 degrees, you might be able to observe it for a few minutes, but it will soon be too high. A target in the west at 80 degrees is safe. Warn the observing specialist before you slew to a target near 80 degrees of altitude. You may be permitted to make the slew while the observing specialist watches the readouts showing the state of the primary mirror. If everything looks all right, you should be allowed to observe. But slewing to such objects without a warning will only annoy the observing specialist. What about lower limits on altitude? The telescope is mechanically capable of pointing to targets below 10 degrees altitude, already a scientifically undesirable airmass. However, we have recently started seeing some primary mirror oscillations at low altitude as well. Let the observing specialist know if you'll be going to a target lower than 30 degrees altitude; we can check the primary mirror readouts and find out if there's a problem. If oscillations do show up, the observing specialist can eliminate them and improve your image quality in just a few minutes, and you will still be able to observe your target. We understand that these altitude limits are more restrictive than those used in the past, but it's important for observers to recognize that this is not an arbitrary rule change. It's based on observed unsafe behavior of the primary mirror -- a very important component of the telescope! Please keep altitude in mind when you choose your targets, and avoid slewing objects that are too close to the zenith. Low altitude targets are usually fine, but you should let the observing specialist know to check for possible trouble when you slew at low altitude. Know your target's altitude BEFORE you slew to avoid unpleasant surprises. Russet McMillan. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 513 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