Hi folks, We've had multiple cases in the last few weeks where some time was lost because observers and/or observing specialists were unwittingly requesting the wrong kind of offset. This occurred with both GRIM and DIS in use, so it's a matter of concern for most observers. We have several different kinds of offsets available so that they can serve as a powerful and flexible observing tool. However, powerful tools can cause problems when used carelessly, so here's a quick refresher: All offsets as requested from Remark are in arcseconds, regardless of type of offset or position on the sky (i.e., offsets in RA include compensation for cos(dec)). [Note: the above statement is not strictly true for very large offsets >10 arcminutes, but those are almost never used in normal observing, so I'm ignoring that issue.] Object offsets are always in RA and Dec space; they will move the telescope east or north regardless of instrument rotation or telescope altitude/azimuth. Positive RA offsets move the telescope east and the stars west; positive Dec offsets move the telescope north and the stars south. Instrument offsets are always along the rows and columns of the detector, regardless of instrument rotation or telescope altitude/azimuth. An X offset always moves along rows, although it may be to the right or left depending on which instrument is in use. A Y offset always moves along columns, although it may be either up or down depending on the instrument. [Exception: with DIS, an X offset moves the target along rows on the slitviewer, but along columns on the detectors.] Absolute offsets are the cumulative offset from the original slew position. Requesting a 20 arcsecond absolute offset, then another 20 arcsecond absolute offset, will put the telescope 20 arcseconds from the original slew position; that second offset did nothing. In most cases, a 0, 0 absolute offset is equivalent to reslewing (but reslewing is better in some subtle ways that I won't explain just now). Relative offsets move the telescope the specified amount from its current position. Requesting a 20 arcsecond relative offset, then another 20 arcsecond relative offset, will put the telescope 40 arcseconds from the starting position. A 0, 0 relative offset does nothing. Slewing normally clears out all of the kinds of offsets that can be requested from Remark. The two kinds of sticky offsets (Guide and Calib) that persist during slews should be adjusted only by the observing specialist. The exception is if the Keep Boresight box is checked in the main slew control window of Remark; if that box is checked, Instrument offsets will also persist during slews. Combining different kinds of offsets is trickier, but here are some general guidelines: switching from absolute to relative offsets usually produces intuitive behavior, while switching from relative to absolute usually produces surprises. Object offsets and Instrument offsets can be combined, but they will behave more intuitively if they are both relative. If more than one user is making offsets, everybody should be using relative offsets. Combining offsets of any kind with changes in rotation in the order offset+rotate+offset is generally a bad idea, but it can be successful if the offsets are relative; reslewing at the time of changing rotation is usually a better idea (that means pressing the Slew button to change rotations, rather than the Set button). When you get non-intuitive behavior from offsets, the first thing to do is check that you have the type of offset you think you have. The next thing to do is reslew, making sure that Keep Boresight is not checked by accident. Many observers think that reslewing means starting over and losing whatever work they've already done, but it usually simplifies behavior enormously and saves time. Here are a couple of instrument specifics: 1) DIS slitview recommendations are always relative instrument offsets. They can be used with absolute instrument offsets by adding the recommendation to the previously accumulated offset. 2) GRIM dither scripts (and SPIcam dither scripts, for that matter) can use either relative or absolute, instrument or object offsets; but the script should always end with a 0, 0 absolute offset or a reslew. If the script is aborted halfway through for any reason, it's wisest to reslew and start over unless your script was deliberately written with such eventualities in mind. 3) When taking sky flats by hand, use relative offsets. That way you can enter the number of arcseconds once, and just press the Offset button each time you want to move. This is not necessary when using the SPIcam "mornflat" and "eveflat" scripts, which take care of all the offseting automatically. 4) For the echelle, just use shift+click. You can use it for guiding as well as centering, with full frames or subframes, and it saves you from worrying about everything else in this message. The only troubles will arise if you do shift+click on an image that is not up to date, if your offsets compete with autoguider offsets, or if two people are making offsets at once (a very bad idea). I hope this helps avoid some confusion! Russet. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 654 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