In response to Christopher Garasi's recent post, I should mention the script that I have been using for GRIM observations for the past few months. It is a TCL script that directs the telescope + GRIM to take on-off pairs with randomly generated offsets. The user can specify how many pairs to take, the exposure time per frame, and a range of acceptable offsets. A copy of the source code resides on tycho, and I will post the source code as a separate message to this mailing list. To use the script, you must be running an "mcnode" window on tycho, and must load it by typing "source ~/peyton/dsrand.tcl" ; the computer should tell you "Loaded dsrand script." To run a batch of exposures, you should first slew to your source, and then need to type dsrand x0 y0 maxoffset1 minoffset2 maxoffset2 npairs exptime (where "dsrand" is literal, and all the other things are numbers.) Basically, (x0, y0) say where the object is relative to the last slew position (allowing for errors in coordinates/pointing/etc); maxoffset1 says how far to dither the telescope for on-source observations; minoffset2 and maxoffset2 say how far away to put the off-source sky frames; and npairs and exptime say how long to integrate. More precisely: The script will first take an exposure at an "off" position, and then will execute npair on-off pairs, so that the total number of exposures per run is 2*npair + 1. Each exposure is exptime seconds long. The script uses absolute instrument plane offsets to move the telescope around. The on-source frames have offsets X, Y in the ranges x0-maxoffset1 < X < x0+maxoffset1 , y0-maxoffset1 < Y < y0+maxoffset1 . The off-source frames have offsets satisfying minoffset2 < abs( X - x0 ) < maxoffset2, minoffset2 < abs( Y - y0 ) < maxoffset2. I have found that this yields a pretty good distribution of pointing locations, and allows the observer to turn some attention to quick-look data reduction on the fly. (I use a home-grow IRAF script to do a quick mosaic, using the recorded offsets in the headers, and have a version with manually generated offsets for later, more exact reduction; I can make those available to interested observers.) Some advice: The script can be used for both point source and extended source observations. For point sources, I usually do something like dsrand 0 0 20 0 20 10 30 thus making "on" and "off" source frames indistinguishable, and giving 10.5 minutes of on-source integration; while for a smallish galaxy it would be more like dsrand 0 0 20 240 300 10 30 giving 5 minutes on source and 5.5 minutes on sky. Probably maxoffset1 and (maxoffset2-minoffset2) should be at least 15 arcseconds, which is about the largest structure I've seen introduced by bad pixel clusters/readout electronics in GRIM data from December 1995. If you want to integrate "indefinitely," you can give a really big number for "npair" and later interrupt the script (using the MCnode feature/bug mentioned as Caveat 1, below). Some caveats: 1. While the script is running, the MCnode window *cannot* be used for *anything* else without interrupting the script. I work around this by keeping at least 2 mcnode windows going at all times. This seems to be a feature of the MC. It can be handy, since it means that you can make the script stop after finishing the command in progress by typing (e.g.) "time", but can also be inconvenient. 2. The script does not check that consecutive exposures lie at different places on the sky. By making the maximum offsets large enough, you can make this improbable. 2a. It is not possible to ask for multiple exposures at each dither position. It would be a trivial change to the script to allow this, but I don't actually think it needed since offsets are a small overhead. 3. The offset commands are of the form "offset $xoff $yoff abs inst nocompute" which means that they are not a large overhead. However, I have once or twice had difficulty observing M31 with degree-scale offsets; the voltages on the drive motors get too high and the azimuth axis stops tracking. For offsets of one or a few arcminutes, this is not a problem. 4. I have never used automatic FTP, and am not sure if it can be done from an MCnode. 5. It is not easy to do clever things like put an interesting point source in the "sky" frames for galaxy observations. 6. I don't know how this compares with Dan's dither script, which I have not used. 7. Use it at your own risk! Anyone who wants to work on improving the script to correct its shortcomings is welcome to, but please don't modify the existing copy on tycho ("/export/apotop/visitor1/peyton/dsrand.tcl"). Also, please let me know if you make a modification that seems successful; I'd like to keep track of the best scripts going. -James Rhoads. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 11 in the apo35-grim archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-grim/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-grim@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO