> Date: Fri, 17 Jan 1997 13:53:51 -0700 > From: "James R. Fowler" <jrf@galileo.apo.nmsu.edu> > To: techstaff > Subject: Grim exposure time > > While testing the Grim today I found out that Grim does not like to > take exposures less than 1.2 secs long. I have a really vague > recollection that this is a feature of the Grim and Jon Brinkman seems > to recall it also from conversations with Bernie Rauscher. I don't > find any documentation about it on the Grim web pages. If this is > indeed a feature of the Grim, then I think we should notate this > promenently in the observing documentation. I could have avoid a > number of hours of testing as well if I had been able to find this > information. > > Jim > > > -- > Think! It's good practice for when the computer is down. > James R. Fowler Computer Operations Manager > jrf@galileo.apo.nmsu.edu Apache Point Observatory > Voice: (505)437-6822 2001 Apache Point Rd. > Fax: (505)434-5555 P.O. Box 59, Sunspot, NM 88349 > With a little browsing through our on-line documentation, I found a note from Bernie on exposure times at: http://astro.uchicago.edu/home/web/nir/GRIM_II/Bugs/expTim.html It makes no mention of a minimum expsure time, but states: BUG: GRIM II exposure times are incorrect The exposure times in all GRIM II exposures taken after 30 October 1994(1) are longer than requested. The indications are that the actual exposure time is equal to the FITS header parameter "OPENTIME" + 1.09 sec. GRIM's exposure timing is controlled by an Apple Macintosh Instrument Control Computer (ICC). During key portions of each exposure, ICC interrupts are disabled. According to Inside Macintosh, one of the ICC's timers is not affected by interrupts. This timer is used as the exposure timer. Contrary to Inside Macintosh's claims, we have found that this timer is affected by interrupts. Bob Loewenstein is attempting to contact Apple Computer for an explanation. ARC does not have Apple Developer status. As a result, the process is less direct than it might otherwise be. (1) Data taken before 30 October 1994 are affected as well. However, the correction depends on the date of the run. The GRIM II FAQ (http://astro.uchicago.edu/home/web/nir/GRIM_II/FAQ.html) also states: QUESTIONS 1.[...] my dark current appears to go DOWN as my exposure time increases. What is going on? ANSWERS 1. [...] my dark current appears to go DOWN as my exposure time increases. What is going on? For short exposures (few minutes), the bias level is a function of exposure time. There are a number of possible causes that are distinct from dark current. This will not limit the precision of your photometry if your DARK exposures have the same integration times as your program exposures. For example, if your program exposures have integration times of 1, 5, 15, and 30 seconds, your DARKs should have exposure times of 1, 5, 15, and 30 seconds. Whenever possible, I take DARKS at the beginning and end of a night. So in general it is a bad idea to take short exposures with GRIM II. I'm forwarding this thread on to Bernie, Mark and Scott. Jon APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 29 in the apo35-grim archive. You can find APO the archive on http://www.astro.princeton.edu/APO/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