Subject: Re: Grim exposure time


Submitted: Fri, 17 Jan 1997 14:10:32 -0700

Message number: 29 (previous: 28, next: 30 up: Index)

> Date: Fri, 17 Jan 1997 13:53:51 -0700
> From: "James R. Fowler" <>
> 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
>                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:

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.

also states:


	1.[...] my dark current appears to go DOWN as my exposure time
	increases. What is going on?


	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

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.

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