Re: Disk 17 FITS headers

[Tom Droege <tdroege@veriomail.com>]

I think there is no way to know what the time is.  I read the clock just 
before I open the shutter and just before I close it.  This is done with a 
a! = TIMER   which should give the time to 1/16 second.  The problem is 
with the call to the serial port.  I think this is done at some arbitrary 
time.  Sigh!  It is still my hope that some software expert somewhere will 
fix this.  Until then, we just have to live with it.

Well, the problem may be (now) with my time delay subroutine which 
generates time delays by an instruction loop.  This could be affected by 
system interrupts which change the number of instructions 
counted.  Depending on how active the system is, this could cause 
variations in the pad time to make 100 seconds.  If this is the case, then 
the times are accurate.  I may try to change the program to avoid the pad 
time scheme.

I would go more by the desired time than by the time reading.  i.e. use 100 
seconds for everything no matter what it says.  This should be just as good 
as the recorded value.  The fact is, we don't know which reading is the 
best.  It is almost always better than it used to be now.  The 10 second 
changes appear to occur rarely.

OK, slight modification.  If it is 100 +/- 5 seconds, then assume it is 100 
seconds.  If the difference from 100 seconds if more than +/- 5 , then use 
the indicated value, or throw the frame out.

Tom Droege


At 01:58 PM 4/7/01 -0400, you wrote:
>c. the EXPTIME values have a significant scatter.  What I mean
>         is that the values in the FITS headers are (for example)
>
>                  101.8, 101.4, 100.7, 101.6, 99.4, 100.2, etc.
>
>         when one might have expected identical values of "100" in
>         each case.  This causes problems for me when I try to
>         create master dark frames.  I'd like to combine dark images
>         with the same exposure times ... but no two images have
>         (exactly) the same exposure time.  Moreover, I'd like to
>         use the 100-second darks to subtract from the 100-second
>         object frames ... but again, the exposure times aren't
>         all 100 seconds.
>
>         I'm dealing with this by binning all exposure times to
>         the nearest 10 seconds before I try to group frames together.
>         It's a minor pain.
>
>         Do we know if the variations are real?  That is, was the
>         exposure time for a frame which says "100.4" in the FITS
>         header really 100.4 seconds?