[Aavso-photometry] RE: Urgent Depth Question

[Arne Henden]

Depends on how you combine the images.  Straight average would use
the starting time of the first image and the ending time of the last
image to calculate midpoint.  Any rejection algorithm, though, for
removing bad pixels, cosmic rays, etc. influences the calculation.
Say for example you combine three images, but the first one gets
rejected consistently.  The midpoint is actually then between images
number 2 and number 3.  In the normal case, each pixel can have
a different midpoint time, depending on which image was rejected
for that specific pixel.
Arne

Kevin Kessler wrote:
> What makes this non-trivial?  In CCDSoft, I combine all the files in a
> directory.  I wrote a script for CCDSoft which then scans through the
> FITs headers of the files in  this directory,  and finds the first and
> last image, adds the exposure time to the last image, and finds the
> mid-point between that time and the start of the first image.  Is
> there something I'm missing?
> 
> Is there some standard FITs header that the mid-point time should be
> stored to?  According to an article I read
> (http://www.cv.nrao.edu/fits/documents/standards/year2000.txt) , the
> IAU stipulates that DATE-OBS is the time of the start of the
> observation, which is what CCDSoft does, and why I had to write the
> script.
> 
> 
> On Thu, 30 Dec 2004 08:24:48 -0700, Arne Henden <aah at nofs.navy.mil> wrote:
> 
>>Brad, be sure to tell Maxim/cyanogen about the error.
>>Obtaining accurate midpoint time for stacked images is actually
>>non-trivial.  If you have any kind of rejection algorithm, that
>>modifies the center time.
>>Arne
>>
>>Brad Walter wrote:
>>
>>>Bill you have hit on a point that causes me some embarrassment. There is
>>>a problem with the times reported by Maxim/DL for the combined images.
>>>I discovered that, when combining a series of images in Maxim/DL, the
>>>program sets a time for the combined image equal to the time of the
>>>series member selected as the reference for the combination, rather than
>>>the earliest member. It also sets the exposure duration as the sum of
>>>the exposure durations of the images being combined. Then the photometry
>>>tool in Maxim/DL  sets the observation time for the corresponding
>>>photometry data table record as the arbitrarily selected image time plus
>>>half the summed exposure time saved in the header for that combined
>>>image file. In my case there were 11 x 40 second exposures so the
>>>exposure duration in the fits header of the combined file was 440
>>>seconds, and the exposure time was shown as the time of the reference
>>>image for the combination plus 220 seconds.  This is the same procedure
>>>that maxim/DL photometry uses when reporting an un-combined image, and
>>>it is correct for an uncombined image.
>>>
>>
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> 
> 
>