[Aavso-photometry] FITS stack results Maxim D/L 4.01

[Brad Walter]

The table below shows the time in the TIME-OBS and EXPTIME fields of the
fits file header of the original files and combined files using a
different file as the reference file for combination. The Table also
contains the time shown in the Maxim D/L 4.01 photometry tool output
table. This table points out the significant difficulty that Maxim has
assigning valid observation times to combined files both in the fits
header and the photometry tool output. The file combination tool in
using the time stamp of the reference file as the time TIME-OBS value,
not the field value of the earliest image of those being combined, and
it uses the total of the EXPTIME fields as the EXPTIME. I verified that
the EXPTIME value is the same regardless of the combination mode used
(e.g. sum, average, etc.) The photometry tool uses the TIME-OBS value
plus one-half the EXPTIME value as the time of the observation. This
would be valid for uncombined images, but can be significantly off for
combined images. for two reasons: 
1) If the reference file isn't the first file in the series, The
starting time of the exposure (TIME-OBS) will be incorrect; and 
2)Adding one-half of the EXPTIME to arrive at the midpoint of the
exposure doesn't account for delay time between exposures. In our
example, it underestimates the mid point of the exposures by 15 seconds
since there is a 10 second download delay between images. If the delays
between exposures are unequal (say the camera or software had a problem
and you had an extra 30 second delay somewhere in the series) or you are
combining a larger number of images, the error becomes worse.
 
My rule for the moment is to always calculate exposure times manually
(or using a correct home grown spreadsheet function) for combined
images. Never use the times generated by the software. 
 
All times in UT +/-0.5 sec	 EXPTIME	 TIME-OBS
Photometry Tool Output time	
HD74156_3372-214v.fit	 20	 4:49:34	 N/A	
HD74156_3372-215v.fit	 20	 4:50:04	 N/A	
HD74156_3372-215v.fit	 20	 4:50:34	 N/A	
HD74156_3372-215v.fit	 20	 4:51:05	 N/A	
4 Combined Ref 214	 80	 4:49:34	 4:50:14	
4 Combined Ref 215	 80	 4:50:04	 4:50:44	
4 Combined Ref 216	 80	 4:50:34	 4:51:14	
4 Combined Ref 217	 80	 4:51:05	 4:51:45	
 

	Brad Walter
	Director Large Systems
	 
	Active Power
	2128 West Braker Lane
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	Phone: (512) 744 9414
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	bwalter at activepower.com