[Aavso-photometry] FITS stack results

[Brad Walter]

When stacking the images try picking each one as the reference and see
how it affects the image time results. I found out on the Christmas
exoplanet campaign that it really makes a difference with Maxim DL 4.01.


I observed and reported to the CCD discussion group on 12/29/2004 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.

Therefore, with Maxim/DL and perhaps some of the other programs,  it
isn't enough to simply report the FITS header time of the combined
image. You also have to report how you stacked the images. Also you need
to know that the exposure time reported by the photometry tool is not
the same as the time reported in the DATE-OBS field of the FITS header.
Try combining the images using a different one as the reference each
time and see what happens to the time reported in the FITS header and by
the photometry tool. This is a real pain with the photometry tool since
time gaps between images that are being stacked, and selection of any
image except the first one as the reference make the image time reported
by the tool an incorrect time for the combined image, and you have to
manually correct the times in the file. It also makes the starting time
in the file header a very questionable value. Even if you use the first
file of the series as the reference, the EXPTIME field shows the sum of
the exposure durations of the summed images. It doesn't take into
account the downloading and delay times between images or any time gaps
that result from software hang ups etc. As a result using the time from
the DATE-OBS field plus half the EXPTIME value as the Mean time of the
exposure doesn't work. You have to manually compute the time of the
resulting image from the DATE-OBS and EXPTIME fields of the component
images. That is a time consuming pain. 

At the suggestion of Arne, I reported this issue to Cyanogen software
support and received a very prompt reply. They understand the issue and
said they would work on an improved algorithm for time reporting in
combined images. 

Note, I have not verified that the EXPTIME field shows the same result
for different file combination methods, i.e. averaging or median
combining vs. summing. until it is checked I would NOT assume that the
exposure duration reported is the same for all combination methods. Has
anyone checked this as part of the exercise?

Brad Walter
Director Large Systems
 
Active Power
2128 West Braker Lane
Austin, TX 78758
Phone: (512) 744 9414
Fax:     (512) 836 4511
bwalter at activepower.com