[Aavso-photometry] FITS and observation time

[arne]

Jeff Hopkins wrote:
> Normally I observe long period eclipsing binaries where precise time 
> to the second or millisecond is not a factor. Recently I have started 
> a mentor project using the fast eclipsing binary sz Herculis. I am 
> using AutoStar and found the FITS time indication is off. It seems to 
> be the time the file is saved. I started a one-shot 15 second 
> exposure at 05:02:30. The end time should have been 05:02:45 or 
> close. The FITS Header indicated 05:02:57.
> 
> If the difference is fairly constant, it could be compensated for. 
> However, things get more complex when images are stacked and during 
> the stacking when some images are not stacked due to poor quality.
> 
> How has this been handled by the AAVSO?
> 
The AAVSO assumes that the submitted time is for the midpoint of
the exposure.  Calculating this correctly is left up to the observer.

Not quite sure how you would get 05:02:57 from your above example.
Does Autostar take 12 seconds to download the image?  Certainly
the written time cannot reflect the end-time of writing an image
since the header gets written first.

Most software packages modify the header time when stacking
images.  There was an extensive discussion about this a couple
of years ago on this list, but I'd presume that each software
package writes things differently today.
Arne