[Aavso-photometry] FITS and observation time

[Michael Newberry]

Arne brings up an important subject and we did discuss it in detail a few 
months ago. For Mira users out there, I will go over the details briefly. 
There are two time issues to consider:

1) The FITS time stamp for a single image, or the effective time stamp for a 
combined ("stacked") image.
2) The time reported for the photometry.

Mira handles all these cases correctly and according to the FITS standard. 
Date and time are combined with the exposure time (and combining issues if 
the image resulted from combining an image set) to get what is saved in the 
image and reported in the photometry measurements.

When doing photometry, in Mira the user can choose to report time referenced 
to the beginning, middle, or end of the exposure. The time of observation is 
reported in the photometry table as the Date, Time, and Julian Date for the 
measurement. These values are correct if the exposure crosses midnight and 
also if the image was made by combining separate images.

Michael Newberry

----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "AAVSO Photometry Group" <aavso-photometry at aavso.org>
Sent: Thursday, July 26, 2007 9:42 AM
Subject: Re: [Aavso-photometry] FITS and observation time


> 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
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