Incorrect DATE-OBS in ZWO ASIImg software ?

[Robin Leadbeater]

A heads up for anyone using ZWO software with their cameras. The DATE-OBS in fits headers created by the ZWO ISIImg software may be the end time rather than the start time as defined by the IAU fits standard. See the discussion here
https://britastro.org/node/26117
and the IAU definition of DATE-OBS
https://www.cv.nrao.edu/fits/documents/ ... ar2000.txt

Cheers
Robin

[Tonis Eenmae]

Hello Robin,

actually FITS standard seems to be permissive, but very clear: https://fits.gsfc.nasa.gov/standard40/f ... 0aa-le.pdf (The current FITS standard) - it shall refer to the start of exposure/observation unless it is not clearly stated differently in the comment field. My experience has shown that well synchronized computer clock and a couple of minutes long exposure is one of the really needed tests when exploring the capabilities of new camera.

I have data from several years, where the control software of a CCD camera didn't add date nor date-obs keywords info FITS headers, timestamp is referred as "DATETIME= Thu Dec 20 23:22:01 2001" in local time. Even worse, that timestamp refers to the end of the exposure + read-out time (ca 20 sec) + random timespan (0.#-2 sec), related how busy the data acquisition computer was :-/ Fortunately that camera was used "just for 12 years" and mainly for spectroscopy and later photometry of long-period variable stars.

Best wishes,
Tõnis

[Robin Leadbeater]

Hi Tõnis

actually FITS standard seems to be permissive, but very clear:

Yes that is the point I am making. Most other software, for example the ATIK acquisition software I am using uses the start time for DATE-OBS (though I have not tested its accuracy) The ZWO software is not IAU fits standard compliant because it uses the end time for DATE-OBS without adding a comment. The BeSS standard specifically requires DATE-OBS to be the start time (no options)
http://basebe.obspm.fr/basebe/Spec_spectres_BeSS_en.pdf
so if you are using the ZWO software the DATE-OBS must be changed to comply with the BeSS standard for submission of spectra to the BeSS/BAA/ARAS/AAVSO databases.

Cheers
Robin