I’m using SG Pro to acquire images for later photometric analysis. When looking for Exoplanet transits it is very important to know the exact start time of each exposure (hopefully better than 25ms).

I notice, in the SG Pro documentation that DATE-OBS (and DATE-LOC) refers to the “time the image was written to disk”, but the HEASARC website which defines the FIT standards suggests that the time should be the start of the exposure, not the end.

My question really is: how accurate is the “time the image was written to disk” compared to the actual end of the exposure? If it is exactly coincident then I can deal with it in my scripts as I can determine the start time using the end time minus the exposure time.

Also, it would be brilliant if the JD field was filled in, in the FITS header (for the start of the exposure) as it is really the Julian Date that we Variable Star observers need.

Thanks in advance for your help
Ian Sharp

SGPro marks the instant the shutter is opened. We failed to update the docs when we changed this.

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Ah, good news!
Many thanks for that.