Local DATE-OBS instead of UTC
Stacking program, astrometric (plate) solver, fits file viewer
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I'm using a DWARF 3 (from DwarfLabs) to make photometry of variable stars. However, I noticed that the "DATE-OBS" in the raw FITS files given by the telescope is in local time, while ASTAP seems to assume that it is in UTC (I realized it after observing large air masses from objects that were near Zenith, due to the 3-hours difference between them, I am in a GMT-3 zone).
For the moment, in my Workflow I use an AI-generated script to change some headers from FITS files, specifically DATE-OBS, DATE-AVG and DATE-END (after calibration but before photometry) but I wonder if there is a feature in ASTAP to correct this.
Also, besides the difference in airmass and JD time of observation, is there any impact on photometry or can I "trust" in the estimations despite that? (e.g. having an airmass of 5+ may impact on estimation of stars in the same frame, DWARF has around 2 degrees of FOV, but I don't know if ASTAP algorithm takes this into account).
Hi Daniel,
Yes you can quickly change the header DATE-OBS:
https://www.hnsky.org/astap.htm#photometry_popup
Select all images in photometry or light tab (ctrl+A). Call up the popup menu with right mouse button. Select change header keyword and type DATE-OBS. Then enter the shift in hours.
It will change the DATE-OBS and delete DATE-MID. If I remember well, it will ignore DATE-END. You could delete all DATE-END keyword by entering DATE-END and as value DELETE (capital letters.
The old DATE_OBS is written to a random keyword which you could rename back.
You could make a backup of the frames anyhow before you make the change.
Normally it is better to uses a COMP en CHECK star then the option Gaia ensemble.
The air mass is normally irrelevant because both the reference and variable are influenced. But if the altitude is very low theoretical the lower part of the image could suffer from a higher airmass. But if your reference/comp and check star are near to each other , I assume the airmass influence is neglectable.
Han