Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 02/10/2022 - 09:58
My camera records the beginning of the exposure under the FIT header DATE-OBS. However, when I process the image in AstroImageJ, it records the Heliocentric JD and the Air Mass at mid-exposure. When I registered my telescope and camera, I chose the beginning of the exposure as the time. Should I change that time to mid-exposure or does VPHOT go by the time listed under the DATE-OBS FIT header?
Tim:
Normally, use the 'Start of exposure' setting in your vphot telescope setup.
VPhot uses the Date-Obs Fit time header and the exposure time header to calculate the mid point time for reporting.
However, IF AIJ changes the Date-Obs fit header value to the mid-point time as you have hypothesized, the reported time may not be correct? Does AIJ actually add another fit header or does it re-write the time in Date-Obs? I would hope it would not, without making a comment about what time point is written??
I would ask that you run an experiment to confirm whether using the VPhot 'start-time' or 'mid-point- of exposure' yields the correct reported time for the image? Make sense?
Ken
I went back to VPHOT and checked one of my earlier submissions:
DATE-OBS 2022-02-09T03:49:50.459
Date/Time (as listed in VPHOT) 2022-02-09 03:51:20
Mid-exposure time given in the FIT header: 2459619.660653461
After putting in the mid-exposure Julian date through an online converter: 2022-02-09T03:51:20
Online converter link: https://onlineconversion.com/julian_date.htm
So it looks like the dates are what they're suppose to be.
Thanks for saving me the anxiety of worrying if all of my data had the wrong time stamp. While it's not a big deal for long period variables (especially since I stack them before doing differential photometry), it is a big deal for the measurements of W UMa, which is an eclipsing binary with a period of 8 hours.
Tim:
That's good to hear. However, I still have a question about DATE-OBS header value. Did AIJ change it?
IOW, is the DATE-OBS header value identical in the image header BEFORE and AFTER AIJ processing of one image?
Ken