[Aavso-photometry] Observation Timing

[Jeff Hopkins]

If you are concerned with the time of your observation using AutoStar 
Envisage to do the imaging, it appears that Envisage add a FITS Time 
of observation to the FITS header that is the end of the first 
exposure. In other words if you use a 30 second exposure and stack 10 
images, the FITS header will report the end time of the first 
exposure (if you started at 00:00:00 the FITS Header time would be 
00:00:30). Since the total exposure time is 300 seconds you must go 
back to the start time which is the FITS reported time minus the 
exposure time (00:00:30 - 30 seconds or 00:00:00) and then add half 
of the total EXPTIME or Total Exposure Time for the 10 stacked images 
worth of 30 second exposures that would be 150 seconds (half of 300 
seconds). The mid-point of the observation is then 00:02:30 (00:00:00 
+ 150 seconds). Note the AutoStar Image Processing software does not 
report mid-exposure time. You must do this calculation apart from 
AutoStar.

The reason I point this out is that if you use another program (e.g., 
AIP4WIN) to do the processing, that software may report a different 
mid-exposure time. With AIP4WIN the software takes the FITS Time as 
being the start of the imaging (which is different from how Envisage 
reports the time - Envisage reports the end of the first exposure 
time). In the above case the resultant stacked image mid-exposure 
time with AIP4WIN is reported as 00:03:00 (00:00:30 + 150 seconds). 
In this case the difference between AIP4WIN and the true 
mid-observation is the time of a single exposure or 30 seconds.

While for most projects this is an insignificant difference, if you 
desire timing accuracy to sub-second precision, you should do some 
experimenting and determine exactly how the times are reported and 
calculated.

Jeff
-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
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