[BAA Comets] Camera time-stamps for astrometry

[Nick James]

Graham,

> Listening to Roger's talk on Saturday encouraging us all to measure our
> comet images I suddenly realised that I did not know for sure when my
> camera creates the time-stamp for each image. I had assumed it was at
> the end of the exposure when the file is created for the data to be
> saved into. I have just done an experiment which reveals that my
> assumption was wrong.
> My Canon DSLR in fact records the date and time of the beginning of the
> exposure.
This is indeed an area where people can get confused but it is generally 
only of significant importance if you are doing astrometry of fast 
moving objects with long exposures or stacks. The time in MPC 
astrometric reports should always be the mid-time of the exposure or stack.

The EXIF data in Canon RAW files (CR2) for my 550D does indeed refer to 
the start of the exposure. IRIS and DSS faithfully translate this to a 
DATE-OBS keyword for the start time in FITS format. As you say, the 
exposure duration in the EXIF is usually slightly different to the 
requested exposure but it is the more accurate value.

The DATE-OBS keyword in FITS headers is not very well defined but it is 
generally understood to be the start of an exposure. Since it is not 
well defined Astrometrica has a setting which allows you to tell it 
whether the time is the start, middle or end of the exposure. I use 
CCDsoft for my camera and this puts the start time in the FITS header. 
Astrometrica then handles the shift to mid time as required by the MPC 
report even if multiple frames are stacked.

The best way to check your set-up is to do some astrometry of a fast 
moving object and then check the residuals using a site like:

http://www.fitsblink.net/residuals/index.html

MPC will not correct your times. Any observations with large residuals 
will just be rejected.

Nick.