[Aavso-photometry] Observation Timing (new question)

[arne]

Yenal Ogmen wrote:
> Jeff,
> 
> Thanx because I didnt know that Envisage records the
> end of first exposure time in FITS file. But what
> about end of total exposure time! How can we calculate
> it? For example, If you start your exposure at
> 00:00:00, 10 images of 30 sec exposure should end at
> 00:05:00 but since some of the frames does not
> included in the combine due to low quality, the actual
> exposure time is not 300 sec. SO how to find total
> exposure time?
> 
Proper midpoint exposure time calculation, when dealing with
stacked images, has always been a problem.  You should be
reporting to the AAVSO the mid-point time of any observation.
We've had this discussion before, so I don't think we need
to reiterate what each vendor's software does - check the
archives and see if the discussion corresponds with their
latest version.  Either don't stack your images, or else
find out how time is handled.

When averaging images, the midpoint is pretty obvious - just
use the midpoint of each subimage, add them together and divide
by n.  If exposures are part of a series, you can sometimes get
by with taking the starting time of the first image and the
ending time of the last image, adding and dividing by 2.

When filtering images, if done image by image, you again add all
included image midpoints and divide by the total number used.
Using first and last image times is not correct if intermediate
images are discarded.

When filtering images through more esoteric means (sigma clip
comes to mind), you may have differing exposure times for
every pixel in the image.  When weighting images in a stack
based on exposure times or fading rate or whatever, exposure time
is also difficult to calculate. That is when things get interesting.

Arne