[TASS] photometric vs. non-photometric

[Stupendous Man <richmond@A188-L009.RIT.EDU>]

  Peter Mount asks:

> What does everyone thing the criteria are that determine what
> type a particular
> night are, and how would you deal with either a night that starts as
> Photometric quality, but deteriorates and visa versa?

  Terminology: "photometric" is an astronomical term which means
"skies are clear and stable".  In practice, it often means
"Fred went outside once an hour and never saw a cloud."

  Technically speaking, it's complicated.  I do this sort of work
for a living, so I'd be happy to go into MORE gory details than any
sane person would want to know -- just ask.  But let me give
two succinct definitions, and skimp on the details for now.

  Rock-solid method to see if night is photometric: collect observations
of stars with known magnitudes at a range of positions around the sky.
Run the observations through a procedure which checks to see if they
obey equations of the form

     instrumental mag  =  known mag +  A  + c*(color)  +  k*airmass

where A, c, and k are constants, one per passband, and "color" is the
color of the star, calculated from its known magnitudes in two passbands.

  If the observations obey the above equation, then the night was
photometric; if not, it wasn't.

  Quick n' dirty method, which can be done during a night:

         Step 1: carry out rock-solid method over a few weeks

         Step 2: calculate mean values for constant A, c, k in each passband

         Step 3: during a night, use the mean values to calculate the
                 expected instrumental magnitude for stars as the telescope
                 takes images.  If the expected magnitude matches the
                 instrumental magnitude, then the night is photometric.


  So, if the plan is for a user to be able to specify, "Please take
this action only if the conditions are photometric", one must

             a. take pictures
             b. reduce pictures to "clean" versions
             c. measure and identify standard stars in the images
             d. compare measured magnitudes to a known model

all during the night!  It's a tough task.

  A better idea is to have a VERY simple program scan each new image
and look for "cloudiness" -- Peter McCullough finds that Stardial
images during cloudy weather are easy to distinguish from those taken
during clear conditions, by several metrics.  If any image shows
hints of "cloudiness", then declare the night "not photometric".

  One might consider classifying two halves of the night (early and late)
separately, or even each hour .... but that's overkill, to my mind.

                                          Michael Richmond