Re: photometric vs. non-photometric

[Peter Mount <peter@retep.org.uk>]

On Thu, 02 Sep 1999, Stupendous Man wrote:
> 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.

Thanks for the explanation.

I think this will be something we will need to discuss later on. Initially I'm
going to get it just to organise a smooth flow of observations. Once that's
done, we can then sit down and plan exactly what we want the MkIV to do.

I mainly asked this one, as it was the only part of TN48 that I wasn't certain
on how we would handle.

Peter

--
Peter T Mount      peter@retep.org.uk                Tel +44 (0) 7990 898443
          Homepage http://www.retep.org.uk           Fax UK +44 (0) 870 0568837
   PostgreSQL JDBC http://www.retep.org.uk/postgres/     US +1 734 661 0110
Java PDF Generator http://www.retep.org.uk/pdf/      More details on the website