[vsnet-chat 886] Re: Magnitudes, sequences, etc.

[Taichi Kato <tkato>]

   Regarding the recipes in [vsnet-chat 879] (Brian Skiff), some practical
tips:

   1) experiences show, unfortunately, extinction values tend to vary by
more than 0.1 mag/airmass even during the cloudfree nights of Ouda Station.
The extinction is larger in evening, slowly getting smaller toward morning.
Even photometric-looking night, we can perceive the variation in extinction
in 1 hour.  Two tips:

   1a) do everything quick!  Return to the calibrating field within 10 min.
       Commanding at computers, recording on notebook, getting out to see
       if the telescope is moving in the right direction, changing filters,
       writing e-mails ;-) ... observing at Ouda is something like a
       quick-shooting game!

   1b) choose calibration stars or fields at the same airmass f(z) as that
       of the target.  It would be an easy task to write some program
       to choose the nearest f(z), closest comparison star at any given time.
       This will lessen the degree of uncertainly introduced by the wrongly
       assumed exrinction value.  Recent Hipparcos/Tycho magnitudes are
       suitable in that they are distributed on the whole sky.

   2) use secondary standard fields around CVs (Henden et al.) as such
      calibrating fields.  Take a shot at this kind of fields (at similar
      f(z)) every time you change the target.  These fields contain
      usually enough number of stars with different colors and brightness,
      which enable color-term determination, as well as yielding "snapshot"
      observations of these CVs (we would be very grateful if interested
      people these snapshots of CVs to vsnet-obs :-).

      We would very welcome standard-field makers choose important variable
      star fields rather than anonymous fields; this would increase the
      opportunity of the observer to find/record "something".

   Such process is not usually so difficult; any serious observer can learn.

   From our experience, as Kiyota-san mentioned, the present difficulty
for amateurs also resides in the process of aperture photometry.  Our
familiar program, CCDOPS, has an integrating funtion in a box aperture.
But observers often wonder and frequently ask how to determine the sky
(background).  Different locations of the aperture at seemingly 'blank'
fields give different background values -- often looks disappointing to the
observer.  Another frequently asked quiestion is: "how should I do in the
presence of a faint close companion?"

    Could someone advise how to handle these problems with CCDOPS, or should
I advise them to buy another software?

Regards,
Taichi Kato

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