[vsnet-chat 879] Re: Magnitudes, sequences, etc.

[bas@lowell.Lowell.Edu (Brian Skiff)]

     A rough outline of what could be done by a backyard (or balcony) amateur
from a typical site not known for having good photometric weather is as
follows:
     1) use assumed extinction values based on one's altitude.  As long as
you don't work at high airmass, even grossly wrong extinction will not lead
to serious errors for ordinary comparison sequences.  And as long as it is
actually cloudfree during the observations, the extinction chosen is likely
to be within about 0.05 of 'truth'.
     2) for zero-point calibration, first take a pair of frames in each
filter (probably V and R will be easiest for amateurs since this uses the
most sensitive wavelength range of the CCD) of either a Landolt field or
other good-quality sequence, such as some of Arne's dwarf Cepheid sequences.
     3) move to the target field and shoot another pair of frames in each
color.
     4) finally, move to another standard field, either the same as in 2),
or perhaps better another similar field simply to get more diffferent stars
to use as calibration.

     These steps would be repeated on another night.  IF there are
inconsistencies---probably because one of the nights was actually not stable
enough---then a third night.
     The reductions would be as normal, usually using aperture photometry.
With only a few calibration stars, however, it would be necessary to have
measured on several good nights the color term of the CCD + filters system.
These could be built up gradually over time, and on any given night the
current "best" value from this work would be used in the reductions.  Even
a few observations on one night of a convenient red-blue pair will give you
a "good enough" value to work with---and such pairs can be found in a number
of Landolt fields, so you can get them often while you're doing the quick
zero-point datasets.
     The advantage of the "just enough" calibration method is that it does
not need to be "photometric" for very long, perhaps 20 minutes or half an
hour depending on how quickly the telescope can be pointed at different
targets.  The results will not be of high precision nor of high systematic
accuracy---the type of stuff Arne does!---but for getting a skeleton sequence
now, it's perhaps the best compromise.  I would try to aim not so much for
best internal precision, but instead to get the observations as close as
possible to the standard system.  Amateurs who have done this type of 
observing using single-channel photometers (and just one or two reference
stars!) get results good to typically +/- 0.03 mag.  Oughta work with a CCD!
\Brian

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