[Aavso-photometry] CCD Observations

[arne]

Steven Orlando wrote:
> Hello All!
> 
> I do my mag observations by imaging two Landolt fields at high and low airmass, and then image the target. I calculate the transforms, ZP and extinction coeff using Photored, and then I use photored's differential photometry routine to get the transformed mags of the target.
> 
> I normally don't use AAVSO charts for this, I use catalogs and The Sky to get mags.
> 
> When we use webobs, we can't submit unless we use an AAVSO chart.
> 
> How should I report this? I sometimes get my mags from "The Sky" using Tycho or UCAC2 Stars, not AAVSO charts.
> 
Hi Steve,
In general, your technique, if I understand it correctly, will yield poorer
results than if you just did differential photometry within a field.  First,
you have to only use photometric nights, so you lose 50percent or more of
your available observing time.  Second, your results will have a zeropoint
error that depends on how well you did your transformations on the given
night (especially extinction).  The only person I've ever see that was
successful in getting good light curves through all-sky calibration without
local comparison stars was Leonid Berdnikov, observing from one of the
most pristine sites in the northern hemisphere - Mt. Maidanak.

The better way is to use those photometric nights to get good mean magnitudes
of comparison stars in the fields of the variables of interest, and then do
differential photometry with respect to the mean magnitudes of those stars.
Your light curves will look much better - that is why we spend so much time
calibrating sequences for the observers.

However, if you want to observe this way, you can certainly submit without
using an AAVSO chart - you just put "not aavso" in the chart field, and
then be sure to put the "absolute" magnitude of the comparison star(s) used
in the comments field.

I would discourage using magnitudes from The Sky, Tycho, UCAC2 unless you
really know what you are doing.  We have a whole sequence team that
determines comparison stars around variables that are carefully trained
in what catalogs to use and the gotchas of each.
Arne