[Aavso-photometry] Reality Check - Basic Concepts (Chart Magnitudes)

[Arne Henden]

Now you start getting into the stickier questions. :)
The V5558 Sgr chart is also a CCD sequence chart - note the line
that indicates how many stars you want displayed in a table.  That
will give magnitudes precise to 0.01mag or so.  Not all charts are
that way, but most of the recent ones are, and quite a few of the
ones that will be available through VSP.  If you run into a chart that
does not have good photometry (only 0.1mag values), then just use
one of those stars as your comparison, calculate the value, and
then be sure to report the magnitude or at least the label of the
comparison star used.  When more precise photometry is available,
someone (you or the researcher) can go back and correct the data.

Yes - all photometry is supposed to be reported as if it were at
airmass = 0.  That is why extinction can be important, and why
for quality results, it is best to not work at high airmass.  All of
the charts with "CCD magnitudes" will be that way.  Some of the
visual charts just have poor photometry, as they were drawn
"in the old days" when you had to use photographic plates or
visual estimates to determine brightnesses.
Arne

On 6/15/07, David Trowbridge <David at tinyblue.com> wrote:
> >> Question #3: Are the published values for Landolt standard magnitudes and
> >> color indexes normalized for air mass = 1 (at the Zenith), or are they
> air
> >> mass = 0 (above the atmosphere)?
> >>
> >air mass 0 (above the atmosphere).  This is always done for standardized
> >photometry to remove all traces of atmospheric extinction.  Air mass = 1
> >values for you will be different than air mass = 1 values for someone
> >on Mauna Kea.
>
> By extension then, I assume that the chart magnitudes (e.g., 1804-18 for
> Nova V5558 Sgr) represent the same standardized photometry, corresponding to
> air mass = 0. Right?
>
> And what about visual magnitudes vs. V-filter magnitudes. Even though this
> is not a CCD sequence chart (indicated magnitudes have only one decimal
> point), can I still use it for getting V magnitudes?
>
> (I guess I have some very basic issues to clear up!)
>
>
> David Trowbridge
>
>