[Aavso-photometry] Choosing Stars for F.O.E.

[Bruce Skelly]

Arne,
 
Is there a cut of limit to the magnitude that one should use with A0 stars?  For example, below a certain magnitude, an A0 star is so distant that there could be too much interstellar reddening.  The stars listed in your book are quite bright, for a CCD.
 
Bruce

arne <arne at aavso.org> wrote:
Extinction is much simpler; the primary choice is a star that is
constant. :-)

There are two common techniques: the Hardie method, which uses pairs
of stars (one at high airmass, one at low airmass) observed nearly
simultaneously. The other is to follow a single star from near the
meridian to high airmass. This latter method requires photometric skies
over a longer period of time.

It is best to choose stars that are nearly colorless (B-V = 0); there
are lots of these around (see my old book for a list of bright A0 stars).
This is not an absolute requirement, but removes any second-order color
effect. The best location of such a star is in the field on which you
are performing a time series so that any possible azimuthal sky variation
is removed. However, my stars are fine for this purpose - you don't
need carefully standardized stars for extinction determination.

More details are in the book.

Regarding SA110, once it gets too far west you lose most of the really
red stars in Landolt's list, so transformations have to be made with
stars with (B-V) < 2 for the most part. This is ok since most of your
observations will be of bluer stars than this, and a color range of
0 to 2 is still pretty wide. I often use the SA98 stars, but the red
ones are pretty faint. Take the Landolt list and sort on color; there
are 5 or 6 bright red stars if I remember right and you can often observe
them separately.
Arne

Greg Crawford wrote:
> Working my way along the path to converting instrumental measurements to
> standard measurements, I seem to have coped with Transformation
> Coefficients. The next step is First Oder Extinction values. 
> 
> 
> 
> I have found that working with a small field of view (11'x 11' - an ST9XE at
> f/10 on a C11) that not every Landolt Equatorial Standard field is as good
> as another. Some put more stars in a standard field. Such was the case with
> SA110. However, SA110 is drifting out of my sky. I got enough use of it
> earlier on to nail T/Cs, but it sets too early now to use for F.O.E.s.
> 
> 
> 
> What are the best choices for First Order Extinction calculations? Must one
> use Landolt Standards? Will Arne Henden's fields (culled for a minimum of 3
> obs) do for FOE? Are colour differences important for FOEs? (I am hoping to
> observe in V and R, or possibly I.)
> 
> 
> 
> I suppose my questions boil down to two:
> 
> 
> 
> Suggested catalogues of stars for FOEs.
> 
> 
> 
> Rationale for using those catalogues.
> 
> 
> 
> - Greg
> 
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