[Aavso-photometry] V filter only - transforms possible?

[Arne Henden]

As opposed to Michael Newberry...
Sure, you can determine your coefficient.  We do that all of the time
for the PEP observers, who also often use a single filter, and I did
it in my M67 experiment paper, as Jim mentions.  The equation looks like

(Vstd - Vins) = zeropoint + Tv*(B-V) + Kv*X

When determining the coefficient Tv, you use the standard (B-V) for
each star, so you don't need two filters to get the coefficient.  It is
when you *apply* the coefficient to a target star where Michael's
"no" might come into play.

To use  the coefficient to determine the standard magnitude for
an object, you need the standard (B-V) for the target object,  You
can easily determine this if you are using both B and V filters; for
just using a V filter, you have to make assumptions.  You may know
the basic color of the star as a function of phase (pulsating stars
get bluer as they approach maximum), and can use that approximate
value for your transformation.  For other stars, the color change may
be minimal throughout its cycle, and can use a mean value.

Certainly, applying a transformation coefficient, along with the comparison
star's known magnitude and color, and an approximate knowledge of the
target's color, gets pretty close and can remove some systematics like
changes with airmass.  It improves a light curve from multiple observers
since some will use comp stars of differing color.  It is not as good as
a true transformation using two filters.  If you use this approximate technique,
you should probably say something in the "comment" field.
Arne
---------------


On Thu, Dec 18, 2008 at 5:48 PM, Tom Krajci <tom_krajci at tularosa.net> wrote:
> I have a CCD/scope rig with only a V filter screwed into the CCD nosepiece.
>  No filter wheel, no other filters.
>
> Here's the $64 question.  Is it possible to determine transformation
> coefficients for this setup with only a V filter?
>
> If I do differential photometry of a Henden calibrated field (V magnitudes
> and color indices are known for the comps, *and* the target), can I
> transform to V magnitudes?
>
> What if I take images of ye-olde standard field, M67, and compare my
> instrumental magnitudes with the catalog magnitudes?  What sort of
> correlation/transformation coefficient will that give me?  (If it's not a
> true transformation coefficient, then what is it called - a pseudo
> transform?)
>
> (I know that if I use this V-filter-only rig on uncalibrated comps and
> targets, i.e. V mags and color indices are unknown,...then there is no way I
> can transform.  Therefore I'm looking for 'easy' target fields that I can
> work to higher precision...hopefully with a proper transform to V
> magnitudes.)
>
> I'm just trying to get the most out of this V-filter-only system.  (And if I
> can't transform to V with this setup...how close can I get?  How do I report
> the magnitudes?...as untransformed?)  (And if I can't transform to V with
> this setup...what do I need to acquire?...a B or R filter?)
>
> Thanks in advance.
>
> --
> -------------------------------------------
> Tom Krajci
> Cloudcroft, New Mexico
> http://picasaweb.google.com/tom.krajci
>
> Center for Backyard Astrophysics (CBA)
> http://cbastro.org/ CBA New Mexico
>
> American Association of Variable Star
> Observers (AAVSO): KTC http://www.aavso.org/
> -------------------------------------------
>
>
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