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

[Michael Newberry]

----- Original Message ----- 
From: "Arne Henden" <arne at aavso.org>
To: "Tom Krajci" <tom_krajci at tularosa.net>
Cc: <aavso-photometry at aavso.org>
Sent: Thursday, December 18, 2008 5:03 PM
Subject: Re: [Aavso-photometry] V filter only - transforms possible?


> 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.

    Exactly.
>
> 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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