[Aavso-photometry] Re: At a crossroads and need some help RE BVRI.

[Bill Goff]

On Jun 11, 2004, at 9:03 AM, aavso-photometry-request at mira.aavso.org 
wrote:

> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 10 Jun 2004 11:23:22 -0700 (PDT)
> From: Tim Crawford <tcarchcape at yahoo.com>
> Subject: [Aavso-photometry] At a crossroads and need some help RE
>
clip

> I am, however, somewhat confused as to how to proceed
> with the BRI filters using the AAVSO Charts with
> extended sequences in that do I have to do
> transformations first?
>
> The AAVSO CCD Observing manual does not address BVRI
> (Section 5.7 is missing).  I did locate some
> information under observing programs “BVRI CCD
> Photometry observing program,”
> http://www.aavso.org/observing/programs/ccd/ccdobs.shtml,
> (which very well may be the missing Section 5.7 from
> the CCD manual); never the less, after studying this 2
> page discussion I am still unable, with a field chart
> for the example, to follow completely what is going on
> and how this can be applied to differential reduction
> using the existing charts with extended sequences.
>
> Are Transformations of my filtered images required
> when using charts with extended sequences or are the
> transformations just required for all-sky photometry?
>
clip

> Tim Crawford
> Arch Cape Observatory
> OR
>
> Ps-will post on several lists
>
>
>
> ------------------------------
>
> Message: 2
> Date: Thu, 10 Jun 2004 14:33:36 -0400
> From: "Walker, Gary" <Gary_Walker at Maxtor.com>
> Subject: RE: [Aavso-photometry] At a crossroads and need some help RE
> 	BVRI.
>

> Hello Tim
>
> AAVSO right now has only 8 stars that require BVRI Transformed data.
> They are on the web page.  Among them are S Per, U Ori, VX Gem, DH Dra,
> VX UMa, W Leo, RU Vir, and RR Boo.  You can get to them by going to the
> web page and clicking Observing, Observing Programs, CCD, CCD BVRI.
>
> Optional transformed measurements are always welcome.
>
> Let me know if you have any other questions.
>
> Gary Walker
> Chairperson
> CCD Committee
>

I experience the same confusion about what is needed for multi-color 
photometry.  While there may be 8 charts with transformed data, there 
are many, many more that show several color indexes, B-V, etc and seem 
ripe from multi-color measurements.  While the color index may be handy 
to quickly understand type of star is it color wise, it's not very 
handy for multi-color differential photometry.  If I shoot a frame in 
V, I have comp stars with V mags.  If I then take frames of B or R etc. 
and only an index is available, I have to subtract out B-V etc to 
arrive at a B or R values for comps in order to make my differential 
measurement.  Now maybe I'm misunderstanding how I'm supposed to arrive 
at B or R values for comps, if so please correct me.  From my view 
point having to reduce data,  it seems that straight multi-color mags 
would be simpler.  I guess some folks have all this is a spread sheet 
somewhere, it just seems to be another step and more chances for error 
and time involved.

But it seems to me that transformed measures are not required to do 
this level of work, recognizing that transformed measures would still 
be more desirable for the long term quality of data viewpoint.

I would also agree with Tom that a clear set of instructions would be 
very helpful.


Bill