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

[Arne Henden]

Tim actually raises some questions that have been around for
a number of years.
   The normal method of submitting observations to the AAVSO is
a bit too general to handle the variety of observations that
amateurs are making these days.  My personal preference is
the following:
      CR   unfiltered (clear), but using comparison star R magnitudes
           for zeropointing
      CV   unfiltered (clear), but using comparison star V magnitudes
           for zeropointing
      U,B,V,R,I  properly transformed observations using proper comparison
                 star magnitudes/colors
      u,b,v,r,i  observations taken through a standard filter, along
                 with the proper comparison star filter/magnitude, but
                 untransformed

Using a B filter and not transforming, you obtain the B magnitudes for
the comparison stars by Bcomp = Vcomp + (B-V)comp

The errors on the comparison star magnitude are then the square root
of the Verr and the BVerr.  Yes, this implies a larger error than if
the comparison star table were given as BVRI (filtered magnitudes)
rather than V, (B-V), etc. (V plus colors).  You can't have everything;
when transforming, you need the color indices rather than individual
magnitudes and that is why most tables use the magnitude/color format.

There is no problem reporting untransformed magnitudes obtained through
standard filters, except in somehow identifying that these are untransformed
by some kind of comment.  The real problem is that I think the current
database has everything mixed together and may be difficult to correct
at some later date.
Arne
-----------

Tim Crawford wrote:
> At a crossroads and need some help RE BVRI.
> 
> I now have my color wheel and have added the BRI
> filters to go with my V filter.
> 
> I have created a listing of AAVSO Charts with extended
> sequences.
> 
> I use AIP4WIN and I am comfortable with differential
> reduction of V filtered fields.
> 
> 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?
> 
> Enduring my 18th continuous day of cloudy sky’s ….
> Guess I have lots of time to study responses  ;  ).
> 
> I am ever reminded that knowledge is very finite while
> ignorance is infinite.
> 
> 
> Thanks for the help folks.
> 
> Tim Crawford
> Arch Cape Observatory
> OR
> 
> Ps-will post on several lists
> 
> 
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