Re: Data reduction methodology for V-I colors

[Thomas Droege <tdroege2@earthlink.net>]

Richard Miles and all,

I am always willing to supply data sets to anyone wiilling to try some new
data analysis.  Just tell me how many days and how many images you need to
do a test of a new method.  The data I have is 2k x 2k images that have
been dark subtracted and flat fielded.  Otherwise they are raw.  They also
have the field center added from the really raw data.

There are about 100,000 images in the stack.  They come 80 on a CD in
various combinations of V and I images.  Best to ask for one half evening
from one telescope pair as that is the way data is taken.  So roughly 1000
square degrees of sky taken in two filters on one evening comes as a data
unit on 2-4 CDs.  You might ask for 5-10 such units to do your experiment. 
I would support such a request.  

Tom Droege


> [Original Message]
> From: Stupendous Man <richmond@stupendous.cis.rit.edu>
> To: <rmiles.btee@btinternet.com>; <tass@listserv.wwa.com>
> Cc: <mwrsps@rit.edu>
> Date: 8/24/2004 3:00:58 PM
> Subject: Re: Data reduction methodology for V-I colors
>
>
>   About two weeks ago, Richard Miles made a suggestion for the 
> photometric reduction of the TASS magnitude measurements.  I'm sorry
> to be joining the discussion so late....
>
> > it occurred to me that TASS may not be making optimal use of some
> > of the data, in particular those pertaining to V-I color index.  The
> > thinking goes like this.  (Someone correct me if I have not got the
> > methodology right):
>
> > The photometric reductions in V and I are performed separately.
> > The last step (step 8) of the data reduction pipeline is to calibrate
the
> > magnitude of each star.
> > Each frame in each color is reduced against a subset of Tycho stars and
a
> > zero-point for each frame is determined.
>
>   No, not quite.  
>
>   Each individual V frame is turned into a set of instrumental magnitudes
> individually -- yes.  Each individual I frame is turned into a set of
> instrumental magnitudes individually -- yes again.
>
>   However, the step of converting instrumental magnitudes to the
> standard V,I scale (really the Vt, It scale) does not treat each
> frame individually.  Instead, it solves simultaneously for the
> zeropoints of each frame, a V-band color term and an I-band color term.
>
>   I believe that the current pipeline already does something along
> the lines that you are suggesting...
>
> > Now here's my concern.  To calculate V-I for each star and frame, the
value
> > of I must be subtracted from the value of V.  However, each of these
have a
> > contribution to their uncertainty contributed by the two independent
> > zero-point values determined for each of the frames.
> > Surely there is a much better methodology for arriving at a more
accurate
> > V-I color index whereby the reduction (for V-I only) is carried out by
> > always working in terms of the differential instrumental magnitudes,
v-i.
> > Qualitatively, changes in sky transparency have a significant effect on
V
> > and I zero-points whereas the equivalent v-i values are relatively
immune to
> > such changes if each pair of images are made simultaneously. 
Therefore, why
> > not add an additional step in the pipeline to derive V-I for each star
by
> > working entirely in color index space?
>
>   I understand what you are saying here -- using color indices rather
> than magnitudes.  It's a standard technique in photoelectric photometry.
> Let's see .... I think what this would mean is that instead of allowing
> the zero-point value of each V-band frame to be a parameter for which
> we solve, and the zero-point value of each I-band frame to be a parameter
> for which we solve independently (thus, solving for 2*N zero-point
> values, where N is the number of exposure pairs), you suggest we force
> the V-band and I-band zero-point values of each pair of simultaneous
> frames to be linked.  For example, if the V-band zero-point drops
> by 0.03 mag on the V-band image of pair 28, then the I-band zero-point
> must also drop by some smaller amount (say, 0.02 mag) for the I-band
> image of pair 28.  That would reduce the number of parameters for
> which we solve to N, rather than 2*N.
>
>   Would this improve our output V and I magnitudes?  Yes, in theory.
> In practice, I _suspect_ that it wouldn't help much, for two reasons.
> First, we would need to figure out the ratio of V-band to I-band
> shift in zero-point, which would be an input to the photometric model.
> Not a big deal, just a few days work with several nights or weeks
> of data.  Second, we are in an overdetermined situation: each frame
> has, typically, 30 or 40 Tycho stars (I believe -- I don't have the
> exact number handy, and it varies with galactic latitude); a typical
> V,I pair, therefore, has 60 to 80 instrumental magnitudes.  The current
> pipeline derives 2 zero-points (plus a fraction of a color term)
> from this mass of data.  Richard's method would reduce this to 1
zero-point
> (plus a fraction of a color term) from the same amount of data.
> I am guessing -- purely guessing, mind you -- that decreasing the 
> number of parameters for which we solve from 2 to 1 won't make a big
> difference in the accuracy of that output parameter.  If we were
> going from, say, 20 to 10 parameters, I'd suspect that the improvement
> would be larger.
>
> >  At present no calibration
> >  advantage is being derived from the contemporaneous nature of the image
> >  pairs (or am I mistaken about this?) 
>
>   You are correct -- we are not using the simultaneous collection of
> V and I measurements to help us.
>
> > It might even be possible to arrive at a more accurate estimate of the I
> > magnitudes themselves via this approach than by basing this on Tycho Vt
and
> > Bt data as used at present.
>
>   Hmmm.  I think that if we discard the Tycho data, we might be able
> to solve more precisely for magnitudes on the _instrumental_ "I"-band
> system, but we would have no idea of the instrumental "I" compared
> to the standard Johnson-Cousins I, or even a pseudo-Johnson-Cousins "It"
> based on the Tycho Bt and Vt.  So we would have a more precise
> measurement of a quantity, but a much larger unknown systematic
> error.  It might help us to find low-amplitude variations, but we 
> could not compare them to I-band values published elsewhere.
>
>                                            Michael Richmond
>