Re: Data reduction methodology for V-I colors

[Doug Welch <welch@physics.mcmaster.ca>]

If the correction is a function of magnitude, then scattered
light may be a factor. This may be scattered light in the
flat and not in an actual image.

Cheers,
Doug

On Tue, 31 Aug 2004, Chris Albertson wrote:

> 
> Could it be that the zero point is really a function of
> (x,y) on the frame and that using one "zero point" for the
> entire frame is only a gross approximation of the true
> "zero function".  It seems unlikely to me that the atmosphere
> really is equally transparent over a degrees wide field.
> 
> Professionals may  have been using zero points for years but
> how many of them use wide feild images?
> 
> One experiment might be to plot the difference between
> instrumental and catalogged magnitutes and see if the differences
> "clump" togeter.  If there were a cloud blocking
> half the frame you would see it.  But more subtle effects
> might also be detectable.
> 
> If the transpaarency does vary across a frame at the few percent
> level then maybe you will have to compute some kind of "zero
> function".
> 
> The "gradient" Tom talked about seems like it might headed in 
> this direction.  "gradient" is a kind of fuction (a first order
> one.) but I supect a tilted plain will not fit much better then
> our current zeroth order function.  The physical atmosphere is
> likey bumpy with sharp discontinuities.
> 
> 
> 
> 
> --- Thomas Droege <tdroege2@earthlink.net> wrote:
> 
> > I will leave this for Michael.  I have just looked at data sets as a
> > whole.
> > Why not join in the fun?  There is lots of data.  Andrew, Michael and
> > I
> > have been working on this.   There is room for more clever minds to
> > puzzle
> > over the data.  As always, I stand willing to copy CDs for anyone
> > willing
> > to compute on the data.  
> > 
> > Tom Droege
> > 
> > 
> > > [Original Message]
> > > From: Michael Koppelman <lolife@bitstream.net>
> > > To: <tdroege2@earthlink.net>
> > > Cc: tass <tass@listserv.wwa.com>
> > > Date: 8/31/2004 12:24:00 PM
> > > Subject: Re: Data reduction methodology for V-I colors
> > >
> > > But aren't you getting a different zeropoint solution for every
> > frame? 
> > > Why not use a single zeropoint for the entire night?
> > >
> > > On Aug 31, 2004, at 12:20 PM, Thomas Droege wrote:
> > >
> > 
> > 
> > 
> > 
> 
> 
> =====
> Chris Albertson
>   Home:   310-376-1029  chrisalbertson90278@yahoo.com
>   Cell:   310-990-7550
>   Office: 310-336-5189  Christopher.J.Albertson@aero.org
>   KG6OMK
> 
> 
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-- 
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 Douglas L Welch     | Res office/voicemail (905) 525-9140 x23186  
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