Re: V image fainter than I

[Arne Henden <aah@nofs.navy.mil>]

droege@snapmail.us wrote:
> Thanks to Michael for the nice discussion below.  I think I am finally
> understanding what is going on.
> 
> It seems to me that the best solution is to minimize the missing detections.
> 
> We could as Michael suggests assign some average (V-I).  I think this
> would be a little smaller than the 3-4 Michael suggests, but the result is
> still probably garbage.
> 
> Another way might be to save all the measurements and then go somewhere
> else to find the (V - I) color term.  Then the (mostly I) measurements
> could be corrected.  This assumes we have such a catalog for the I stars. 
> Seems to me that one of the problems with our survey is that we don't have
> a good I catalog.  Sigh!
> 
> So I vote to adjust something to make the detections match for the two
> telescopes. We have:
> 
> a)Exposure
> b)Aperture
> 
> to make them match.  (Anything else?) Which is best to use?  It is really
> more convenient to adjust the aperture.  This assumes that a simple
> objective end mask will do it.
> 
I will rephrase my answer once again to see if I can make it clearer.
Let's take a simple example from one of Tom's earlier messages:
   time    Idet    Vdet
34452056  1827    465

If you decrease the I-band camera aperture but leave the exposure for
both V and Ic identical, then you will still have 465 V detections, but
some smaller number of Ic detections - lets say you change it so that
Vdet = Idet.  There are two problems with this:
   (1) the V/Ic matches do *not* necessarily match a V-band measure with
an equivalently bright Ic measure.  You will lose some matches with faint
Ic measures.
   (2) you lose the ~1300 Ic detections you currently have that are
unmatched with V.  These *are* good detections; the software finds them,
they are just not matched (but presumably saved).  The net effect of
decreasing the I-band camera aperture is to reduce the amount of science
coming from your camera.  Software does just as good a job of "removing"
the unmatched stars as would your aperture mask.

One obvious solution is to change the exposure length of the V-band camera,
making its exposure a factor of two or more longer than the I-band camera.
This is the best solution to gain matches, but complicates the survey
process.  The best solution is to find out why your current V-band measures
are so poor.
Arne