Re: V image fainter than I

[droege@snapmail.us]

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.

Tom Droege




>
>   The current pipeline does detect objects which appear in the I-band
> image alone; they are listed with their proper (RA, Dec) positions,
> but a purely instrumental magnitude, in the .ast files.  I do not
> know if these files are stored anywhere, or if they are discarded
> as intermediate products.
>
>   However, the penultimate step in the pipeline, the "collate"
> stage, is where they are dropped.  The list of objects found in
> V-band is compared to those found in I-band, and only those detected
> in both bands are sent to the final step of photometric calibration.
>
>   A number of people very reasonably request that the pipeline
> be modified to keep objects which are detected only in I-band.
> Could you please tell me what to do with the instrumental
> measurements of the stars?
>
>      - the final step, photometric calibration, does two things
>               to the instrumental magnitudes: it shifts them
>               to match the standard magnitude system, and it
>               also uses the (V-I) color to correct for
>               a color term in the detectors.
>
>      - the orphan I-band measurements will have no V-band magnitude.
>               If one performs a shift on them, without a
>               color correction, then they will be systematically
>               off in magnitude; this simple procedure would treat them
>               like stars of color (V-I) = 0, which are relatively
>               hot, blue stars.
>
>      - moreover, these true (V-I) color of these stars might very
>               well change if the stars are variable.  If one does
>               not correct for that changing color, one gets
>               incorrect I-band magnitudes, on top of the problem
>               mentioned earlier.
>
>   The easiest thing would be to assign I-band orphans some very red
> color -- say, (V-I) = 3 or 4 -- and use that to make the color
> corrections.  One could them send these stars through the pipeline
> with those detected in both bands.  One would have to flag the
> orphans, but there is a facility for flags.  I'm not sure how
> all the software in the pipeline and, more important, in the
> database stages, would handle a non-existent V-band measurement.
> I could assign V=99.0, the standard value I use to mean "no real value",
> but I'm not sure if that would play well with the database.
>
>                                        Michael Richmond
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