Re: V image fainter than I

[droege@snapmail.us]

Arne and all,

We are not (now) keeping the unmatched I detections.  We have been keeping
the dark and flat corrected images, so they could be reprocessed to get
the unsaved detections.

Looking at the V and Ic filter bandpasses superimposed on the CCD qe
curve, it makes sense to me that their should be roughly twice the I
sensitivity as
v.

This is supported by the sky background of the I images which is about a
factor of 2 larger than the V images.

For Arne's argument below, it should be noted that stars are lost at each
end.  We lose I detections because they are faint and there is not
matching V detection.  We lose I detections because they are too bright
and thus the good matching V detection is thrown out because the I
detection is saturated.

Arne writes:

> 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.

Seems like there are three posibilities:

1) Put a neutral density filter in the I path
2) Use different V and I exposures.
3) Stop down the I camera.
4) Use the lumigen coated CCDs in the V cameras.

Arne says 2) "complicates the survey".  In what way?  One complication is
that the present software can't do it.  Are there others?

I don't like 1) beacuse I have had not so good experience with neutral
density filters in the past, and it adds another pair of surfaces to keep
clean.

So far I like 3) best.  It is easy to do to any level.  One can stop down
just enough to save most of the bright I stars.

I have two lumigen coated CCDs.  These should pick up the V response a
little (looking at the qe curve).  But I only have two and there are five
dual systems.

Now is the time to make such decisions.  The engineering run is over. 
This is the "real" run.  If we are to do things like save the unmatched
detections, then we need software to do it now.  So "Heros" please step
forward.

Tom Droege


> 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
>
>
>