Re: V image fainter than I

[droege@snapmail.us]

OK, here is some (not so typical) data:

Time      I detections  V detections
34452056  1827          465
34452073  2074          476

34452616  1994          148
34452634  1986          146

The first two rows are tracking one field and the second two another.

Something is really funny with the V camera.  I expect fewer detections in
the V camera but it is usually only a factor of two.  Perhaps this has
something to do with spring weather.  Since I can't see very well in the
dark, it is hard for me to assess the clearness of the sky.  The result
above could be from looking through thin fog.

Note the ratio is a factor of 4 in the first case and over ten in the
second.  I really don't believe these results and have changed the V
camera but I don't see anything wrong.  OK, part of the problem is that
the V camera keeps moving out of focus.  Rob claims that he never moves
the focus but I am not so sure.  I will solve this in the future by
unplugging the motors between doing focus runs.

In any case, for several years the ratio of I detections to V detections
has been a factor of about two.  At the moment, I can't find the q.e.
curves for the CCD442A.  I recall that the factor of two is explained by
the curve.  I will find the curve and report back.

Tom Droege



> This is no different than the case of going through a reddened region,
> where you see more Ic stars than V stars.  Just leave everything alone
> and have a few more unmatched Ic measures.
>    In general, the throughput between V and Ic is about equal for most
> CCD cameras.  Stars tend to be brighter at V and the QE is higher at V
> (not Ic, as Tom suggests), so the net effect is to compensate for the
> wider Ic bandpass.  My usual exposure times are about 1.2x longer
> at V than Ic.
> Arne
>
> droege@snapmail.us wrote:
>> As I am setting up the camera for the coming run I notice the V camera
>> is
>> even fainter than usual from the I camera. I don't know why and have
>> changed out the camera.  We shall see if it is a camera problem when I
>> get
>> some clear sky.  I have done all the obvious things like look through
>> the
>> lens for dead bugs and hanging chad.
>>
>> There has always been a bad match.  Many stars are lost because they are
>> too bright in the I images and others are too faint in the V.
>>
>> Rob tells me that different exposure lengths are a pain.  The
>> alternative
>> is to stop down the I camera.
>>
>> The I filter has about twice the bandwidth of the V filter, further the
>> CCD is more sensitive in I.
>>
>> At the moment, the I camera has a great focus.  I really hate to throw
>> away all those photons now that they pass through the optics so well.
>>
>> Should I stop down the I lens?
>>
>> Any comments?
>>
>>
>>
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