Re: V image fainter than I

[droege@snapmail.us]

My thinking is to try to go a little fainter with the present setup. 
Let's say increase the exposure to 200 seconds.  This will work fine on
the V camera.  The I camera will have a sky background problem at this
exposure.  Further, a lot of stars that are not saturated in V will be
saturated in I so we will lose bright stars.  I admit, not a big loss
since these are well measured.

By stopping down the I camera the sky background will be reduced and it
will better match the V camera through the range.

Tom Droege


> I don't see what you gain.
> If you leave the exposure as it currently is, you have lots of
> unmatched Ic stars.
> If you stop down the Ic camera, then you have the same number of
> matched V/Ic pairs, but you've lost the unmatched stars.
> If you increase the exposure, you pick up the current unmatched
> stars with matches to the longer V exposure, but now you lose
> the 2/3 stars that would normally appear on the full-aperture Ic
> frame.
>    So the only gain with stopping down the Ic camera is to remove
> the unmatched Ic stars.  Software removes them already, but gives
> you the option of sometime in the future using these unmatched
> objects in some other way.  Why not just leave things alone?
> Arne
>
> droege@snapmail.us wrote:
>> Richard,
>>
>> This does not work.  A star that is saturated in I is not saturated in V
>> and so is lost.  There are lots of I stars for which there is no V star
>> detected, like a factor of 3.
>>
>> The scheme would be to stop down the I camera and take longer exposures
>> so
>> that at the faint level there was a rough match of detection.
>>
>> Tom Droege
>>
>>
>>>Speaking as a lurker, I'd change the V vs I parameters in the data
>>>reduction
>>>process to accept the difference.  Don't throw out (or block) good data.
>>>
>>>Richard
>>>
>>>-----Original Message-----
>>>From: owner-tass@listserv.wwa.com [mailto:owner-tass@listserv.wwa.com]On
>>>Behalf Of droege@snapmail.us
>>>Sent: Friday, 18 March, 2005 13:20
>>>To: tass
>>>Subject: V image fainter than I
>>>
>>>
>>>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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