Re: A Comment on the Lenses (part 2)

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

As I said, the test would "perhaps be simpler".  It depends
on how much trouble it would be to replace the camera window.
That is in your regime, but since you bought a stock Omega
filter, the Custom Scientific filter may be similar enough
in size and thickness to almost be a drop-in replacement.
This is a test that only needs to be done on one camera just
to see if the long-wavelength tail is the problem.  Adding
internal stops in a camera lens would appear to me to be
just as time-consuming!
   As for adding something to remove the long-wavelength response:
this would be another piece of glass, and I doubt you have room
in the current camera to do this.  You can have someone coat the
current filter with an interference coating, but that might be
more expensive/difficult than just buying someone else's stock filter.
   Tom, I'm not trying to make things difficult for you!  You had
a problem and were looking for solutions.  Changing the filter is
a viable experiment that you may or may not decide to do.  A theoretical
test would be to run a ray-trace using the real bandpass of the
filter/CCD combination and see if it shows significant image
aberration compared with a true Ic bandpass.  A person running
something like Zemax could also check to see what effect stopping
down the lens would have or where to put an internal baffle.
Arne

Tom Droege wrote:
> Those not familiar with the design should know that the filter is used 
> as the camera window.  Part of the camera body, the camera window, and 
> the shutter are all one unit.  Changing the filter means building a new 
> camera.  You all should remember that I give these things away, so I 
> have tried to keep the cost down in every way possible.
> 
> Is it possible to just add something to just cut off the tail of the IR 
> response?  Is this even desirable?
> 
> Having learned a little, my present interest (for the systems that I 
> run) is to just take specific fields night after night attempting to 
> cover the whole sky.  I can now see that this has absolute photometry 
> problems.  At my location I could probably not solve these even if I 
> tried.  It does appear that I can do field specific relative photometry 
> with considerably better precision.  This will detect variable stars in 
> two simultaneous filter.  We may never be able to get good calibration 
> on the either the V or the V-I photometry.  Sigh!  That may be life in 
> Batavia.  I am hoping that a large catalog of such measurements will 
> still be useful.
> 
> Tom Droege
> 
> At 09:07 AM 11/5/02 -0700, you wrote:
> 
>> One option would be to order an interference Ic from Custom
>> Scientific, and try that on your I-band camera.  This has the
>> proper bandpass and would not have any out-of-band transmission.
>> I am not sure how well such a filter will work in an f/4 beam,
>> but you can ask David Marcus (owner).
>>   This test would perhaps be simpler than experimenting with
>> internal aperture/pupil stops.
>> Arne
>>
>> Tom Droege wrote:
>>
>>> The filters are "Bessell I" from Omega Optical.  I assume they are 
>>> "stock" since there were no special instructions on the order.  This 
>>> is something that Michael Richmond selected after talking with the 
>>> expert at Omega Optical.
>>> Tom Droege
>>
> 
> 
>