Re: A Comment on the Lenses (part 2)

[Tom Droege <tdroege2@earthlink.net>]

One place to test this would be at the image end of the lens.  Here the 
opening diameter is a little over 2" so anything larger than a 2" diameter 
filter would do the job.  This would save tearing up a camera, but would 
add two more surfaces to the optical path.  There is room for something up 
to 1/2" thick here.

Tom Droege

At 10:07 AM 11/5/02 -0700, you wrote:
>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
>>
>