Re: A Comment on the Lenses (part 2)

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

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
> 
> At 08:33 AM 11/5/02 -0700, you wrote:
> 
>> Dave makes a good point.  I don't remember where you got the I-band
>> filters from and whether they were "custom" or "stock".  The Bessell
>> Ic prescription, for example, is just RG9 glass, with the red edge
>> of the passband formed by the CCD response.  In other words, it has
>> a significant red tail in comparison with the Cousins I passband.
>> Arne
>>
>> dave garnett wrote:
>>
>>> I've just looked at the lens design files again. They are designed for a
>>> 50mm dia stop just in front of the front lens, but it is noticeable that
>>> some of the internal rays pass very close to the edges of lenses, so 
>>> it is
>>> possible that the real stop may be internal.
>>> I note that the design spot size is small for wavelengths in the 
>>> range 0.715
>>> to 0.880 - outside of these values the spot size increases rapidly. 
>>> So, for
>>> good quality images you need to be sure that the filter is doing its 
>>> job.
>>> Presumably the ccd chip is very sensitive at this end of the 
>>> spectrum, which
>>> will exaggerate any filter leakage ...
>>> Dave
>>> ----- Original Message -----
>>> From: "Tom Droege" <tdroege2@earthlink.net>
>>> To: <tass@listserv.wwa.com>
>>> Sent: Tuesday, November 05, 2002 5:19 AM
>>> Subject: jA Comment on the Lenses
>>>
>>>
>>>> As noted earlier, the wide open I lens is down 5% at the edge (on a 
>>>> circle
>>>> tangent to the sides of the CCD) and 10% in the corner.
>>>>
>>>> I note that now I have stopped down the I lens, it is down more like 
>>>> 20%
>>>
>>> in
>>>
>>>> the corner.  This surprised my feeble knowledge of optics.  I thought
>>>> everything got better as you stopped down lenses.
>>>>
>>>> Possibly an optics expert can respond?  In any case, we are still
>>>> considerably superior to camera lenses which are often 50% down in the
>>>> corners of images of our size.
>>>>
>>>> Tom Droege
>>>>
>>>>
>>>
>>>
>>
> 
> 
> 
>