on 7/15/01 4:01 AM, Chuck Norcutt at norcutt@xxxxxxxxxxxxx wrote:
> Someone commented the other day about severe vignetting from a Celestron
> (whose focal length I don't recall). Celestron makes mostly telescopes
> with Schmidt design optics. (Back in the 60's they made a manufacturing
> breakthrough and figured out how to automate the grinding and polishing
> of the complex, shallow curve in a Schmidt corrector plate.) It may be
> that the lens in question was meant primarily as a telescope and
> secondarily as a camera lens. As a telescope you only need to avoid
> vignetting across a diameter of 25mm or even less. This allows for a
> smaller secondary mirror and less obstruction. Fully lighting the full
> 43mm image circle of the 35mm film diagonal would take a large
> secondary.
>
> Chuck Norcutt
> Woburn, Massachusetts, USA
Chuck -- 'someone' is me... great info on the mirror lens design! The
Celestron lens I have is definitely designed as a camera lens - its a 300mm
f5.6 in a T-mount, for which I have Olympus and Pentax-K adapters. I got my
pictures back, and they are acceptable... Someone commented that this lens
has low contrast, and I think they are right. But it is light and small and
therefore useful for what I want it for.
I think my vignetting problem is due to some strange interaction between the
Oly 1-7 screen and the output of this lens, perhaps related to its mirror
construction with a 'large' central obstruction. I also have an Olympus
500/8.0 mirror, and with the 1-7 screen there is a slight indication of the
same vignetting in the 1-7 screen image, but only in the extreme corners of
the view. So I am thinking that this effect is probably a combination of the
focal length, central obstruction, and fresnel/clear field construction of
the 1-7 screen. I'm going to just chalk it up as non-compatible and leave it
at that. The pictures from the Celestron 300 are evenly exposed and sharp,
even though a bit low in contrast.
--
Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney...
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