Thanks, I remember something about the solar filter being off axis but
probably never knew why. A variable aperture at the back end is probably
the only real option.
AndrewF's suggestion for rear filter apertures sounds like a good way to
test the idea! Unfortunately the filter on a Vivitar Series 1 resides
inside the T-mount so changing the filter involves taking the camera off
but it should be possible to test the idea. If it works coming up with
something easy to do in the field will be pretty challenging. The
Celestron Mirrors have much more room to work with though.
Thanks!
-jeff
----- Original Message -----
From: "Donald Shedrick" <shedridc@xxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
> You can't stop down by a hole in the center, as that is where the
> central secondary mirror is and there is no light there. The mirror
> diameter as a percent of the main mirror diameter is what will
> determing the contrast possible with this type of lens, whether it is
> a camera lens or a telescope. These small camera types have probably
> at least 400bstruction. The general rule for astronomy scopes of
> this type is no more than 200bstruction for the best contrast. You
> should be able to cut an off axis hole to reduce the amount of light,
> but the reduction will be large since there is not much diameter to
> work with in the annular space around the central secondary mirror.
> This technique works well with much larger astronomical scopes to
> reduce the light when viewing bright objects like the moon or the sun
> (proper solar filter still required).
>
----- Original Message -----
From: "andrew fildes" <afildes@xxxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Tuesday, August 26, 2003 4:56 PM
Subject: Re: [OM] Sigma 600mm F8 Mirror Lens
> >Its a mirror lens. I've yet to see a snappy aperture on a mirror
lens.
> SNIP
> > It sounds like
> >something to try. I suspect a large lens cap with a large hole cut in
it
> >could give a one stop aperture to make up for focusing errors. More
> >things to try...
> >
> >-jeff
> >
>
----- Original Message -----
From: "andrew fildes" <afildes@xxxxxxxxxxxxxx>
> The usual method would be to use ND filters on the rear filter mount
> as anything reducing the front element would cause problems - a hole
> in a cap would cause vignetting and increase the proportional impact
> of the secondary mirror reducing contrast, etc. Sticking a larger
> disc on the secondary mirror at the front would reduce the light
> coming in and would be more effective, but again would probably
> degrade the image significantly. I suspect that the best of these odd
> options would be to make a series of Waterhouse stops - filter ring
> for the rear mount with a set of different size hole masks as was
> done before the invention of iris variable apertures. This is
> probably not the best position for placing fixed aperture
> restrictions but a cheap and simple DIY project if you've got the
> time and inclination and it's much easier to cut small, precise holes
> with clean edges.
> AndrewF
>
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