At 19:24 16.03.01 -0500, John Raymond wrote:
I would say with the mirror the viewfinder appears to be
atleast an f8 or slower. I am puzzled.
That is puzzling. f/8 means you you loose 65-700f the light somewhere in
the optical path. The mirror coatings would have to be *very* bad to
account for that, and this thing is only like 20 years old.
The confusion I have is that I have read from other mirror users that this
loss of light is common and the f stops are not accurately described on
mirror lenses. I thought then I wasn't alone.
Nothing is perfect, but I would assume a lightloss about 0.3 stops,
certainly nowhere close to 1.5 stops. Check out the measurements at
http://brashear.phys.appstate.edu/lhawkins/photo/mp-cat-tests.txt
The worst is the Vivitar 600/8 at f/8.9, and that's almost exactly 0.3 stops.
The only thing I can see possibly causing problem (this
might be huge) is that the lens I have doesn't have the UV filter on the rear
end. I am not sure that it is part of the optical formula as it would sit
right against the last real element anyway.
A missing *flat* filter will not cause lightloss, on the contrary, but any
rear mounted filter should be part of the optical formula, as it otherwise
will cause spherical aberration. You should always leave a filter in there.
Also, the t-mount doesn't stop in accordance to the built in-non movable
tripod mount. If it were to be on a tripod I would have to tilt the platform
quite a ways to bring it to "flat".
I have heard that some t-mounts can be adjusted to get this alignment-maybe I
have a bum t mount as well? : )
I will also research this.
A T-mount adapter will usually have an inner ring that can be rotated after
losening 3 or 4 screws. Is it possible that your T-mount has a very small
opening thereby causing serious vignetting? If not, take the whole thing
back to the shop or have another shop look at it. You've probably paid good
money to have f/4.5, not f/8.
Thanks Thomas for your thoughts and any others who have responded or will.
You're welcome, hope you can sort this out! Now, if anyone knows of another
fast mirror lens, a Celestron Comet Catcher, for sale, (500mm f/3.64) I'd
love to hear about it.....
Regards,
Thomas Bryhn
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