See comments below...
regards,
m.------------------------------
>>>>Date: Wed, 22 Dec 1999 08:35:13 -0500
From: Chuck Norcutt <norcutt@xxxxxxxxxxxxx>
Subject: [OM] Alternate 500mm telephoto lens
Mike Butler asked:
<snip> I have been on the look out for a 400mm or longer lens to take to
the race track (I
don't get a Press Pass). A friend of mine suggested a small telescope
e.g. Celestron C-5, C-8 or similar catadioptric. <snip> I then came
upon the Orion 500 mm f5.6 for about US$280.00. It is a bunch bigger
than a Zuiko but it is also stop faster.
<snip>
- ---------------------------------------------------------------------
Hi, Mike. I posted a query about the Orion 500mm f5.6 several months
ago but got no response from anyone on the list. What appears to be the
same (Russian?) lens is also sold by Adorama under their Pro-Optic
brand. Orion sells it principally as a spotting scope so it comes
packaged with an eyepiece, erecting 45-degree diagonal and carry case.
Adorama sells it as a 500mm telephoto lens and sells the eyepiece and
diagonal (and I think the case) as optional extras.
<snip>
I spent a lot of time searching the web for anyone who had actual
experience using one. I finally found one guy who had posted some bird
pictures and had some brief correspondence with him. He thought it was
pretty good for the price but the build quality was not robust. Since
it's an f5.6 mirror lens the central obstruction from the secondary
mirror is pretty large. So, it has lower contrast than a slower mirror
lens and is probably a little bit slower than the nominal f5.6 would
indicate (all mirror lenses are to some degree). The guy I was
communicating with eventually sold the Orion f5.6 for something entirely
different but not because he was dissatisfied with the product at the
price. I fully intend to buy one myself but just haven't gotten around to it.
I should do it now to take advantage of the sale.
<<<<
Beware: I have actually looked through one of these Orion/Pro-Optic 500 mm f5.6
telescopes; it's a Maksutov-Cassegrain design, as opposed to a
Schmidt-Cassegrain. The secondary obstruction is not just large, it's VERY
large, it has to be to get the f 5.6 focal ratio. Accordingly, it exhibits
unusually low contrast, even by catadioptric standards. I didn't like it.
>>> As to other mirror lenses/telescopes. Celestron stuff is very good but
you can forget about using a C-8 except under very ideal conditions. I
have owned one for 25 years and don't use it very much since the tube
assembly alone weighs 25 lbs. Someone else mentioned using the C-5 with
telecompressor to give a 750mm f6.3. I don't have a telecompressor for
my C-8 so can't give first hand experience but my guess is that there is
some appreciable vignetting at the edges of the field due to the small
diameter of the secondary mirror for the C-5's normal f10.
<snip>
<<<
I was the one who mentioned the C5/telecompressor combo. Believe it or not, no
vignetting, even when using a 32mm eyepiece. There probably will be some at the
corners of a 35mm frame, though. Note that this is not exactly an inexpensive
solution, though it is more available (and less expensive) than long
conventional telephoto lenses. The slow focus comments are correct; none of
these telescopes was designed for speedy usage, and critical focusing at
infinity with 7mm or less eyepieces gets to be a pain. I wish the focusing on
my C5 was even slower.
regards,
Mahlon
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