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[OM] Alternate 500mm telephoto lens

Subject: [OM] Alternate 500mm telephoto lens
From: Chuck Norcutt <norcutt@xxxxxxxxxxxxx>
Date: Wed, 22 Dec 1999 08:35:13 -0500
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.

Finally to my question, does anyone (maybe one of our Astro
Photographers) have any experience using a Catadioptric telescope for
terrestrial photos?  How is the build quality, sharpness ect?  Better
than A Tamron 500mm F8 for example?  TIA,
---------------------------------------------------------------------

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. 

Orion currently has theirs on sale at $239.  Adorama's regular price
with eyepiece, etc. is about $10 less than Orion's sale price.  For the
difference, I think I would buy from Orion as they are a no hassle place
to do business.  I only know that Adorama has the optional finder scope
package since I saw it in one of their print ads in a camera mag.  It's
impossible to find it on their web site and I had a tough time getting a
current price from them via e-mail since I had to convince the guy on
the other end that they actually carry the item.

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.

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.  The
secondary mirror size on reflecting telescopes is always a trade-off
between vignetting (trying to cover a wide field of view with a small
diameter secondary) and loss of contrast due to refraction around a
large secondary.  The only way to have your cake and eat it too is a
Sheifspiegler (an off-axis design that gets the secondary mirror out of
the way) but they're long focal length designs that you'd have to build
yourself.

Sorry I can't give you much more help.  I'm not concerned about buying
one of these 500mm f5.6's since my primary intention is a bright
spotting scope.  If I also happen to get a usable 500mm lens so much the
better.

ps:  The C-8 is definitely very slow to focus.  I expect that the 500mm
f5.6 will be as well.  It has a focusing knob extending out the back of
the mirror cell just like the C-8. The focusing knob causes the primary
mirror to move back and forth on a threaded, hollow shaft.  Not as quick
as grabbing a focusing grip up front like most lenses.

Chuck Norcutt
Woburn, Massachusetts, USA

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