There are multiple types of Cassegrain telescope designs but only two
that have survived within the affordable consumer market... the Maksutov
and the Schmidt-Cassegrain. For telescopes of 90mm diameter or less the
overwhelming design choice is a Maksutov which uses a spherical primary
and a very thick meniscus corrector lens. This is your typical camera
mirror lens design which (if it's done correctly) can have the secondary
mirror simply be an aluminized spot on the back side of the Maksutov
corrector lens. The Maksutov corrector is easy to make in diameters up
to 90mm. Above 90mm the cost equation swings toward the
Schmidt-Cassegrain design which also uses a spherical primary mirror but
with a Schmidt corrector plate in place of the Maksutov. The Schmidt
corrector plate is thin and nearly flat. It is not easy to make but
less costly and troublesome than a Maksutov in diameters over 90mm.
Secondary mirrors are also more complicated than a Maksutov since they
require mounting in an adjustable cell on the back side of the corrector
plate. One of the reasons that large Maksutovs are expensive is the
difficulty of casting a large, thick piece of glass with no bubbles.
You can buy a Maksutov up to 7" in diameter from Questar but, if you
know that name, you know you will pay dearly for it.
Celestron built its business in the 60's selling military grade
Schmidt-Cassegrain telescopes to the military. Between themselves and a
Japanese lens maker they figured out how to automate the grinding and
polishing of a Schmidt corrector lens which had heretofore been done by
hand. If fact, it was considered a major breakthrough in lens
manufacture when Bernhardt Schmidt figured out how to do it even
manually in 1930. The cross section of a Schmidt corrector plate has a
sort of sinusoidal pattern from center to edge so you might appreciate
how difficult is would be to create that when normal grinding and
polishing methods produce a spherical shape. The curve is also
extremely flat and difficult to see with the eye. Then the Vietnam war
started winding down and Celestron's military revenues started down with
it. They then reinvented the company as a low cost provider of high
quality consumer telescopes. They did it by utilizing the terrific cost
advantage they had on the Schmidt optics (low cost corrector plate and
spherical primary mirror) and also by value re-engineering of all the
mechanics of the tube, mount and tripod structure. Early Celestrons are
made of very thick wall aluminum tube with heavy, machined aluminum
cells for the optics which are screwed onto the tube. Later Celestrons
(such as my 1974 C8) have lighter weight cells made from castings with
minimal machining and a rolled steel tube that's epoxied into the cells.
Very rigid, lighter weight and is still perfectly well together after
34 years.
Then along came Meade after Celestron's patents expired. They produce
very similar designs with many accessories that are fairly
interchangeable between the brands. The also produce very good stuff
and are very competitive. But both companies also produce some very low
cost stuff for the Christmas market. Avoid it.
So, no matter who you buy from you are likely to get a Maksutov design
at 90mm diameter or less and a Schmidt-Cassegrain over 90mm. At 1000mm
focal length and above you're talking about f/10 (usually) optics. They
are true telescopes with secondary mirrors sized for visual use...
meaning a small image circle. Probably OK on an E-body but with some
evident vignetting on 35mm size. But it will be equipped with a finder
and means for sturdy mounting and have options for removal of the visual
back and replacment with a larger focusing tube and T-mount for a camera.
I'm not sure about Meade but Celestron has built some purely
photographic mirror lenses in the past which you can sometimes find on
ebay. These are characterized by shorter focal lengths (about 750mm
IIRC) and with larger secondary mirrors to combat vignetting.
Chuck Norcutt
Maarten Schulte wrote:
> Thanks for your thoughts.
>
> I have had a number of "cheap" 500mm mirror lenses, and didn't like the
> quality of them.
> As you both remark, it is quite difficult to get sharp images.
> I also thought about a true telescope (do you suggest a cassegrain type or
> something else?).
> But since I need the carry the thing with me, a heavy tripod is not really
> an option.
>
> The Yashica lens is quite expensive (I think) almost 600 euro's including
> the adapter to fit my e510. I might be able to talk it down a bit. But it
> remains (especially in these credit stress times) a significant amount.
>
> I'm curious what type of telescope, you would suggest.
> I have had a close look at the Celestron c90 Mak spotting scope.
>
> Any ideas are welcome.
>
>
> Maarten
>
> -----Oorspronkelijk bericht-----
> Van: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] Namens Moose
> Verzonden: woensdag 22 oktober 2008 11:28
> Aan: olympus@xxxxxxxxxx
> Onderwerp: [OM] Re: Yashica 1000mm lens
>
> I agree with what Chuck has to say.
>
> I would add that actual lens performance is usually not the limiting
> factor at greater distances. Atmospheric contamination, thermal movement
> and subject movement all have an effect and the donut OOF highlights are
> a constant bother.
>
> I have a Meade telescope that is also a 1000/11 lens. With the proper
> scope to T and T to OM mount adapters, it makes a nice, solid
> connection. The finder scope is a real help. Focusing is not bad in
> bright light with standard 1-13 screen. I found the 1-7 and 1-8 screens
> both helped. I liked the 1-8 better.
>
> Taking pictures of nesting hawks, I found the swaying of the trees in
> even tiny breezes was a real problem.
>
> I think you may have a problem with focusing on any of the E-thingies,
> alhough the E-3 should be best. Maximum aperture of F11 is really
> outside their design criteria.
>
> Moose
>
> Chuck Norcutt wrote:
>> I checked the old Modern Photo tests of mirror lenses and there's no
> Yashica of any focal length and the only 1000mm tested was a Meade. But
> Yashica generally made good stuff so I wouldn't be too concerned about
> purchasing it if it's in good condition and you get a good price.
>> But some other thoughts. There's a reason why you only see Meade listed
>> in the 1000mm range. At 1000mm you are basically in telescope
>> territory. Given the narrower angle of view of the e-camera it will be
>> more like using a 2000mm lens on a 35mm camera. Been there, done that
>> with my 2000mm Celestron 8. When you get that long and with that narrow
>> a field of view it is extremely important to have
>> 1) a massive tripod and sturdy head
>> 2) a wider field finder to be able to locate your viewing target.
>>
>> If you're intent on 1000mm or above I think I'd skip the "lens"
> designation and go straight to a true telescope which is designed to resolve
> points 1) and 2) above. Meade and Celestron are the principal
>> and low cost purveyors of same.
>>
>> Chuck Norcutt
>
>
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