Interesting. It doesn't look like either one. Nor does the Zuiko 500mm
for that matter. Here's a Wiki article that illustrates the Maksutov
corrector plate <http://en.wikipedia.org/wiki/Maksutov_telescope> Note
the thick and steeply curved shape. From this article I see that the
design wasn't commercialized until 1957 and that Perkin Elmer held the
rights for some time. Perhaps the Yashica is old enough that the
Maksutov design couldn't be used. That doesn't explain he 500mm Zuiko
design though. It doesn't conform to current conventional design
either. But it also performs better than most. But it uses lots of
pieces of glass which accounts for its high cost. Here's a
Schmidt-Cassegrain.
<http://en.wikipedia.org/wiki/Schmidt-Cassegrain_Telescope>
Note the simplicity of the basic designs of both. In particular, both
use spherical primary mirrors which significantly reduces the cost of
production.
Chuck Norcutt
Andrew Fildes wrote:
> Here's the design - which is it?
> http://www-unix.oit.umass.edu/~coreya/yashica/1000mm.jpg
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 23/10/2008, at 8:16 AM, Chuck Norcutt wrote:
>
>> 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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