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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