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[OM] Re: Yashica 1000mm lens

Subject: [OM] Re: Yashica 1000mm lens
From: Andrew Fildes <afildes@xxxxxxxxxxxxx>
Date: Thu, 23 Oct 2008 08:44:54 +1100
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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