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Classic Cassegrain Popularity?

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#1 Bomber Bob

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Posted 18 October 2014 - 04:51 PM

After the holidays, I'll start restoring this vintage Tinsley Classic Cassegrain.  Forty years ago when I was a teen, CCs were fairly popular competitors to large refractors; but with SCTs & MCTs, I wonder if today they're in the same niche as large achro refractors, given the quality and declining prices of APOs...

 

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#2 Ed Wiley

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Posted 18 October 2014 - 10:45 PM

To your specific question: "I wonder if today they're in the same niche as large achro refractors, given the quality and declining prices of APOs..." I think the answer is "no" once you get into the 8" and above class. My 8" Royce DK (not a CC, but at F22 who cares) is inexpensive compared to a 8" APO refractor (or even an F15 8" acro) and DKs in the 10"-16" range must be super inexpensive compared to APOs of similar size. However, I suspect they will remain in the "specialized" niche compared to SCTs like my C11 Edge (which has very good optics) simply because DKs are, by nature, long focal length scopes.

 

Ed


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#3 jrcrilly

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Posted 19 October 2014 - 02:16 AM

After the holidays, I'll start restoring this vintage Tinsley Classic Cassegrain.  Forty years ago when I was a teen, CCs were fairly popular competitors to large refractors; but with SCTs & MCTs, I wonder if today they're in the same niche as large achro refractors, given the quality and declining prices of APOs...

They really don't compete, as they don't do he same job. When I had a classical cass (8"), I mounted it beside my apo refractor (7") so I could use whichever was appropriate for what I was doing at the time. You just can't get those long focal lengths out of a refractor that will work well. These days, with faster cass variants readily available, folks are going to much greater apertures to get the long focal lengths


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#4 Jon Isaacs

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Posted 19 October 2014 - 05:20 AM

I think what Bob was wondering if the classical cassegrain has been passed by in the same way the achromat has been passed by, that both were once the optimal solution but now there are better designs. 

 

Jon


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#5 Eddgie

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Posted 19 October 2014 - 09:45 AM

After the holidays, I'll start restoring this vintage Tinsley Classic Cassegrain.  Forty years ago when I was a teen, CCs were fairly popular competitors to large refractors; but with SCTs & MCTs, I wonder if today they're in the same niche as large achro refractors, given the quality and declining prices of APOs...

 

To expensive to make and still make as much profit on as the much easier to make SCT or MCT.   Lots of complex curves if you want it to perform well both on and off axis.   If you make it f/10, to get it to match the off axis performance of an SCT, you have to do a lot of work on the secondary,   The obstruction could be made slightly smaller, but at a hugely more complex and difficult set of curves to grind and polish.

 

Here is an example.  I you made a cassegrain at f/10 and used f/2 and f/5 secondary so that it would be about the same physical size as an f/10 SCT,  the secondary obstuction could be made only 45mm in diameter (22%) but the level of complexity required to deal with off axis aberrations would mean that this would be a far more complicated design to grind and polish.  Otherwise, the field curvature would be extremly bad.  (RC = - 160 for the Cass, vs an RC+ -270mm for the SCT, and this is the standard 200mm SCT.   The EdgeHD is more like RC =  - 810mm, which is much flatter than you are going to get with the classical cassegrain). 

 

Now, if you took the easier route (probably the one chosen for the scope in the pitcure), things change.  By using an f/5 primary and an f/2 secondary, you have the longer tube (less compact, heavier, harder to mount than the f/10 SCT), and the curves will be far easier to grind and polish.  The RC will also be a very flat -3290mm, which is about as flat as fields get, which is great for imaging.  

 

Ah, but in this design, the central obstruction is now 76mm, or 38%.    And this does not include the secondary spider vane diffraction, which now goes to total obstruction as about 40%.

 

Now this design could be tweaked to be somewhere in between, but unless you make the total obstruction meaninfully smaller than it would be for the SCT, then what would be the point.   The only inhibitor to SCT performance at the center of the field is the obstruction size.  The design othewise if 100% diffraction free, and a Cassegrain with the same size obstruction would be no better.

 

This is why the classical cassegrain is no longer in serial production. It is difficult to make to bring it to the same compact size as the commercial SCT, and if you abandon that compact size to make it easier to produce, you also suffer a much bigger central obstruction. 

 

This is why you used to see so many large Cassegrains in professional observatory work.  The can have a very flat field at f/10 using the f/5, f/2 forumula.  You see some compact cassegrains (UT McDonald has one of these) but they have the labor involved in correcting the curves needed to tame the off axis performance issues is very high..  OK for an professional observatory, but unacceptable for mass produced instruments.  Just to hard to make.

 

I would be curious to know the primary and secondary diameters of your scope.  I would guess by looking that it is f/8 to f/10, and that the primary is maybe f/3 or f/4 and the secondary is maybe f/3 or f/4.  Another words, somewhere between the two examples I used above.  My bet is that the secondary is also about 30%. 

 

Maybe I am wrong, but my bet is that this scope is about the same obstruction and performacne as a standard SCT (Flatter field though) but in a package twice as long.

 

And there you have it.  Bigger, harder to make (which means more expensive to make), and in the end no better than a standard SCT for visual work because the obstruction would be about the same size.  Would be better for imaging though, because the field would be much flatter.

 

I would be curious to know the primary and secondary diameters though...

Data used came from the book "Telescope Optics." If there is an error, I apologize.  Just repeated data from my reference work.


Edited by Eddgie, 19 October 2014 - 09:47 AM.

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#6 Bomber Bob

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Posted 19 October 2014 - 10:36 AM

My mirror set was quoted as f/20:  A 6" f/4 primary, and an exactly 2" secondary.  I'm wondering if it isn't actually f/16.  I'll confirm the system FL later, but I have measured the primary FL as 25.2" +/- 0.1 or f/4.2  I don't know the maker or history of the optics, except that it was recoated (probably in the 1970s) by Delta-Lambda (now Reynard) in California.  The mirrors are in good shape overall -- definitely usable for initial star testing.

 

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#7 Bomber Bob

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Posted 19 October 2014 - 10:38 AM

I think what Bob was wondering if the classical cassegrain has been passed by in the same way the achromat has been passed by

 

Yep.  It seems to me that long FL scopes in general have fallen out of favor -- victims of digital imaging / cost (especially the mounts!) / light pollution (requiring transport to dark sites).


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#8 Eddgie

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Posted 19 October 2014 - 03:38 PM

Dob't take this as being bad mouthing of your telescope.  I can't wait to see the finished product, and I hope that it is rewarding to do.

 

With respect to your question though, hopefully you can see my point.   Here is a scope that is at best f/16, and i has a obstruction that is only a tiny bit smaller than an f/10 SCT, but once you add the spider diffraction, the total obstruction and the total effect on contrast transfer is going to be about the same,

 

They could have made it f/10 with a flat field, but again, this would have required a 38% obstruction (40% with the spider vane diffraction added in) and the tube would be twice as long as for the SCT.

 

Or they could have kept the tube lenght the same as the SCT and gotten a 23% obstruction, but at the expense of very high levels of field curvature and the amount of figuring needed to make it work well would be far beyond what it takes to make an SCT, where machines do pretty much 100% of the work.

 

So here we have a scope that in the example pictured, could never out-perform a 6" SCT with 33% obstruction, and this scope wil have a great field limitation vs an f/10 SCT due to the focal ratio.  It will also be longer and make more demand on the mount.

 

As for your question though, I think that yes, the design simply became obsolete. 

 

As much bad mouting as the SCT gets, it remains one of the most successful commercial designs ever produced.  Dismissing department store telescopes, I would guess that the C8 is perhaps the best selling scope of all time.  I know that it has one of the longest production runs.

And the reason is simple.  It is inexpensive to make, and offers a lot of performance in a small, compact, comfortable to use, and flexible package.


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#9 Bomber Bob

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Posted 19 October 2014 - 04:27 PM

Dob't take this as being bad mouthing of your telescope.

 

Not offended at all.  I appreciate honest opinions & assessments, which is why I ask the questions.  I'm a Refractor Dude (45+ years now); and, while I've looked through a number of CCs, SCTs, and MCTs, I'm a novice with folded reflectors.

 

As for your question though, I think that yes, the design simply became obsolete.

 

And that's a shame.  I know I'm biased, but I think there's still a place for long FL scopes with lunar & planetary observing & imaging.  If this scope is indeed f/20, I'll get 240x with a Meade RG OR12.5 -- only 40x / inch! -- with a decent apparent field; and 333x with the OR9.  Yet another reason to get it up & running for Jupiter's opposition.

 

I'm still looking for a Tinsley ad or catalog listing the 6" -- the 1930s copy I have lists 5 models 10 - 20" and they're all f/16.  I measured the Tinsley's mirror cell placements, and it looks like the original 6" primary was f/3, so maybe it was an f/12 or f/15 system?  If so, I'll have to modify the spider, but I'm going to try and avoid new holes in the tube.


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#10 Eddgie

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Posted 19 October 2014 - 05:17 PM

Well, I would be surprised if it were f/20, but not terribly so.  You could find out though if someone knows how to measure the power of the secondary.

 

I thought you just multiplied the numbers together.

 

As to liking a longer focal length, it seems that the marketplace wanted ever shorter focal lengths.  APOs need to be f/8 to be desirable these days, and people complain about the SCT being f/10 all the time....   I bet I have read a dozen posts in the last 15 years where someone complained about why Celestron can't make the SCT to be f/6 or something.

 

While you may love it, the market does not love it enough to support serial manufacture.  Remember, the maker has only so many grinding machines, so many coating machines, and so many workers.  The manufacturer wants to use all of those in the most efficient manner possible.   Making telescopes with little demand in the marketplace works against them.  They produce what the market wants.

 

I look forward to seeing the fruits of your labor though. This could result in a really beautiful and unusual telescope....


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#11 Jon Isaacs

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Posted 19 October 2014 - 05:25 PM

 

And that's a shame.  I know I'm biased, but I think there's still a place for long FL scopes with lunar & planetary observing & imaging.  If this scope is indeed f/20, I'll get 240x with a Meade RG OR12.5 -- only 40x / inch! -- with a decent apparent field; and 333x with the OR9.  Yet another reason to get it up & running for Jupiter's opposition.

 

Back when, short focal length eyepieces with a reasonable amount of eye relief were not available, coatings were not as effective as they are today, telescopes were not transported as much and scopes were generally quite small.  These days, quality short focal length eyepieces are available, coatings are very good and large apertures make slow focal ratios more impractical. 

 

By the way, when you finish that one and want to start on a more ambitious project, there was a guy around here trying to sell a 12 inch Classical Cassegrain that needed some real attention, he was asking $350.

 

Jon


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#12 Jeff B1

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Posted 19 October 2014 - 05:55 PM

My 12.5" f/30 Classical was a really nice instrument to use and fiddle with.  Not sure how many of them I designed but they are fun telescopes to plan and make.  The optics were superb and if I had not gone of the deep end my 16" mirror would have been a 16" f/50 Classical Cass. Just think about eye relief!  The depth of focus on the 12.5 was great and at f/50 the 16" would be really good.  Problem was my focusing a camera sucked!  A close friend, who can take a photo like no other, loved to use it and make some fine images.  Oh well, good thing I liked to draw :)  

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#13 Stephen Kennedy

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Posted 19 October 2014 - 06:37 PM

I think that the classical Cassegrain and classical Gregorian have largely been supplanted by the better corrected aplanatic Cassegrain (Richey-Cretien) and the aplanatic Gregorian. All major recently built and planned observatories use one or the other designs since they are free of both spherical aberration and off-axis coma. The aplanatic Gregorian has slightly more advantageous optical properties than the RC but requires a longer tube which means a more costly mount and dome. As a result more RCs are in operation than aplanatic Gregorians.

Amateurs hardly ever use the same telescopes as professionals (aplanatic Cassegrains and Gregorians) while professionals hardly ever use the same telescopes as amateurs (refractors, SCTs and Newtonians).
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#14 Jon Isaacs

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Posted 19 October 2014 - 09:43 PM

I think that the classical Cassegrain and classical Gregorian have largely been supplanted by the better corrected aplanatic Cassegrain (Richey-Cretien) and the aplanatic Gregorian. All major recently built and planned observatories use one or the other designs since they are free of both spherical aberration and off-axis coma. The aplanatic Gregorian has slightly more advantageous optical properties than the RC but requires a longer tube which means a more costly mount and dome. As a result more RCs are in operation than aplanatic Gregorians.

Amateurs hardly ever use the same telescopes as professionals (aplanatic Cassegrains and Gregorians) while professionals hardly ever use the same telescopes as amateurs (refractors, SCTs and Newtonians).

 

"The Ritchey–Chrétien design is free of third-order coma and spherical aberration, although it does suffer from fifth-order coma, severe large-angle astigmatism, and comparatively severe field curvature. When focused midway between the sagittal and tangential focusing planes, stars are imaged as circles, making the RCT well suited for wide field and photographic observations"

 

Ritchey–Chrétien telescope
From Wikipedia, the free encyclopedia

 

Nothing is ever quite as perfect as one would hope.

 

Jon Isaacs


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#15 Bomber Bob

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Posted 20 October 2014 - 09:01 AM

there was a guy around here trying to sell a 12 inch Classical Cassegrain that needed some real attention

 

Yes... and my post-retirement Dream Scope is a 10" - 12" CC on a permanent pier vintage GEM in my Observatory...  I see Cave & StarLiner CCs come up for sale often enough to give me hope that when the timing is right, I'll be able to get one.  Let's see how this Tinsley goes -- that may change my tune!

 

My 12.5" f/30 Classical was a really nice instrument to use and fiddle with.

 

f/30?  9500mm FL?  Mercy!  380x with an OR25...  boggles my mind.


Edited by Bomber Bob, 20 October 2014 - 09:06 AM.

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#16 tim53

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Posted 20 October 2014 - 10:10 AM

As Jeff said, long focus cassegrains are really hard to beat when it comes to planetary observing and imaging.  Imaging, in particular, perhaps.  Think of a Cassegrain or Gregorian as comparable to a Newtonian with a barlow, only with just two reflective surfaces.  The advantage here for planetary imagers is that you can take infrared and UV images, whereas systems with lens components in the way will cut out the IR and much of the UV, depending on the glass used.

 

With my 12.5" f/23 Classical (Ed Beck mirrors), I don't need a barlow to get a decent image scale on a ccd camera with small to average-size pixels.

 

My Cass on a Tak EM-500 (you need a big mount for big scopes, but hey, you WANT one anyway, so...) next to my 8" f/7 Cave lightweight deluxe for scale:

post-6788-14073021096735_thumb.jpg

 

-Tim.


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#17 Ed Wiley

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Posted 20 October 2014 - 12:36 PM

Some very thoughtful comments! Another thought: Much depends on what you want from the scope. Certain kinds of observing are better with high f-ratio scopes where narrow fields are acceptable. DKs give this when the major interests are double stars, planets, Luna, (visual and lucky imaging), and small DSOs (visually). For these programs one can purchase a much larger DK for the same money as a smaller APO and one that is much more compact than a F15 Acro. Are they "overall" scopes? Certainly not. That is why I seem to collect scopes. Its almost like photography: how many lenses do you have? I probably have too many but don't seem to have enough!  :)

 

Ed


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#18 Jeff B1

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Posted 20 October 2014 - 12:55 PM

there was a guy around here trying to sell a 12 inch Classical Cassegrain that needed some real attention

 

Yes... and my post-retirement Dream Scope is a 10" - 12" CC on a permanent pier vintage GEM in my Observatory...  I see Cave & StarLiner CCs come up for sale often enough to give me hope that when the timing is right, I'll be able to get one.  Let's see how this Tinsley goes -- that may change my tune!

 

My 12.5" f/30 Classical was a really nice instrument to use and fiddle with.

 

f/30?  9500mm FL?  Mercy!  380x with an OR25...  boggles my mind.

 

Funny story:  once when doing a 'show and tell' for a local TV station about some comet passing by the reporter, in his zeal for a photo-op, wanted a photo of me attaching my camera to that f/30 Cass!  :lol:   Well, he would not listen so I did and had this Cheshire cat smile on my face and the appropriate middle finger exposed when he snapped the camera.  I told him astronomers would laugh at it, but he didn’t care. :crazy:    Oh well, he had no clue.



#19 Jeff B1

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Posted 20 October 2014 - 12:58 PM

BTW, used many a telescope at 45,720mm efl.  A Clave 55mm eyepiece makes for a nice 830x for Mars.


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#20 Jeff B1

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Posted 21 October 2014 - 12:15 PM

Tim, it may have been comet Austin or the like back in 1980 or 82 that Don and I observed it passing by stars at 3,000x.  Yeah, it never occurred to us that was impossible :)  Actually, I used the wrong Barlow and blew up the image much higher than I thought.


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#21 vahe

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Posted 21 October 2014 - 12:55 PM

My 12.5" f/30 Classical was a really nice instrument to use and fiddle with.  Not sure how many of them I designed but they are fun telescopes to plan and make.  The optics were superb and if I had not gone of the deep end my 16" mirror would have been a 16" f/50 Classical Cass. Just think about eye relief!  The depth of focus on the 12.5 was great and at f/50 the 16" would be really good.

 

There a was a discussion on TEC user group on the practical limitation of building long focus Maksutovs, TEC in the past offered 8” and 10” F/20 Maks, in response to the question of why not F/30 or even F/40, Yuri stated that for a high quality system optical testing becomes extremely difficult at these high F ratios making the F/20 the practical limit from manufacturing standpoint, not to mention extremely narrow and long baffles as well as tube assemblies with little benefit to the final image.
Maks and Cassegrains are two different animals but this problem applies to both.

 

Vahe



#22 Jeff B1

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Posted 21 October 2014 - 03:02 PM

Back when number crunching and theory was of interest to me that would have given me pause for thought; however, when the local glass pusher did my f/30 optics we tested it with his equipment and knowledge and yes, it was tedious.  After getting the system adjusted and all that I observed with the scope for some years (forget how long now) and found that everything about it from calculating the parameters, baffles, assembly, collimating, all of it was easier that the slower version.  Image contrast, sharpness and limits of magnification increased; everything the "experts" said would come about -- maybe it was magic but they were wrong. 

 

While the Lowell scopes were designed for photography at f/75, and not so much visual,  the times I used them with the eyepiece the images were superb.  When seeing was good the 24" Clark at Lowell at f/75 was very good and we even put in a 3x Barlow with the 55mm Clave with nice results.  The 24" f/75 Cassegrains at the other sites used the 55mm Clave at 830x and at times the images were outstanding.  Clyde Tombaugh, an ATM curmudgeon of ancient times,  was one of the foremost knowledgeable optics guys I ever met and he told me and others many times that people should try things before publishing stuff that they had little or no clue of.  He said that a reflector at f/100 would not be too far out in left field.     :confused:   Don't ask me; he said it.

 

While my glass pushing ended early in life and testing mirrors was fair game some of us stumbled through it and somehow made mirrors that when used at star parties had the longest lines.  Don Parker still has a 6" f/8 that is some well figured that not even the Chicago gang (Joyce, et al) can find fault with it :)  Everyone at the WSP marveled at viewing with that telescope; it is a peach. I helped him test it between trips around his barrel and I still do not understand how it is done.   He has the touch.

 

If I ramble on just humor me :)


Edited by Jeff B1, 21 October 2014 - 03:03 PM.

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#23 pogobbler

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Posted 21 October 2014 - 03:14 PM

I don't have any experience with a classical Cass, but I'll admit when I think about my winning-the-lottery scopes, the 20" classical Cass that D&G offers often pops into my head-- among a dozen or so other scopes, too, of course.


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#24 tim53

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Posted 21 October 2014 - 04:41 PM

 Don Parker still has a 6" f/8 that is some well figured that not even the Chicago gang (Joyce, et al) can find fault with it :)  Everyone at the WSP marveled at viewing with that telescope; it is a peach. I helped him test it between trips around his barrel and I still do not understand how it is done.   He has the touch.

 

If I ramble on just humor me :)

That was the scope I used in 2000 when I went watching for Mars flashes with you folks in the Keys!  One of the best figured 6" scopes I've ever used.

 

-Tim.



#25 Jeff B1

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Posted 21 October 2014 - 06:44 PM

 

 Don Parker still has a 6" f/8 that is some well figured that not even the Chicago gang (Joyce, et al) can find fault with it :)  Everyone at the WSP marveled at viewing with that telescope; it is a peach. I helped him test it between trips around his barrel and I still do not understand how it is done.   He has the touch.

 

If I ramble on just humor me :)

That was the scope I used in 2000 when I went watching for Mars flashes with you folks in the Keys!  One of the best figured 6" scopes I've ever used.

 

-Tim.

 

He had a 10" Cass once and it was a good one except the primary baffle was not quite right.  Can't remember who figured the mirrors, but they were very well done.  My 12.5-inch had two other sister primaries that our glass pusher made and showed us his testing, whenever method he used I forget, and they seemed to be very close to the same -- if memory serves.  When my primary was ruined I tried to buy one from the owners, but no way would they sell them.  Just loved that telescope and wished it was here now.

 

When we changed the back focus, used a 1.97" secondary blank for f/30 the guy took my primary around the barrel a few times to take our a slight zone and man, that did the trick. Images were as sharp and contrasty as any scope I have used, except for my current 16".  Then he made my 12.5" f/7 that has a tiny zone, but that too is an excellent telescope -- still in use when the spirit moves me.

 

A Classical or DK Cassegrain is IMHO the perfect compromise in telescopes.  Even some DSO stuff is really good in those things.     


Edited by Jeff B1, 22 October 2014 - 07:08 AM.

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