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12.5 inch classical cass from Adler Planetarium

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#101 Jeff B1

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Posted 08 November 2018 - 06:48 AM

I know that Celestron made a 22" SCT.  I have always wanted one and look through one.  I had a lead on one and was tempted.

Some group near Claremont, CA or maybe in L.A., (1987)  had a 22" Celestron and it was not very good.  Saturn looked like one of Galileo's drawings.  



#102 CHASLX200

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Posted 08 November 2018 - 07:06 AM

Some group near Claremont, CA or maybe in L.A., (1987)  had a 22" Celestron and it was not very good.  Saturn looked like one of Galileo's drawings.  

Most of the C scopes were not very good back then. Saw a C16 that was soft and any Cass i have ever looked thru was not good at all.  I am sure there are knock your socks off sharp Cass's out there but i have yet to see one.



#103 Jeff B1

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Posted 08 November 2018 - 07:20 AM

Most of the C scopes were not very good back then. Saw a C16 that was soft and any Cass i have ever looked thru was not good at all.  I am sure there are knock your socks off sharp Cass's out there but i have yet to see one.

It may have been all the people standing around under and in front of the 22”, plus it was on a concrete pad and the seeing generally sucks out there anyway.  Someone had a 36” three mirror Cassegrain there too.  While I was observing something low in the sky some creep stuck his head in the front of the scope and that produced some thrilling and blurry images.  A gathering like that is no place to test telescopes. confused1.gif



#104 starman876

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Posted 08 November 2018 - 07:24 AM

It may have been all the people standing around under and in front of the 22”, plus it was on a concrete pad and the seeing generally sucks out there anyway.  Someone had a 36” three mirror Cassegrain there too.  While I was observing something low in the sky some creep stuck his head in the front of the scope and that produced some thrilling and blurry images.  A gathering like that is no place to test telescopes. confused1.gif

Thermals from the ground and just from body heat can do wonders for viewing conditions.


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#105 Jeff B1

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Posted 08 November 2018 - 08:45 AM

Thermals from the ground and just from body heat can do wonders for viewing conditions.

Yeah, when the guy stuck his head up inside the trusses, next to the secondary, I thought a nuke went off on Saturn. The images swelled up like a pumpkin hit with a 12 gauge. lol.gif


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#106 DAVIDG

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Posted 08 November 2018 - 10:02 AM

 I've worked a on number of cassegrains through the years, the largest being a 24" were I refigued the primnary the problem is  that you have to get the  spacing between the primary and secondary set correctly,  like no more then 1/8" from the ideal position. The commercial scope made in the 50's thru the 70's just don't address this issue. The other problem I keep seeing is if the optics are of classic type ie. a parabolic primary and hyperbolic secondary, the maker of the optics did not have the proper equipment to correctly figure the secondary to the needed accuracy. The result is that you find Cass. wasting away in basements, back of garages or college and university observatories were they are never used. A scope with good optics will get used,  ones that give poor images just fade away. There have been a number of examples that have shown  up in this forum and up in the ATM section. 

      To make a convex hyberbolic secondary you need a Hindle sphere. Once the secondary and primary are figured you then need to assemble the OTA and test the complete system and then touch it up. 

  My friend Dick Parker who in my opinion is one of very best ATM's around has made a number of cassegrains and he does it right. This is how it should be done http://mirrorworksho...grainStory.html Note that both the primary and secondary are made using null testing techniques. The primary using double pass and the secondary using a Hindle sphere, then the Dick tests the complete system and touches it up using double pass. 

   I hope Darren's cass turns out to the exception to what I have seen and I'll  be happy to help so it has the best image possible.

 

                          - Dave 


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#107 Jeff B1

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Posted 08 November 2018 - 10:42 AM

On the second round of my 12.5” f/30 Cass the optician used a secondary tool that was out of whack, so figuring it a slight zone near the edge developed and made it way too long to the focal point. I had to get on the floor 3 feet behind the scope to finally see a focused image.  Oops he said.  Yeah, measuring the separation takes patience.  In my lust to get it right I intentionally went too far back and forward to see the effects.  It is quite obvious when it is set out of tolerance; plus neck stretching to boot.  While I was not all that interested in the testing phase of the optics, that tweaking taught me a lesson.


Edited by Jeff B1, 08 November 2018 - 10:43 AM.

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#108 Darren Drake

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Posted 08 November 2018 - 11:13 AM

Ha ha...

Attached Thumbnails

  • IMG_20181108_30923.jpg

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#109 Jeff B1

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Posted 08 November 2018 - 11:55 AM

lol.gif   That's it Darren!

 

Here is a diagram of my 12.5" f/30:

 

12_f30_3.jpg

 

Adjustable baffle:

 

MyCass7.jpg


Edited by Jeff B1, 08 November 2018 - 11:58 AM.

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#110 clamchip

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Posted 08 November 2018 - 11:59 AM

Does this help, it looks like its based on J. Texereau's work:

http://www.astrophot...ain_e_main.html

 

Robert


Edited by clamchip, 08 November 2018 - 12:00 PM.


#111 DAVIDG

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Posted 08 November 2018 - 12:23 PM

 What I have seen happens is that ATM's that get  good at making Newtonian primaries decide to make a Cassegrain. It sounds like it isn't too much more complex then making a Newtonian primary. What gets lost is that you now have a two element system in which both surfaces have power.  So you have to get the radii, spacing and figures too a  much tighter tolerances than a Newtonian.  Note that on  Dick Parkers write up he made a test plate for the secondary and used that to first polish the secondary so it matched in radius using intereference  to get the radius dead on the money. This was done before he figured it. You need to pay attention to this level of detail when you make multi-element systems when they have optical power. This is another reason why many Cassegrains just don't perform, they just aren't made to the needed  accuracy. 

  This is why I say when you find one of these, covered in dust in the  basement of some college or a  garage or in storage unit  and dreams of this amazing image come to mind, you need to take a step back into reality and get it up on the test stand and see what is really going on. Like I have said scopes with good optics get used, one with issues live in basements.

 

                      - Dave  


Edited by DAVIDG, 09 November 2018 - 10:15 AM.

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#112 Jeff B1

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Posted 08 November 2018 - 02:11 PM

Yeah, I watched Fagin figure my optics and it was a long drawn out process.  My patience is more adapted to mechanical design and not optical testing, so watching him was an education.  He recognized the slight zone he left in my primary then polished that out along with figuring the secondary as he explained the two mirrors were the whole system.  It takes a lot of time.  After he finished I really paid attention to al the mechanical details, tweaking everything up until it was perfect. 

 

Before going from f/16.5 to f/30 and tweaking everything up the telescope performed very well; but was missing the sharpness and image contrast that Parker’s Newtonian gave us.  However, after that my scope really worked great and was every bit as good as his scope; plus I could use larger eyepieces without a Barlow with great eye relief and still observe Mars up to 1,000x. Our seeing was usually that good.

 

Wished I would have Danny J. make my 16” into Cass; I would still be swatting mosquitoes, chasing off critters and enjoying sitting under the scope instead of hanging off a ladder.  Besides the challenge and fun of making that scope would have been well worth it. An f/50 is a stretch, but for planetary observing that works out well.  We used 24” scopes at f/75 in the Lowell’s planetary patrol and images of Mars or Jupiter at prime focus are really good.  A 24” Cass with 1.6” secondary!


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#113 starman876

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Posted 08 November 2018 - 02:26 PM

Ha ha...

been there done that when assembling refractorslol.gif



#114 CHASLX200

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Posted 08 November 2018 - 07:20 PM

Just never seen a good Cass yet. Maybe one year i will.



#115 clamchip

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Posted 08 November 2018 - 08:34 PM

If the Classical Cassegrain primary was made f/4.5 or slower than the scope could be 

used as a Newtonian. I have owned many f/4.5 newts and have no complaints of this 

fast speed.

I would like it possible to replace the Hyperbolic secondary with a diagonal flat in a

quick-change cassette style design.

I'm sure this has been done commercially, Tinsley comes to mind, or Parks? but I think

these may still have  f/4 primaries.

 

Robert


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#116 starman876

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Posted 08 November 2018 - 09:31 PM

If the Classical Cassegrain primary was made f/4.5 or slower than the scope could be 

used as a Newtonian. I have owned many f/4.5 newts and have no complaints of this 

fast speed.

I would like it possible to replace the Hyperbolic secondary with a diagonal flat in a

quick-change cassette style design.

I'm sure this has been done commercially, Tinsley comes to mind, or Parks? but I think

these may still have  f/4 primaries.

 

Robert

there have a reasonable number of commercial optics with the quick change kit.   I had a 12" cassegrain that was awesome.  detail on the planets was mind blowing. even detail on Jupiters moons.  However it was 4 feet long and just to much of a beast to take in and out of the house each time I wanted to use.  I sold the optics.  


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#117 Jeff B1

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Posted 09 November 2018 - 01:16 PM

lol.gif   That's it Darren!

 

Here is a diagram of my 12.5" f/30:

 

attachicon.gif 12_f30_3.jpg

 

Adjustable baffle:

 

attachicon.gif MyCass7.jpg

Well;  Except for the 5” aluminum secondary holder obstruction on the 24-inch f/4 – f/13.3 - f/75 Classical Cassegrain made by Boller and Chivens, the optics appeared to be first rate.  Observing with four of these telescopes my impression was the optics were figured very well; however, they were convertible where the upper housings had two secondary configured for f/13.3 and f/75.  When using one for photographing Mars it would have a Clave 55-mm Plossl eyepiece rendering 830x magnification or a plate scale of 4.5 arc-seconds per millimeter.   In 1990 when observing Mars was only 17 arcsec but in the eyepiece the globe filled much of the view with reasonability high contrast images.  Viewing a fairly large dust storm gave me distinct and well defined dust cloud details. 

 

 

While I will agree many of the Cassegrain scopes I’ve observed with are dogs, many are superb.  My 12.5” f/30 is a case in mind; where I used it for 5 or 6 years before the primary was ruined so I switched to a Newtonian. From the quality of Dan Joyce’s telescope mirrors that I have personally used, why I chose a long Newtonian for my 16” still bugs me.   My plan was a 16” f/50 Classical Cassegrain where the 9.4% obstructed 1.5” secondary would provide an effective focal length of 800” (20,320 mm) or plate scale of 10.2 arc-seconds per millimeter.  With my old eyepiece collection that would give me from 452x to 1,270x magnifications.  If torture was my forte a neat camera would produce some nice images.

 

 

16_f50_3.jpg



#118 steve t

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Posted Yesterday, 12:11 PM

Reading this thread got me wondering; If the spacing between the primary and secondary,  in a cas. system, is critical. Does the same applies to a SCT or other cat. optical systems that moves the primary mirror for focusing? 

Thanks

Steve 



#119 Jeff B1

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Posted Yesterday, 12:23 PM

Reading this thread got me wondering; If the spacing between the primary and secondary,  in a cas. system, is critical. Does the same applies to a SCT or other cat. optical systems that moves the primary mirror for focusing? 

Thanks

Steve 

It’s been too long ago for more details but a typical SCT, as I recall, uses an oversized primary mirror so moving it keeps the ration the same while changing the parameters in calculating separation.  It’s been a while, but I used to understand it. Will sleep on it smile.gif

 

Here are some CN links

https://www.cloudyni...ation-for-scts/

https://www.cloudyni...primary-on-sct/


Edited by Jeff B1, Yesterday, 12:30 PM.



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