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SS 76mm Maksutov Cassegrain

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#1 Arjan

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Posted 02 October 2019 - 01:41 PM

Has anybody ever tried the Surplus Shed optics set for a 76mm MCT, that is currently on pre-order?

 

 

I have a hard time modelling the system based on the data on the SS web page:

- F primary = 138mm

- F corrector = -4000mm

Which should lead to a 1520mm (F/20) system.

 

I doubt that the -4000mm of the meniscus is correct...



#2 MKV

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Posted 02 October 2019 - 03:05 PM

Not enough information. It would help to know the meniscus thickness and suggested separation . The spot looks like it's about 25 mm, which would place the corrector at about 115 mm from the primary. Given that the corrector  is 78 mm in diameter,  I suspect the meniscus is about 8 mm thick. It may be an interesting and inexpensive project to play with, but I wouldn't expect too much out of it. Personally, I would be more interested in testing the optics. 

 

Mladen



#3 DAVIDG

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Posted 02 October 2019 - 03:46 PM

 I have one of these sets and benched tested  it via double pass.  I bought mine back in the Spring so they many not be what Surplus Shed is selling now but it sounds like  the same . My set is very well corrected. I machined a PVC tube to mount the optics but haven't had change to finish it.  It is very sensitive to the spacing, a small change  makes a large change in the back focal length which is known with cassegrain optics.  Here is  an OSLO file for my set. The radii are not exact on the meniscus. I measured and tested the primary which in my set is  a clean sphere via a Foucault test but my spherometer is set up for larger diameters  then the meniscus so I wasn't able to precisely measure them.  

   A good friend also bought a set  and made wooden cells with All-thread spacer to adjust the spacing and also reported good results but added that alignment is critical, hence the reason why I went with a machined tube to hold the meniscus and will machine a mirror cell.

   Attached is the OSLO file, again it is not exactly what I have  but close.

 

                     - Dave 

      Attached File  SShed_Mak_Cass.len   1.21KB   68 downloads   


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#4 Arjan

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Posted 02 October 2019 - 04:27 PM

Thanks Dave!
Mladen, I deduced similar figures, but calculating best R of the meniscus yielded a very steep value of about 50mm must have done something wrong...

#5 MKV

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Posted 02 October 2019 - 06:06 PM

Sounds like an ideal pet project for a 3D printer :o)



#6 MKV

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Posted 02 October 2019 - 06:25 PM

 I have one of these sets and benched tested  it via double pass.  I bought mine back in the Spring so they many not be what Surplus Shed is selling now but it sounds like  the same . My set is very well corrected. I machined a PVC tube to mount the optics but haven't had change to finish it.  It is very sensitive to the spacing, a small change  makes a large change in the back focal length which is known with cassegrain optics.  Here is  an OSLO file for my set. The radii are not exact on the meniscus. I measured and tested the primary which in my set is  a clean sphere via a Foucault test but my spherometer is set up for larger diameters  then the meniscus so I wasn't able to precisely measure them.  

   A good friend also bought a set  and made wooden cells with All-thread spacer to adjust the spacing and also reported good results but added that alignment is critical, hence the reason why I went with a machined tube to hold the meniscus and will machine a mirror cell. 

Thanks Dave.

 

The configuration you linked is an f/11 instead of f/20, and your clear aperture is 72 vs 76 mm.

 

Mladen



#7 DAVIDG

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Posted 02 October 2019 - 07:19 PM

Thanks Dave.

 

The configuration you linked is an f/11 instead of f/20, and your clear aperture is 72 vs 76 mm.

 

Mladen

 Yes, both my meniscus and my friend's  has a clear aperture of 72mm. I just measured mine again  and the CA is 72mm and the physical OD is 76mm.  We both tried to get the design to work with the radius we measured. We both got similar results but like I said I couldn't use my spherometer to get exact values but with what we measured we found   F/20  wouldn't work. 

 There are a  number of small Maks spotting scopes that look to possibly be were these optics are being used and they are in the F/10 to f/15 range but we couldn't find a F/20 example. 

   As I said before we got these sets back in the Spring while SS stated they were F/20 as well, ours might be different then what is being offered now. 

   Here is a picture of my friend Matt Considine's  setup to test the optics.

 

              - Dave 

 

mattSS76mmMak.jpg


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#8 MKV

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Posted 02 October 2019 - 11:35 PM

 Yes, both my meniscus and my friend's  has a clear aperture of 72mm. I just measured mine again  and the CA is 72mm and the physical OD is 76mm.  We both tried to get the design to work with the radius we measured. We both got similar results but like I said I couldn't use my spherometer to get exact values but with what we measured we found   F/20  wouldn't work.

Hi Dave,

 

A while ago, I made a number of spherometers just for that reason alone -- knowing that sooner or later I will run into a situation where the one I have will be just a tad too big, and in retrospect I never regretted the extra effort. Here is a small sample. In addition to what's seen here I have one that's 9.25" and 11.5 inches in diameter and a couple of 1.500 and 0.7500 inch tube spheormeters that I turned on my lathe made of brass. I fell in love with the 2-point spherometers made of hardwood. You can make those literally in less than 15 minutes with a band saw and a drill press. And if you have access S&T famous papers on Maks, you'll find that they used 2-point spherometers for their Maks! 

 

Cheers!

Mladen

 

spherometers_LR.jpg


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#9 DAVIDG

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Posted 03 October 2019 - 08:56 AM

 Mladen,

    Having the right tools for the job, is very helpful. As for the Surplus Shed Mak, since both Matt and myself tested our sets via Double Pass and found them to be very well corrected, knowing the exact design was secondary.  The challenge for anyone who purchases a set will be to mount them in a way to maintain the critical alignment that is required. 

 

          - Dave 


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#10 Arjan

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Posted 03 October 2019 - 12:18 PM

Sounds like an ideal pet project for a 3D printer :o)

Well, I happen to have half a meter of 80mm aluminum tube...
But yes, you obviously need more than that!

#11 Arjan

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Posted 03 October 2019 - 12:22 PM

Dave, what exactly is critical in the alignment other than separation? Is it corrector tilt that needs adjustment too?

#12 DAVIDG

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Posted 03 October 2019 - 02:05 PM

Dave, what exactly is critical in the alignment other than separation? Is it corrector tilt that needs adjustment too?

 Since the primary is very fast, yes the tilt of both the primary and the meniscus is critical or your going to get oval shaped stars.  From what I saw when doing double pass the separation was critical to fully illuminate the secondary spot but like I said before if you changed the separation by just a few mm, the back focus would change by a large amount like 20mm or more.  No surprise since this is known with cassegrain type systems. 

  On my set the primary and the meniscus have different diameters  The meniscus is larger. So I machined out a piece of PVC pipe to fit the meniscus and plan to make a mirror cell that I can adjust the tilt of the primary. Also you need to be able to get the elements optically centered.

   Hopefully this Winter I'll finish mine. 

 

                - Dave  


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#13 Arjan

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Posted 03 October 2019 - 02:43 PM

Just got the reply that the set currently on sale is the same as they sold before. So it indeed seems to be a nice project to do!



#14 MKV

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Posted 03 October 2019 - 03:59 PM

Dave, what exactly is critical in the alignment other than separation? Is it corrector tilt that needs adjustment too?

What Dave said is of course correct, but for a specific design SYNOPSYS can do professional tolerance analysis. 

 

_________

PS

 

In S&T Bulletin C, John Gregory listed tolerances for his 6" Mak-Cass configurations. However, he doesn't  specify tip/tilt, separation, glass homogeneity, or centration tolerances.

 

Greg_Mak-Cass_Tol.jpg


Edited by MKV, 03 October 2019 - 04:36 PM.


#15 ed_turco

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Posted 03 October 2019 - 04:35 PM

I think there is some confusion about the lens.  Wile it has a fl of -4000mm. the focal length doesn't have much with the focal length of the total system.  Rather, it is the aluminized spot on the lens that makes the system like a Cassegrain but with better correction overall.

 

Hope this helps.

 

 

ed



#16 MKV

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Posted 03 October 2019 - 04:57 PM

Assuming the lens central thickness tc = 0.1D, where D is the primary mirror's diameter, and glass substrate is N-BK7 one can calculate the corrector R1 and R2 for an f/20 configuration.based on the   equations published by D. D. Maksutov in JOSA (1944). 



#17 DAVIDG

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Posted 03 October 2019 - 05:15 PM

  What you do for a Gregory type Mak ( spot on the rear of the corrector) is use the standard Cassegrain formulas to calculate R2 on the back of the meniscus for a given primary ROC and the back focal length you want. That radius is the same as what it would be for a given F-ratio   and back focal length for the secondary in a typical cassegrain system since the rear surface of the meniscus is the secondary   Then you calculate R1 on the meniscus and the thickness of meniscus for a given glass type to correct for spherical for the system and also make the meniscus achromatic. 

 

                   - Dave 


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#18 BGRE

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Posted 04 October 2019 - 04:36 AM

Since the primary is very fast, yes the tilt of both the primary and the meniscus is critical or your going to get oval shaped stars.  From what I saw when doing double pass the separation was critical to fully illuminate the secondary spot but like I said before if you changed the separation by just a few mm, the back focus would change by a large amount like 20mm or more.  No surprise since this is known with cassegrain type systems. 
  On my set the primary and the meniscus have different diameters  The meniscus is larger. So I machined out a piece of PVC pipe to fit the meniscus and plan to make a mirror cell that I can adjust the tilt of the primary. Also you need to be able to get the elements optically centered.
   Hopefully this Winter I'll finish mine. 
 
                - Dave


Tilt about what point?
For spherical surfaces how do distinguish between tilt and decentration?

#19 dbird

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Posted 04 October 2019 - 05:04 AM

I'm a newby here.

 

I brought this topic up as a new thread, and was directed here. There was another response first though which might be worth reproducing here from BGRE as follows:

 

"Since the only user variable parameter is the spacing between the corrector and the primary, optimising the performance of the optics as supplied should be fairly easy.
Just measure the residual aberrations and plot them against the separation to locate the minimum.
The cost is certainly low enough that its worth trying."

 

It seems to me that getting the alignment right should be fairly easy with simple machining assuming that the optics are exactly centered/aligned with the dimensions of the parts. So, my question is: Is there an easy way to measure the aberrations maybe short of a star test? 



#20 BGRE

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Posted 04 October 2019 - 05:16 AM

For initial testing an autocollimation setup using a Ronchi test should facilitate coarse adjustment.
A Foucault test setup in autocollimation should allow measurement of axial SA.
Alternatively a Roddier test (essentially a quantitative star test) using an autocollimation setup could be done to measure axial SA etc.
A PDI (point diffraction interferometer) could also be used with an autocollimation setup.
A Hartmann test of an autocollimation setup is another option.

#21 dbird

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Posted 04 October 2019 - 05:38 AM

It's been a long time since I've looked at autocollimation setups. Are there any that don't require a large precision flat?



#22 luxo II

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Posted 04 October 2019 - 06:29 AM

Vertically facing down over a pan of oil should work and be relatively easy to do with this small scope. Dye the oil black.

Edited by luxo II, 04 October 2019 - 06:30 AM.


#23 BGRE

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Posted 04 October 2019 - 08:04 AM

The oil flat should be at least 3" larger in diameter than the telescope aperture to minimise curvature due to capillarity effects at the edge of the oil flat.

#24 Chuck Hards

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Posted 04 October 2019 - 08:33 AM

I didn't see any mention on the SS website, is the corrector AR coated?  Can't tell from the pic.



#25 DAVIDG

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Posted 04 October 2019 - 08:34 AM

It's been a long time since I've looked at autocollimation setups. Are there any that don't require a large precision flat?

 The flat just needs to be optically smooth. The math will show that a few waves of power adds something like 1/50 wave of error to the test. So a smooth 1/2 wave flat is more than good enough especially in this case were you using it to align the optics. 

 

                     - Dave 


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