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12" F:5/20 classical Cassegrain refit.

Classic Cassegrain
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#1 Astognosis

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Posted 23 February 2021 - 05:12 AM

Hello,

Returning after a long absence.

 

A few years ago I managed to obtain a 12" F5/20 classical Cassegrain/Newtonian OTA and mounting. I wish to use the Cassegrain configuration mainly for lunar and planetary imaging.
Unfortunately the previous owner left the telescope in the Newtonian configuration and hence unable to determine the Cassegrain  inter mirror spacing. I'm looking through the various formula and software to try and determine the best mirror separation.  I hope to test out the set up in it's original OTA and if results are satisfactory I would like then to make a new carbon fiber truss OTA for it.

 

If I set out some of the measurements I'm hoping someone would have the skill and knowledge to determine the best spacing's for planetary imaging.

 

Aperture.   12"
Primary focal length,  60 "
Aperture secondary mirror 3"
Primary focal length 60"
Secondary amplification x 4

Effective focal length 240"

Secondary mirror radius of curvature 20".    Secondary freshly aluminized and clear flat  back so reversed mirror and measured CR approximately to be 19.5", but with refraction through the thickness of the class  I reckon 20" would be correct and will specify that.

Primary mirror thickness.  1.5"
Primary centre hole diameter 2.25"

Existing Primary mirror cell gives a distance of mirror thickness + cell  = 1.5" + 2" = 3.5" to which the length of focuser needs adding which would be a moonlite with flange, i.e. 4.5".

So total is 8"  Some allowance range for focusing  in prime focus and Barlow/Powermate projections.

 

These are the measurements I'm working with.  I don't think I will need a big field of view since I'm mainly imaging and planetary cameras have a small chip size so I was thinking min 8.5min/arc to 17 min/arc.

I'm new to this, but I'd like to have a good go at the project if anybody can come up with the required measurements I'd be extremely grateful and full acknowledgment given.

Best wishes stay safe,

Eric  (UK)



#2 rob1986

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Posted 23 February 2021 - 05:27 AM

true casses use hyperbolic secondaries. , hence the secondary has two focal points. the focal point of the primary needs to be set the same as the concave focal point in order to maximize correction by the hyperbolic secondary. otherwise you will induce SA into the system. spherical secondaries are jow SCTs raise qualityby simplifying figuring of the mirrors.

it might also be a dall kirkham. you need to figure out whether the mirrors are spherical, parabollic, eliptical, or hyperbollic. the differences are quite important for correcting the system.

Edited by rob1986, 23 February 2021 - 05:37 AM.


#3 TOMDEY

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Posted 23 February 2021 - 05:40 AM

With those nominals you list up there, it is easy to solve for the nominal back-focus relative to the PM. So build to that, with adjustable PM-SM spacing. Tune that to perfection by driving out the spherical aberration in the image of a star. [Even better would be an interferometer set to autocollimation... but twelve-inch AC flats (and interferometers) are uncommon in our community.] Doing it on a star is the way to go. Mounting the mirrors for convenient high-finesse adjustability is the key.    Tom



#4 rob1986

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Posted 23 February 2021 - 05:43 AM

With those nominals you list up there, it is easy to solve for the nominal back-focus relative to the PM. So build to that, with adjustable PM-SM spacing. Tune that to perfection by driving out the spherical aberration in the image of a star. [Even better would be an interferometer set to autocollimation... but twelve-inch AC flats (and interferometers) are uncommon in our community.] Doing it on a star is the way to go. Mounting the mirrors for convenient high-finesse adjustability is the key. Tom


Thought of saying this. if its a true cass or RC, use an off axis star. if its a dall kirkham then on axis, those designs have really bad coma, and compensate with extra long fl

#5 Astognosis

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Posted 23 February 2021 - 05:43 AM

Hi Vostok,

 

Do you mean the secondary has the concave front for which we need the radius of curvature. For which we would need a spherometer.  An optician told me yesterday  to measure the focused reflected light from the rear which would also give me the radius of curvature, but yes that would be convex emm Perhaps I misunderstood him.  Possibly there may be a relation ship between these to foci ?

 

Any suggestions.

Eric



#6 rob1986

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Posted 23 February 2021 - 05:47 AM

Hi Vostok,

Do you mean the secondary has the concave front for which we need the radius of curvature. For which we would need a spherometer. An optician told me yesterday to measure the focused reflected light from the rear which would also give me the radius of curvature, but yes that would be convex emm Perhaps I misunderstood him. Possibly there may be a relation ship between these to foci ?

Any suggestions.
Eric

Yes, if its a classical cass they need to be co-focal. if its a dk I have no idea.

#7 Astognosis

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Posted 23 February 2021 - 06:01 AM

Vostok,,

 

It is a classical Cass for sure; made by what was Astronomical Equipment (AE) at Luton, Bedfordshire , England. late 70's early 80's.

 

Are you saying in that case that measuring for the centre of radius from the back reflective surface is incorrect method and gives wrong value?

 

Eric



#8 rob1986

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Posted 23 February 2021 - 06:49 AM

Vostok,,

It is a classical Cass for sure; made by what was Astronomical Equipment (AE) at Luton, Bedfordshire , England. late 70's early 80's.

Are you saying in that case that measuring for the centre of radius from the back reflective surface is incorrect method and gives wrong value?

Eric

among other things. yes. first you need to find what kind of curvature.

#9 TOMDEY

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Posted 23 February 2021 - 07:54 AM

Vostok,,

 

It is a classical Cass for sure; made by what was Astronomical Equipment (AE) at Luton, Bedfordshire , England. late 70's early 80's.

 

Are you saying in that case that measuring for the centre of radius from the back reflective surface is incorrect method and gives wrong value?

 

Eric

There's no need to fiddle around with trying to measure radii etc. etc. That it was configured as a Newt with a matched Cass SM provided entirely confirms that it is a Classical Cass, not a Ritchey, not a Dahl nor any other in that continuum of spherically-corrected positive negative systems. To amplify my earlier reply --- given the amplification of the SM and the radii of both mirrors, is more than enough to solve for the nominal offset of the Cass focal plane behind the PM. Put it there and then tune that separation to finesse out any slight remaining spherical aberration to perfection. It's as simple as that. The tip-tilts of both mirrors are also needed to finesse in the field symmetry. It is a five degree of freedom alignment. (Dahls are 3 DOF).    Tom


Edited by TOMDEY, 23 February 2021 - 07:55 AM.

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#10 Garyth64

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Posted 23 February 2021 - 09:11 AM

Something doesn't seem right about the RC of the secondary.

 

In punching in your numbers into some formulas, the position of the secondary  = 

                                                                                                                               

assuming a 10" BFL,  =  f.l  +  BFL   =  60  +  10    =   14"

                                      Amp + 1         4  +  1

 

And that will be, p.   p'  = Amp x p  =  4 x 14  =  56".

 

The RC of the secondary, from those two parameters, =  2 x p x  Amp  =  2 x 14 x 4  =  37.33"

                                                                                         Amp -1              4 - 1

 

I then plugged those number into the online calculator, and it agrees.

 

Cass sys.jpg

 

So, I'm not so sure about your secondary having a RC of about 20". 

 

here's the online calculator:  http://www.astrophot...ain_e_main.html


Edited by Garyth64, 23 February 2021 - 09:21 AM.


#11 Jeff B1

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Posted 23 February 2021 - 12:13 PM

B = 46.0"  +0.6975" / -0.8525"


Edited by Jeff B1, 23 February 2021 - 02:58 PM.


#12 dogbiscuit

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Posted 23 February 2021 - 02:25 PM

Actual 30" ROC would be refracted through flat back of secondary to about 20".


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#13 DrewFamily

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Posted 23 February 2021 - 02:36 PM

Not to go off on a different tangent, but didn't  OP say that he had the original OTA? If so, there will be two sets of holes in the tube, one for the Newtonian spider and another for the Cass. If you have that, you have a good starting point. Load up the Cass secondary and point it at a star...Jay



#14 DrewFamily

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Posted 23 February 2021 - 02:39 PM

Oops, to clarify I meant to say "there often will be two sets of spider holes."



#15 PETER DREW

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Posted 23 February 2021 - 03:59 PM

The AE skeleton tubes, one of which I believe the OP's to be, had hexagonal formers that could be slid up and down the tubes to account for the spacing difference for Newtonian and Cassegrain systems. The former carried both the secondary holder and the focuser when used as a Newtonian..  I have a similar F5-F20 Dall-Kirkham set with unknown spacing, I just set them up in a tube and fitted the mirrors at a spacing that provided access to a focuser and diagonal.  Fine tuning can be done on a star.  Initial guesswork spacing to achieve this produced a pretty good image in the first trial.


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#16 Astognosis

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Posted 23 February 2021 - 06:39 PM

Thanks to the many respondents to which I shall reply later.  Couple of late night  so earlier to bed tonight.

 

Just a thought on the method I used to measure the RC from the back.  Would not then the RC of the front of the mirror be twice because from the front the RC is behind the mirror, i.e. 2x 19.5"  to  40"  ?  Which would also make the focal length of the  -19.5"  to -20" range?

 

I had a conversation to day with a mirror aluninizer who said;

 

As a very simple rule, if a sec. amplifies 4x it is abouta 1/4 of the primary focal length   If 5x amp. about 1/5  focal length  , etc. etc.It all depends on desired back focal length . for the system.I would have guessed it to have a focal length  of say 15 to 18 inches. 19.5 " sounds OK. Possibly calculate for 19". If you did calculations for say 18.5 " focal length and 20.5 " focal  length this would give you the most likely range of secondary to primary distances.

 

If primary is 60 inch focal length  you will probably find they are 48 to 50 inches separation. Half inch movement can change the final  focus by more than a few inches.

 

There were no other indications on the skeleton tube of  secondary mirror positions other than that for the Newtonian secondary.

 

Thank you all for help so far.

 

Eric



#17 Garyth64

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Posted 23 February 2021 - 07:30 PM

What are you saying?  You want to know the numbers for your systems if the secondary has a focal length of 18.5" to 20.5"?

 

Go ahead and plug your numbers into the calculator and see what you get.  It is really easy to use.  Everything is there for you to play with.

 

I like to figure out those numbers by hand on paper, but that's just me.  And they usually agree with the online calculators, and they use the same formulas.



#18 Garyth64

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Posted 23 February 2021 - 08:00 PM

"If primary is 60 inch focal length  you will probably find they are 48 to 50 inches separation. Half inch movement can change the final  focus by more than a few inches."

 

smile.gif   In designing a Classical Cass, with the parameter you originally gave, I would bet the design if very close to what I said, minus slight deviations in the RC of the secondary and it's placement.  I would say that the original placement of the secondary is very close to being at 14".  If it was my scope, and I wanted to assemble it, that is where I would place the secondary.

 

Think about it, if the secondary is placed at 12" , and you want the image to be formed 10" behind the mirror or 58" away from the secondary.  The amp of the system is 4.8x and the system becomes a f/24.  The RC of the secondary would be 30".

 

If you want the secondary placed at 10", and the image formed behind the mirror 60" away, then the amp becomes 6x.  The system becomes an f/30, and the RC of the secondary would have to be 24".

 

Slight changes in the secondary's RC, and it's position, mean everything to the overall system.



#19 dogbiscuit

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Posted 23 February 2021 - 10:26 PM

That was a rough calculation of the 30 inch ROC if you found the refracted null near 19.5 or 20". 

I suspect your actual secondary ROC will be very near the 37.3333" shown in post 10 design.  The refracted Foucault null would be ~24" from flat back surface for that... but why bother with that.  Best assemble the scope, space the secondary 46" from the primary, find the focus, adjust space + or - for best image quality. If the numbers you have for primary focus and Cass focus are reasonably correct that will start you reasonably close, probably within 3/4" one way or the other depending on BFD.



#20 macdonjh

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Posted 23 February 2021 - 11:02 PM

There's no need to fiddle around with trying to measure radii etc. etc. That it was configured as a Newt with a matched Cass SM provided entirely confirms that it is a Classical Cass, not a Ritchey, not a Dahl nor any other in that continuum of spherically-corrected positive negative systems. To amplify my earlier reply --- given the amplification of the SM and the radii of both mirrors, is more than enough to solve for the nominal offset of the Cass focal plane behind the PM. Put it there and then tune that separation to finesse out any slight remaining spherical aberration to perfection. It's as simple as that. The tip-tilts of both mirrors are also needed to finesse in the field symmetry. It is a five degree of freedom alignment. (Dahls are 3 DOF).    Tom

+1

 

If the scope has a Newtonian configuration which works well, the only Cassegrain configuration which can work is the classical Cassegrain.  It's the only variant which uses a paraboloidal primary mirror, like the Newtonian.

 

It would be really impractical to have two sets of spider mounting holes.  It would be way too much effort to actually change spiders.  Also, remember, centering the secondary mirror is very important in classical Cassegrain systems, so swapping spiders would introduce a lot of variables.  It seems much more likely the spider stays put, and only the secondary mirror holders with their mirrors get changed.  That's how my Parks 10" HIT works, that's how the Takahashi CN-212 works.

 

I had a heck of a time with the initial assembly of my classical Cassegrain, and I had the prescription of the mirrors and recommended spacing.  It didn't form an image, though.  I ended up doing exactly what TOMDEY recommends: made myself a jig so I could vary the mirror separation and then slid the mirrors closer together and farther apart until an image formed with a practical back focus distance.  Then I build my OTA and used a spider/ secondary holder with a mounting bolt so I could continue to make small changes to the separation distance.  After lots of trial and error I finally got it.  Wonderful scope I enjoy using a lot.

 

Astognosis, I hope you get your scope working.  These "convertable" scopes are lots of fun.  Especially during the summer months when there is a large selection of both large objects and small objects so you'll make use of the choice between f/5 and f/20 several times during the same evening.


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#21 mconnelley

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Posted 24 February 2021 - 03:54 AM

Hello:

  I also have a 'mystery' cassegrain secondary mirror.  Here is how I measured it's radius of curvature:

 

http://amrita.olabs....6&sim=243&cnt=2

 

  Here's a short version of how I did it.  I used my iPhone and a magnifying glass.  

1) Set up the phone standing up, and turn on the flash light

2) Set up the magnifying glass a bit farther from the phone than the focal length.

3) Put a screen (a white box or book works great) at the image of the iPhone's light made by the lens

4) Put the mirror into the light path after the lens, so that the mirror reflects the light back through the lens.  Adjust the mirror

    until it makes an image of the iPhone's light just next to the light itself.  

5) Measure the distance from the mirror to the screen.  That's the radius of curvature.  

 

    Once you have an accurate measurement of the ROC of the mirror, then figuring out the right spacing is easy.  

 

Cheers

Mike



#22 Astognosis

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Posted 24 February 2021 - 04:06 AM

Thanks to all for your advice and suggestions.  It would seem that a 48" separation as a trial start is a good beginning.

When I first approached the problem I had a similar opinion to Macdon. If classical Cassegrains are a bit of a "pain" to set right then the manufacturers would try and keep things simple particularly if Cass/Newt were to be interchanged relatively frequently.  I took some rough measurements at the time and from memory there is some similarity in what suggestions are being put forth.  I shall double check this with more accuracy.

 

The existing tube is essentially a test platform and I think I can find a distant horizon to try out initial test mirror  positioning.  Yes stars would be good to, but I have been able to observe only twice since November 22, 2020.  Been particularly bad this winter.

 

OK I've got something to get on with. I shall report back as I go along.

 

Great attitude guys. Well done.

 

Eric 



#23 Astognosis

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Posted 24 February 2021 - 04:19 AM

Viking 1,

 

neat as I've got the mirror out I'll try that later at night in a dark room-for the sake of completeness.

 

Out of interest what project are you involved with using Cass mirror configuration.

 

Eric



#24 Garyth64

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Posted 24 February 2021 - 08:49 AM

If you want to take a look, I converted the the OTA of my Gregorian to hold an optics set for a Cass.  I made it to be used either as a newt or a Cass.  The secondary vanes would be in one place, and the secondary housing for each secondary was made to make them interchangeable.

 

https://www.cloudyni...n/?hl=gregorian




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