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Recommended opticians for Convexe Secondary Mirrors.

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#1 StellarField20

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Posted 13 October 2020 - 10:51 AM

Hi all,

 

I'm new to the forums,

 

I'm presently looking to build my first Classical Cassgrain or an Ritchey Chretien.

I'm wondering, does anyone know of an optician that can source or provide the secondary convexe mirror?

I would do the primary myself.

 

Anyhelp would be appreciated.

 

Dearly,

 

Charles.H

 

 



#2 luxo II

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Posted 15 October 2020 - 04:52 AM

Zen in Venice can supply complete sets (primary and secondary).

Edited by luxo II, 15 October 2020 - 04:53 AM.


#3 PETER DREW

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Posted 15 October 2020 - 05:42 AM

I think most classical Cassegrain optics are figured as complete sets.  If you are making your own primary, a supplier of the secondary may need your primary to make a good match.


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#4 msheald

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Posted 17 October 2020 - 06:47 AM

Hello! When I thought about a Cassegrain, I thought that it  might be possible to purchase a convex lens that matched what I needed from a company, such as Melles Griot and then grind and polish by primary to match. With a star test, using both components but only tuning the main mirror, the system should perform. I have not tried this, but it is a thought that might avoid producing a convex mirror with those testing requirements. Best regards.

 

Mike


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#5 Benach

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Posted 17 October 2020 - 08:42 AM

Hi all,

 

I'm new to the forums,

 

I'm presently looking to build my first Classical Cassgrain or an Ritchey Chretien.

I'm wondering, does anyone know of an optician that can source or provide the secondary convexe mirror?

I would do the primary myself.

 

Anyhelp would be appreciated.

 

Dearly,

 

Charles.H

Wonders: it is not that difficult to make your own. Why haven't you tried it?



#6 PETER DREW

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Posted 17 October 2020 - 01:48 PM

I think, from an ATM perspective, you would have a better chance of success with the Dall-Kirkham design. Still a Cassegrain.
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#7 Benach

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Posted 17 October 2020 - 03:47 PM

There are various methods to make a nice secondary mirror but one of them is this:

1) make a concave mirror with the same curvature as your secondary, often the grinding tool.

2) Finish it with the same conical constant as the desired convex secondary mirror.

3) Test this with traditional methods such as Foucault and/or Bath interferometry

4) Finish the real convex mirror with a new, plaster, tool.

5) Test the convex mirror with Newton rings.

6) Assemble the entire telescope.

 

For more accuracy:

7) Take a Dobson telescope of at least the size of the primary mirror

8) Put a fiber in the primary focus of the Dobson

9) Illuminate the fiber with a small LED.

10) Align the telescopes so that you will see a Airy disc in the center of the FOV of the Cassegrain

11) Do a Foucault and/or Ronchi test in the Cassegrain. It should be a null thus no shadows on the wavefront must be visible and/or Ronchi bands should be straight.

12) If they aren't, check you secondary.



#8 Steve Dodds

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Posted 17 October 2020 - 04:18 PM

There are various methods to make a nice secondary mirror but one of them is this:

1) make a concave mirror with the same curvature as your secondary, often the grinding tool.

2) Finish it with the same conical constant as the desired convex secondary mirror.

3) Test this with traditional methods such as Foucault and/or Bath interferometry

4) Finish the real convex mirror with a new, plaster, tool.

5) Test the convex mirror with Newton rings.

6) Assemble the entire telescope.

 

For more accuracy:

7) Take a Dobson telescope of at least the size of the primary mirror

8) Put a fiber in the primary focus of the Dobson

9) Illuminate the fiber with a small LED.

10) Align the telescopes so that you will see a Airy disc in the center of the FOV of the Cassegrain

11) Do a Foucault and/or Ronchi test in the Cassegrain. It should be a null thus no shadows on the wavefront must be visible and/or Ronchi bands should be straight.

12) If they aren't, check you secondary.

That is the hard way.  All professional opticians who make Cassegrains use a hindle sphere to make the sec.  A hindle sphere is just a fast sphere with a hole in the center.  A C8 primary works well for this.

Once the Sec is made and coated they match the primary to the sec by autocollimation.  Amateurs without a full size flat can skip this, but without matching them any slight error in the sec is magnified.



#9 Benach

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Posted 17 October 2020 - 04:45 PM

Could be, but I find this quite straightforward.



#10 Steve Dodds

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Posted 17 October 2020 - 04:53 PM

Could be, but I find this quite straightforward.

It is straightforward, it's just 20 steps when you can do it with 1


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#11 Arjan

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Posted 18 October 2020 - 01:16 AM

I did it the hard way, i.e. with a test plate. You don't need to figure a conic into this, sphere suffices. Then use DFTFringe to analyze the Newton fringes when figuring the convex part.

#12 davidc135

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Posted 18 October 2020 - 03:14 AM

Charles, Can I ask; what is your experience? Any kind of Cassegrain sounds quite a challenge.  David




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