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Maksutov Secondary mirror design?

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#1 Leafus

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Posted 07 December 2022 - 07:21 AM

I just got a celestron 127 maksutov tube delivered and it being my first maksutov - I’ve got an SCT - I am confused why there is an external pointing mirrored surface on the secondary. At first I thought the front plate had been removed and put back wrong but images of other examples seem to show the same thing. But as I cannot find anything written in this I’d appreciate an explanation as it seems odd.

Thanks
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#2 vtornado

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Posted 07 December 2022 - 07:59 AM

Celestron (synta) mak's are Greogory or Spot Mak's the secondary mirror is a aluminized spot applied directly to the back of the corrector.  What you see is the back of the secondary.

 

An SCT has the mirror in a plastic holder. The secondary mirror is not visible from the front of the scope.


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#3 Leafus

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Posted 07 December 2022 - 08:26 AM

That’s what I thought.

But on the celestron 127 5inch Mak I have just received there is a mirrored surface on the secondary assembly that points outwards from the centre of the corrector not inwards. That’s what’s confusing me. Thanks Lea

#4 vtornado

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Posted 07 December 2022 - 08:46 AM

A picture is worth a thousand words.



#5 Minuam

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Posted 07 December 2022 - 09:28 AM

It indeed looks like mirror surface but there is nothing wrong with it.

 

Attached Thumbnails

  • 127Mak.jpg

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#6 gene 4181

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Posted 07 December 2022 - 11:48 AM

 On the inside its a first  surface mirror reflecting  94-98 % of the light. The outside is considered second surface mirror , less reflectance, google it .



#7 Protheus

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Posted 07 December 2022 - 12:18 PM

That’s what I thought.

But on the celestron 127 5inch Mak I have just received there is a mirrored surface on the secondary assembly that points outwards from the centre of the corrector not inwards. That’s what’s confusing me. Thanks Lea

It's easier to just put an aluminum spot on the back of the glass than it is to make a lens with a hole in it and attach a second mirror.  The back of the lens is basically the correct shape anyway.  It just has a mirror spayed right on.  This means that you see the shiny from the front of the scope, even though that is actually the back side of the secondary you're looking at.  The business end faces the visual back.

 

Chris


Edited by Protheus, 07 December 2022 - 12:19 PM.

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#8 Mossonarock

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Posted 07 December 2022 - 01:32 PM

because the secondary mirror is fixed in place in this manner, you never have to collimate the secondary mirror. not all mct's are made this way but the less expensive ones are.


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#9 Leafus

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Posted 07 December 2022 - 07:36 PM

Thanks everyone.

So if I read the comments correctly there is no reason that the outer facing of the secondary - as per the photograph above - is mirrored or mirror like. It’s just a side effect of how the secondary cell is produced and fitted.

Still seems odd it should be so mirrored without any intent.

#10 JoeInMN

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Posted 07 December 2022 - 08:19 PM

The corrector in a Gregory type of Mak is shaped so that an aluminized spot in the center of its inner surface has the correct curve to function as the secondary mirror, eliminating the need for a separate optic to be attached there. There's no actual intent or purpose behind the outward-facing reflectivity; there's just no reason to go to the extra effort of covering it up. You're seeing the same reflective coating that bounces the light back down the tube, just from the other side of the glass. 



#11 Tony Flanders

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Posted 07 December 2022 - 09:23 PM

Still seems odd it should be so mirrored without any intent.


Not at all. The reflective metal is essentially spray-painted on the inside of the glass, and it's reflective on both sides exactly the same way that any other thin sheet of metal is reflective on both sides. You'd have to go out of your way to prevent it from reflecting outward.
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#12 BFaucett

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Posted 07 December 2022 - 10:00 PM

Still seems odd it should be so mirrored without any intent.

 

On my 90 and 102 Maks, I just think of it as decorative trim. IMO, it adds a little pizzazz to the scope's appearance. 

 

Cheers! Bob F. smile.gif



#13 Protheus

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Posted 07 December 2022 - 11:50 PM

Still seems odd it should be so mirrored without any intent.

 

But that's the point.  It's there intentionally.  It's probably the easiest and cheapest way to get a proper secondary in the system.  That it's shiny on the outside is just a side-effect of the process, though.  That side doesn't do anything, but getting rid of it would be more expensive than keeping it, and would serve no purpose but aesthetics.

 

This is somewhat like asking why the mirror in the diagonal covers space that's not exposed to the focus tube.  In that case, the answer is that it's cheap and easy to make a square mirror, and cutting the edges off would add cost while also serving no useful purpose.

 

Chris



#14 SeattleScott

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Posted 08 December 2022 - 02:15 AM

You can always cover the external shiny part with a round smiley face sticker.
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#15 Asbytec

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Posted 08 December 2022 - 07:58 AM

The corrector in a Gregory type of Mak is shaped so that an aluminized spot in the center of its inner surface has the correct curve to function as the secondary mirror, eliminating the need for a separate optic to be attached there. 

I hear ya, and that's basically true except for which component has the correct radius. The secondary has no choice but to have the same radius as the back side of the meniscus. The secondary is what it is because it's the meniscus that has the correct curve to balance spherical aberration. Though there may be a little compromise between the two.

 

A separate secondary, as in the Rumak design, with more freedom to select a correct radius for the secondary would offer better flexibility with correcting aberrations. But the Gregory works just fine despite that design limitation. A bonus is, as mentioned above, the Gregory (Spot) Mak secondary is permanently collimated to the meniscus optical axis, also, because it shares the same radius as the meniscus. 


Edited by Asbytec, 08 December 2022 - 08:04 AM.


#16 gene 4181

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Posted 08 December 2022 - 11:23 AM

You can always cover the external shiny part with a round smiley face sticker.

  lol.gif



#17 vtornado

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Posted 08 December 2022 - 11:43 AM

You can always cover the external shiny part with a round smiley face sticker.

Or Mr. Yuk lol.gif



#18 Mossonarock

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Posted 08 December 2022 - 12:35 PM

You can always cover the external shiny part with a round smiley face sticker.

Or a googly eye would be even more fun.

https://www.michaels...-eyes/876270980

 

 

I'm trying real hard to not tempt myself into doing this.


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#19 Pingu

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Posted 08 December 2022 - 03:59 PM

You can always cover the external shiny part with a round smiley face sticker.

Dang it man!  You know this is about as bad as, "Don't push the red button".


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#20 Leafus

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Posted 08 December 2022 - 08:17 PM

Oh right. I finally get it. I had not realised the mirrored finish was applied directly to the glass. I thought the mirror housing was stuck on and a separate mirror was in that. Makes sense now I know the mirror coating is direct to the glass as it would be harder to apply a black layer then the mirror layer.

Thank you all for the explanations.
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