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Refractor-like Mirror cell?

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#1 mikeDnight

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Posted 26 May 2019 - 10:31 AM

Hi Guy's,

 

I thought I'd ask this question here as some of you may have given serious thought to this problem.

 

Refractors have a well deserved reputation for being able to maintain collimation decade after decade, even when treat relatively roughly. So why has no-one been able to produce a Newtonian primary mirror cell that can do the same? Lens cells don't pinch or put pressure on the optics of even large refractors, so surely it would be in the realms of possibility to manufacture a refractor-style cell for a mirror? Has anyone been successful in doing this?



#2 Astrojensen

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Posted 26 May 2019 - 11:00 AM

It's definitely been done. I have an old newtonian from the 1950'ies that has held collimation for many decades. 

 

It's mostly a matter of cost. People want newtonians to be cheap, because they can be cheap. They generally don't want them to be well made, because that would mean that they would suddenly become expensive. 

 

 

Clear skies!
Thomas, Denmark


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#3 Jon Isaacs

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Posted 26 May 2019 - 11:27 AM

Refractors do go out of collimation. That can mean a trip to the factory. 

 

I disagree with Thomas.. it's not about the Imoney. I would not want a 22 inch Dobsonian that would not need collimation. It would necessarily be very heavy.

 

The beauty of the Newtonian is that it has the least sensitive collimation tolerances of all the designs. One curved surface,one flat surface. Collimation takes a few moments. Full collimation, that is collimation beginning with a pile of parts,  is easily doable by the owner.

 

Jon


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#4 TOMDEY

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Posted 26 May 2019 - 11:27 AM

It's also the mechanics of it... a mirror's performance is hypersensitive to stresses; a lens hardly. Stress a mirror and the distortion is doubled in the wavefront; stress a lens, and the two surfaces almost entirely compensate each other's wavefront changes. Consequently, a mirror has to be mounted ~almost loose~, whereas a lens can be gripped... making it far less likely to flop around and get out of alignment.    Tom


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#5 ed_turco

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Posted 26 May 2019 - 03:03 PM

I've made any number of push-pull mirror cells that aren't too shabby about hholding collimation.  And, each only needed the added weight of three 1/4-20 screws.


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#6 MitchAlsup

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Posted 26 May 2019 - 03:42 PM

Refractors have a well deserved reputation for being able to maintain collimation decade after decade, even when treat relatively roughly. So why has no-one been able to produce a Newtonian primary mirror cell that can do the same? 

It can be done, but the cost would put it in the same ball park as the cost of the refractor.

 

Would you be willing to pay this kind of cost for a newtonian?



#7 mikeDnight

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Posted 26 May 2019 - 04:33 PM

It can be done, but the cost would put it in the same ball park as the cost of the refractor.

 

Would you be willing to pay this kind of cost for a newtonian?

I can imagine large aperture reflectors becoming too costly, but scopes 6" to 10", if they are optically excellent, might be worth spending more on. I love refractors and have spent a lot of money on them over the years, and few people would argue about the lunar and planetary prowess of say a Vixen FL102 or similar Takahashi. Yet I've seen a 4.5" F11 Newtonian significantly out-perform the legendary FL102. Many folk would look down on such a humble reflector, imagining it to be little more than a toy, but in the hands of the right observer it would be a formidable instrument. I'd be willing to pay significantly more for a less temperamental mirror cell if the scope was excellent, and the fact it was a Newtonian wouldn't really bother me. 



#8 hakann

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Posted 26 May 2019 - 11:20 PM

We do t live in a perfect world.
Lets say one do the best of secondary design and a full aststic mirror cell and a astatic edge support that will cost a bunch in design and work ( ir ATM way do it one self, but it still needs collimation.
Comlimation is not a issue.


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