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Why are bottom mirrors not fixed?

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31 replies to this topic

#26 Mike Spooner

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Posted 29 July 2021 - 10:31 AM

Generally reflectors have a tiny diffraction limited field of view. Refractors mostly are more forgiving and can tolerate minor misalignments and still give an acceptable visual image. I’m generalizing and  most scopes benefit from accurate collimating. Refractor lens elements are generally closely spaced; reflector and compound designs often have large separations of their elements and basic triangulation over distance can cause visible problems at the focal plane. I suppose a reflector could be designed to perform with only a factory adjustment but affordability would likely be out of reach for the perceived benefits. Aspheric optical systems can have different tolerance envelopes compared to spherical systems. 
 

Mike Spooner


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#27 rob1986

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Posted 29 July 2021 - 11:44 AM

Generally reflectors have a tiny diffraction limited field of view. Refractors mostly are more forgiving and can tolerate minor misalignments and still give an acceptable visual image. I’m generalizing and  most scopes benefit from accurate collimating. Refractor lens elements are generally closely spaced; reflector and compound designs often have large separations of their elements and basic triangulation over distance can cause visible problems at the focal plane. I suppose a reflector could be designed to perform with only a factory adjustment but affordability would likely be out of reach for the perceived benefits. Aspheric optical systems can have different tolerance envelopes compared to spherical systems. 
 

Mike Spooner

and the first time it took a bump or sat in the sun because you fell asleep, you'd have to return it to the factory for tuning.


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#28 Steve Harris

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Posted 29 July 2021 - 11:56 AM

There are a couple of different things that "fixed" could mean.  One is between the cell and the mirror - that the mirror is held rigidly in place by the cell, and that's usually not good because it causes stresses on the mirror as mentioned in other replies.

 

But another meaning of "fixed" could be that the cell itself not be adjustable for collimation, ie between cell and lower part of scope. So the cell can be simpler that way as it doesn't need to incorporate adjustability.  That would require doing the primary tilt collimation another way of course.  If you have 6 poles, the poles themselves can do this!  I think Oberon's scope has a fixed cell and collimates by twisting poles, which is doable at the focuser!

 

https://www.cloudyni...t-16-dob/page-7


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#29 LIVS

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Posted 29 July 2021 - 12:16 PM

Wonderful responses from all, I had no idea there was so much to discuss on this topic. I bought my dad a Collapsible Dob and he has been trying to fix the bottom mirror in place, he has a homemade one with a fixed mirror, and he thinks the fixed mirror creates a clearer planetary view but I think it is just the difference of atmospheric distortion from 20 years ago to today. I started to wonder why it wasn't fixed in the first place and a google search did not provide any answers. You guys are all awesome on here.


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#30 ShaulaB

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Posted 29 July 2021 - 12:40 PM

If a reflector had a focal ratio of 15 or so, and the scope was built for kids or outreach, then a fixed primary mirror would be understandable. But for a reflector with a more reasonable focal ratio, having a primary mirror that can be adjusted is essential for good viewing.

#31 gwlee

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Posted 29 July 2021 - 03:15 PM

So, like a musical instrument, these comparatively inexpensive, light weight,  large-aperture instruments can be easily and quickly tuned up to give their best possible performance each time they are used. I suspect that a newt with a structure sufficiently robust to allow its mirrors to be permanently aligned at the factory, like small refractors are built, would be far too heavy and too expensive to be practical in anything other than the smallest apertures. 


Edited by gwlee, 29 July 2021 - 03:24 PM.

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#32 MitchAlsup

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Posted 29 July 2021 - 05:00 PM

Why are the bottom mirrors in a reflector telescope not fixed? Why do they need to be adjusted? I have seen homemade telescopes that have fixed bottom mirrors. 

A fast primary (F/3) may need to be able to cast its point of optimal (i.e., on axis) focus within 0.003" of the center of the focuser, and this is not a "number" that can simply be machined.

 

A fast refractor is F/6 and at this speed its optimal point of focus can be anywhere in 0.070" which can be simply machined. Also the aberrations of the refractor at off axis distances is way smaller than the fast Newt, lessening the requirement for perfect collimation.




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