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Why are bottom mirrors not fixed?

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#1 LIVS

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Posted 29 July 2021 - 06:36 AM

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. 



#2 Supernova74

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Posted 29 July 2021 - 06:52 AM

We’ll if the primary mirror was completely fixed in the cell etc and didn’t have any lateral movement at very least the scope wouldn’t be able to be collimated to the secondary mirror.im no expert in reflectors however I can imagine air flow for cooling also does play a major part.



#3 rob1986

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Posted 29 July 2021 - 07:03 AM

because they wont stay in position. materials deform over time, and they deform faster with greater forces acting upon them. the non fixed primary is essential to maintain collimation over time. nano-meters matter when it comes to 



#4 Sandy Swede

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Posted 29 July 2021 - 07:03 AM

If you are referring to a Schmidt Cassegrain telescope, focus is achieved by movement of the primary mirror which is located at the rear (there is a focusing knob on the rear of the scope).  Different with the Newtonian design. 



#5 kathyastro

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Posted 29 July 2021 - 07:06 AM

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.


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#6 Supernova74

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Posted 29 July 2021 - 07:17 AM

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.

Well to a degree thay are however on some of the more premium apo refractors you can at least collimate them.

however tho still needs a certain level of skill so you don,t pinch the optics etc.


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#7 wrvond

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Posted 29 July 2021 - 07:17 AM

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. 

Great question! I've never actually heard a real explanation of why the primary mirror of a reflector telescope isn't fixed in place. There are definitely advantages to not having it fixed in place - such as collimating as the ambient temperature and temperature of the glass changes - but there are also disadvantages - such as mirror shift as the viewing angle changes.

When a large mirror is not fixed in place, it can be removed and safely boxed for transport, so that's an advantage too. Also, mirrors that are locked down tend to suffer from pinched optics. I guess the reality is, a mirror that is not permanently fixed in place is free to expand and contract without adversely affecting the view. It's easier to remove for cleaning, maintenance and transportation too.

 

 

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.

Not all reflector lenses are fixed. They may not have the range of motion a larger mirror has in it's cell, but a collimatible refractor cell uses the same six screw (three push/ three pull) design -sans springs - as a reflector mirror cell.

 

Edit: I imagine you are referring to the edge retention system of the refractor cell vs. mirror cell. I believe refractor lenses are still subject to pinched optics and are allowed to move in the cell to a certain extent to avoid the effects of expansion.


Edited by wrvond, 29 July 2021 - 07:21 AM.

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#8 rob1986

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Posted 29 July 2021 - 07:18 AM

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.

except they usually aren't. they're just usually smaller (and thinner!)

 

the meade 7" apos had just this problem, and it wasn't fixed till they added screws.



#9 havasman

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Posted 29 July 2021 - 07:18 AM

Almost everywhere on the planet on almost every night the temperature changes over the course of a night. Glass is not stable over a range of temps but exhibits thermal expansion and lags behind ambient temperature changes. So the glass is never "fixed" but moves at a molecular level. If we limit movement of a primary mirror by constraining it in the cell we do not change the thermal expansion properties of the material but create directional limits on it. When it moves these limits force the material to warp instead of expanding/contracting smoothly over its entire mass. This warping distorts the reflective surface. 

 

Additionally, gravity works on a mirror from different relative angles as the scope travels in altitude and support needs to respond to the vectoring of this force. The support required moves from the rear to the edge of a mirror and back. Failure to allow this will induce sag and also distort the reflective surface.

 

Last, "fixing" a mirror by clamping it into a device will create aberrations of the reflective surface caused by the pinching of the clamps.

 

Often mirror support systems in homemade telescopes are less than cutting edge.

 

 

A good discussion can be found here toward the end of the article  -  http://www.loptics.c.../starshape.html

and here  -  http://www.loptics.c...rorsupport.html

and here in Section 5.4. FORCE-INDUCED SURFACE ERRORS: THERMAL, PINCHING, GRAVITATIONAL PULL  -  https://www.telescop..._surface_errors

 

 

Collimating a Newtonian telescope is straightforward and easy. It is an essential task that takes less than 5 minutes in all cases if properly and efficiently executed. It enables the optical system to function well. Good mirror support and thermal management essentially enables collimation stability.


Edited by havasman, 29 July 2021 - 07:56 AM.

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#10 xvariablestarx

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Posted 29 July 2021 - 07:23 AM

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.

I think the Orion Goscope 80mm, focuses by moving the objective lens, but that is hardly a premium refractor.



#11 Alex65

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Posted 29 July 2021 - 07:24 AM

Not all reflectors have adjustable primary mirrors.

 

My AstroScan (a 4.25" reflector) primary mirror is fixed and hasn't moved in over forty years, though I am well aware that I've been lucky with it in this regard as so many have moved in their fixed settings over that period of time. My AstroScan is still perfectly collimated.


Edited by Alex65, 29 July 2021 - 07:25 AM.


#12 rob1986

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Posted 29 July 2021 - 07:32 AM

just have to remember that out "solid" matter is made of >*10^26 tiny particles all which which are connected by various methods, few of which are truly "fixed" with respect to each other, and all are held together by something akin to magnetism or friction.

 

tubes stretch, bend, and get out of shape from from gravity, heating, cooling, being lifted, all the time, even if the unaided eye can't appreciate it.



#13 Jon Isaacs

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Posted 29 July 2021 - 07:37 AM

I understand why mirrors are not fixed: any piece of precision equipment needs to be adjustable to maintain its precision.  Given that, I wonder why refractor lenses are fixed.

 

Refractors objective cells are carefully designed so as to avoid pinching the optics.  Matching the cell to the objective is part of the expense of a quality refractor.   Over the years, Roland Christen of Astro-Physics has been very open about the design of his refractors but the one thing he does not share is the design of the objective cell..  

 

Newtonian mirror cells are also tricky, particularly with thin mirrors.  The edge support and the back support as well as the friction on the back support are critical in maintaining the figure. It only takes a little to pinch the optics.

 

Jon



#14 Jon Isaacs

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Posted 29 July 2021 - 07:39 AM

Not all reflectors have adjustable primary mirrors.

 

My AstroScan (a 4.25" reflector) primary mirror is fixed and hasn't moved in over forty years, though I am well aware that I've been lucky with it in this regard as so many have moved in their fixed settings over that period of time. My AstroScan is still perfectly collimated.

How often do you check the collimation?  Are you getting clean, even diffraction rings at 200x without any focal plane tilt?

 

Jon



#15 rob1986

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Posted 29 July 2021 - 07:50 AM

How often do you check the collimation?  Are you getting clean, even diffraction rings at 200x without any focal plane tilt?

 

Jon

I imagine it's something with how she positions the scope for storage, equally distributing the forces over the tube and mirror mounting. I have heard that meade maks need to be stored on both sides to prevent unequal application of gravity on the secondary's glue



#16 Alex65

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Posted 29 July 2021 - 07:54 AM

How often do you check the collimation?  Are you getting clean, even diffraction rings at 200x without any focal plane tilt?

 

Jon

I check the collimation every time I use it. Out of focus star still presents a black center.

 

I don't use that high power on my 'scope, it isn't designed to carry off 200x! Highest I go is 25x per inch.



#17 ButterFly

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Posted 29 July 2021 - 07:54 AM

Thermal expansion is the issue with the larger mirrors that we have access to.  With smaller mirrors, they can cool faster, and the resulting astigmatism is hardly an issue for very long.  With even larger mirrors, active optics help with gravitational distortion.  With my 15" in a very open cell, I can watch the astigmatism develop as the mirror adheres to the pads, then jiggle it out.  Thermal over/undercorrection can be Paracorr'ed out.

 

Bear in mind that there is an inherent degree of freedom with respect to primary mirror placement and focuser placement when collimating.  Collimation involves proper adjustment to both tilt and placement.  In practice, the primary support cell further constrains the remaing degree of freedom.  The primary is constrained by its placement in the cell, and the focuser is constrained by the placement of the primary,



#18 TOMDEY

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Posted 29 July 2021 - 08:09 AM

Primary mirrors are "loose" in their cells because even a small amount of constraining stress will strain the wavefront out of shape and degrade the image. It takes very little to exhibit this overconstrained performance defect. We're talking very small gaps to allow for differential expansion and contraction of the mirror-mount interfaces. Like the thickness of a piece of paper or less. The best approach comprises passively-adaptive whiffle-trees and side supports... like Ryan's "Ladder Sling" shown here. Some people actually glue their primary mirror to a board and expect it to perform. That's the worst approach. We did experiments at work where we intentionally overconstrained mirrors. Every one induced objectionable astigmatism into the mirror.    Tom

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  • 59 36-inch diam x 5-inch thick mirror in Ladder Sling.jpg

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#19 rob1986

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Posted 29 July 2021 - 08:44 AM

did anyone but me notice the "why does it need to be adjusted" clause?

 

he's not talking about the complicated floating point mirror supports. (and probably doesn't yet appreciate that aspect), he's talking about the screws on the back of the tube. he doesn't understand why provision for collimation is necessary. 



#20 Supernova74

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Posted 29 July 2021 - 08:59 AM

I imagine it's something with how she positions the scope for storage, equally distributing the forces over the tube and mirror mounting. I have heard that meade maks need to be stored on both sides to prevent unequal application of gravity on the secondary's glue

Well that definitely seems unorthodox at best ?



#21 rob1986

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Posted 29 July 2021 - 09:02 AM

Well that definitely seems unorthodox at best ?

which?



#22 Supernova74

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Posted 29 July 2021 - 09:13 AM

which?

Being stored a certain way regarding the glue.ie secondary glue first I’ve heard of this just seems a little odd that’s all.



#23 rob1986

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Posted 29 July 2021 - 09:16 AM

Being stored a certain way regarding the glue.ie secondary glue first I’ve heard of this just seems a little odd that’s all.

I saw it here on CN, it must be true!

 

evidently the glue is bad and the secondary baffle slips. Someone here stores it on opposite sides to prevent slippage.

 

(dad didn't have his long enough for the glue to slip so I haven't seen it.)



#24 charlieb123

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Posted 29 July 2021 - 09:17 AM

Expansion, contraction, brittleness.



#25 andysea

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Posted 29 July 2021 - 09:19 AM

I believe the Astro-Physics RH305 is not collimatable. All its mirrors are fixed in place. It’s a Cat tho, not a pure reflector.


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