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Tensioned primary mirror - 8" F4 Newtonian

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#1 lucian nicu

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Posted Yesterday, 08:50 AM

Have you ever heard of this term: "Tensioned primary mirror"?

 

I'm a beginner, so I don't rule out the term, but I don't understand how you can bend a piece of glass. It seems that my 8 'F4 Newtonian (bought used) cannot be used properly due to the tense primary mirror.
I understand the following:  before I bought it, a person tightened the screws that support the mirror and then stored the telescope for a certain period, long enough for the mirror to twist and stay that way. If I did not understand well, please correct me.

But is this possible?

 

The effect is that the stars have a triangular shape, in about the same direction, not as a coma (from the inside to the outside). I've already checked for pinched optics, and I've even loosened the screws more than I would have liked. I'm attaching a picture taken by someone who knows these things better than I do.

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  • focus + out of focus.png


#2 KBHornblower

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Posted Yesterday, 10:43 AM

A triangular distortion like that can be caused by three clips pressing down too hard on the edge of the mirror.  You can bend anything if you force it hard enough, and it takes only a hair-splitting amount of deformation to spoil the image like that.  A hair is about 50 microns thick, while the distortion needed to cause this fault is less than a micron.  If the fault persists with no pressure from the clips, that is bad news.  


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#3 lucian nicu

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Posted Yesterday, 11:33 AM

Thanks KBHornblower. I really don't like this answer ( smile.gif  ), but i appreciate the reply.

Can I turn easily this bad news into good news ?



#4 PETER DREW

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Posted Yesterday, 12:41 PM

I would be surprised if it is possible to permanently distort a piece of glass with the diameter to thickness ratio of a telescope mirror. Ideally there should be no pressure on the mirror other than its support, the clips are just to prevent the mirror falling out. Try the mirror loose, if the deformation persists try rotating the mirror to see if the effect rotates with it.


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#5 lucian nicu

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Posted Yesterday, 02:12 PM

I hope it doesn't sound like a silly question but could a side pressure create this effect? I ask because in the mirror holder there are 3 curved pieces of bronze (or something similar), partially covered with rubber, which seem to support the mirror on the side. These pieces can be pushed from the side by small screws so that the pressure can be adjusted. As an idea, without these supports the mirror could move sideways over a distance of 3-5 mm.

 

Regarding your recommendations, I will try to rotate the mirror to see if the direction of the stars changes. I didn't think of that :)
Thank you very much, Peter Drew.



#6 macdonjh

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Posted Yesterday, 03:14 PM

Thanks KBHornblower. I really don't like this answer ( smile.gif  ), but i appreciate the reply.

Can I turn easily this bad news into good news ?

lucian nicu,

 

Welcome to Cloudy Nights.  

 

Can you post some photos of your scope?

 

I agree with Peter Drew, it's unlikely your mirror is permanently deformed.  I think you'll have to take your telescope apart to fix your problem.  At the primary mirror end I expect you'll find three large screw heads, each 120o apart, on the side of your telescope tube.  Those probably hold your primary mirror cell and mirror in the tube.  If you remove those three screws you should be able to slide your primary mirror cell out of the tube.

 

Now you should be able to adjust the screws you say are pushing against the side of your primary mirror, or the mirror clips holding your mirror in place.  If you have the screws pushing against the side of your mirror, loosen them a half turn or so, such that there is a small gap between your mirror and the ends of the screws.  If you have mirror clips, which "wrap" over the front of the mirror's edge, loosen them such that there is a gap between the underside of the clips and the face of your mirror.  The gap only has to be about the thickness of a couple pieces of paper.  That should relieve the stress in your mirror and make your triangle image go away.

 

Then put your scope back together and recollimate it.  Note: if you have your Newtonian mounted on an equatorial mount, be careful about where it points if you don't have mirror clips to prevent your primary mirror from falling out.  If your scope in on a Dobsonian mount you don't have as much concern- it's unlikely you'll point it lower than horizontal.

 

Good luck.


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#7 Eddgie

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Posted Yesterday, 03:23 PM

As others have said, the most likely cause is the mirror clips are too tight.  Loosen them and things will be fine. Glass does not deform.  It will spring back.

And while we thing that glass is inflexible, that is not really true. It will bend before braking, but if the pressure is relieved it will want to be flat.

 

And we are dealing with fractions of a wavelength of light here. While glass bends, it does not bend that much before breaking.  In this case though, the deformation is really microscopic.



#8 Asbytec

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Posted Yesterday, 05:26 PM

I hope it doesn't sound like a silly question but could a side pressure create this effect? I ask because in the mirror holder there are 3 curved pieces of bronze (or something similar), partially covered with rubber, which seem to support the mirror on the side. These pieces can be pushed from the side by small screws so that the pressure can be adjusted. As an idea, without these supports the mirror could move sideways over a distance of 3-5 mm.

Pushing from the side is likely to cause some astigmatism or trefoil. Trefoil looks (and acts) a little like pinched optics. I would not put any pressure on the edge of the mirror, but also not let the mirror slide 3 to 5mm, either. 



#9 KBHornblower

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Posted Yesterday, 09:09 PM

It will be interesting to see what happens when you rotate the mirror.  My 17.5" f/4.5 Coulter mirror has the same sort of fault, and when I rotated the mirror 60o in its cell the pattern rotated with it.  This means it is figured into the mirror rather than being any fault with my 9-point cell.  That tells me the mirror was improperly supported during the figuring.  In good seeing its resolution is roughly equal to that of my 6" f/8 mirror, so for what I use it for under a dark sky it still blows me away on globular clusters, M17, M51, etc.


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#10 lucian nicu

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Posted Today, 12:52 AM

Thanks to everyone for their help.
Because until Wednesday I don't have the time to apply what you told me, I will come back with photos and feedback by the end of this week.
A great week for everyone!
Lucian.



#11 areyoukiddingme

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Posted Today, 01:17 AM

The mirror clips are just there to stop the mirror from rolling out of the cell, So they shouldn't be touching the mirror.

 

The standard advice is to be able to slide a business card between the mirror and clips.

 

If the mirror is siliconed on the bottom, that can also induce some astigmatism, but probably not the pattern in the picture. 




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