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gluing primary mirror to cell

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#1 dmcrane

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Posted 25 May 2013 - 11:41 PM

I got a Discovery split tube PDHQ 12.5" several years ago. I don't remember gluing the primary mirror to the cell when I put it together. It has retainer clips. I don't use it that often and always transport it vertically. How important is it that the mirror is glued to the cell? If the mirror was glued, would it be safe to transport the tube on its side? A small picture of the cell is on discovery's web site. I don't know what brand it is.

#2 Jon Isaacs

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Posted 26 May 2013 - 02:30 AM

I got a Discovery split tube PDHQ 12.5" several years ago. I don't remember gluing the primary mirror to the cell when I put it together. It has retainer clips. I don't use it that often and always transport it vertically. How important is it that the mirror is glued to the cell? If the mirror was glued, would it be safe to transport the tube on its side? A small picture of the cell is on discovery's web site. I don't know what brand it is.


Dave:

You don't want the mirror glued to the cell, you want it floating. I transport a tube dob on it's side, I just make sure I am comfortable that the mirror clips will keep the mirror in place.

JOn

#3 dmcrane

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Posted 26 May 2013 - 09:14 AM

I remember that the clips on the side didn't look that sturdy; there were 3 clips that were held on by a screw into the side of the cell. If 1 screw fails, the mirror could move. I thought maybe I missed something because other mounts do glue the mirror like Aurora's primary mirror cell used by Teeter.

#4 Mirzam

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Posted 26 May 2013 - 09:30 AM

I don't like to glue primary mirrors as it (1) can cause astigmatism and (2) makes it harder to remove the mirror for storage, cleaning or recoating. Instead of just loosening the clips to take out the mirror you must saw or cut it loose from the cell. This increases the risk of damage.

A better solution for you may be to modify the mirror cell to provide more secure mirror clips. If you could post a photo of your cell we can offer some suggestions about how to do this.

JimC

#5 Geo31

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Posted 26 May 2013 - 09:50 AM

I would think gluing a primary to a cell would be a BAD move as the glass and the cell material probably have different rates of thermal expansion. That would in all likelihood distort your images.

#6 careysub

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Posted 26 May 2013 - 03:34 PM

You can make an estimate of how important this would be by looking at the results of PLOP calculations of "glued in cell".

In sufficiently small mirrors having about one to two pounds of force being exerted in shear at each support point has a negligible effect on surface figure (this is the case of a glued-in-cell mirror tilted 90 degrees to vertical).

The shear force exerted at each support point by cell contraction after cooling will be determined by the actual displacement, and how thick and elastic the layer of silicone rubber glue is (the elasticity is about 100,000 times larger than glass).

If we assume a steel cell and a BK7 mirror, a 20 C temperature change, and a 20 cm separation of two support points, then the differential contraction will be 24 microns.

Each point has to take up half of that change, or 12 microns. The question is, how much force applied to the rubber blob is required to stretch it 12 microns? Since the shear modulus of rubber is about 100 psi, and the stretch is 1/2000 of an inch, the force is far less than the gravity loading of the tilted mirror.

In other words - the effect of mirror distortion from tilting the mirror will always be larger than the thermal contraction stress and will be the controlling factor about whether it is acceptable.

#7 gatorengineer

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Posted 26 May 2013 - 04:45 PM

3 dobs of glue are no big deal, (assuming you used silicone) if you troweled it on then that would be problematic and different. Don't rely on it in a hot car or out in the hot sun..

#8 gatorengineer

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Posted 26 May 2013 - 04:47 PM

Never rely on silicone for transport..... That being said from a mechanical standpoint 3 dobs of glue are no big deal, (assuming you used silicone) if you troweled it on then that would be problematic and different. Don't rely on it in a hot car or out in the hot sun..

The shear of silicone adhesive is probably more on the order of 20 psi (I haven't met a silicone secondary that's been tough to get off), and I hope that you have dobs less than an inch anyway.

#9 Pinbout

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Posted 26 May 2013 - 05:16 PM

unless the cell is design to have the mirror glued. Teeter uses the Aurora Precision 6 point cell and it is designed to be glued to the mirror.

#10 Geo31

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Posted 26 May 2013 - 08:23 PM

Why mess with adhesive when it's so not necessary?

#11 davidpitre

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Posted 26 May 2013 - 09:26 PM


You don't want the mirror glued to the cell, you want it floating.

This is really an over generalization. As Carey explained very well, larger mirrors can suffer astigmatism when glued. On the other hand Portaball telescopes use a glued mirror on their 12" scopes, and I have never heard of anyone in many years experiencing primary mirror astigmatism caused from the mounting in one of these scopes. Carl Zambuto swears by the set-up. As mentioned, Rob Teeter also is using glued mirrors in some of his scopes. That being said, I generally feel more comfortable with floating mirrors in my scopes.
To see if it is an issue in your scope, you need to star test it. It is very easy.

#12 zytrahus

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Posted 26 May 2013 - 10:43 PM



You don't want the mirror glued to the cell, you want it floating.

This is really an over generalization. As Carey explained very well, larger mirrors can suffer astigmatism when glued. On the other hand Portaball telescopes use a glued mirror on their 12" scopes, and I have never heard of anyone in many years experiencing primary mirror astigmatism caused from the mounting in one of these scopes. Carl Zambuto swears by the set-up. As mentioned, Rob Teeter also is using glued mirrors in some of his scopes. That being said, I generally feel more comfortable with floating mirrors in my scopes.
To see if it is an issue in your scope, you need to star test it. It is very easy.

I always wondered about gluing combined with soft (nylon for example) lateral supports such as it is done by ASA (I have a link somewhere but I think it was >15"). Shouldn't it help with astigmatism?






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