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obsession mirror support fixes/upgrades

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#1 the_chemist

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Posted 11 October 2021 - 11:37 AM

hello all, I have an obsession mirror support question. I have an 18" classic, and it suffers from the (apparently) common ails of the mirror support, including a stretchy belt and sticky felt pads. I can get it collimated OK, and views are OK, but every so often when viewing I can hear a little sound (clunk) and the image shifts in the EP.  Also, when I have the scope tilted during collimation, if I push on the mirror it moves easily, but it also often times shifts from one side to the other a few mm, and then says there. after some viewing, the central laser spot/ring has shifted from the center to near the edge (using the barlowed laser method). I'm guessing these are symptoms of both issues. BTW, the belt is adjusted properly, and the mirror isn't touching the posts (most of the time anyway, that might be the clunk).  These sound like symptoms of the mirror support issues I have read about with these scopes, and I am interested in fixing them to the degree its possible with the existing frame. So I think the Glatter cable support upgrade is in order, and as long as I'm in there I think it would be a good time to upgrade the mirror support pads to delrin or acetal. The cable I think I understand, but for the mirror pads, do I just need to get roughly the same same diameter and thickness, and then mount them in the same location as the felt pads are? is there anything more to consider? any preference between silicone adhesive and the ones with posts? Those would need drilling, but I think I can handle that if needed. Does anyone one know of a "kit" or package that has the right parts that anyone sells? I could track down the parts on McMaster, but I would rather get the right parts from someone who has put some thought into it. thanks for all the great ideas on this forum.

 

Eric



#2 Bob4BVM

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Posted 11 October 2021 - 12:01 PM

I prefer the type with a post or pin, for the simple reason that the pin allows one to locate the pad center precisely at the PLOP- calculated location on the triangles.

I bought a bunch of these drawer glide pads with pins. Not sure of the material, but it's a very hard,, about 1/16" thick, and VERY slick surface that my mirrors glide on with complete freedom of movement. The pads are hard so do not deform under weight of mirror.

If you want, PM me and i will send you a set.  I assume you have an 18-pt cell ?

CS

Bob

 

cell mirror pads w bushgs.jpg

cell trngles no brace.jpg

subassys mntg preps-cell8.jpg


Edited by Bob4BVM, 11 October 2021 - 12:28 PM.

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#3 ngc7319_20

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Posted 11 October 2021 - 12:24 PM

One other idea is to collimate, and then run the scope up and down to the elevation limits a couple times to sort-of shake out the collimation and get everything to settle, and then check the collimation again.  Assuming the scope is on a level surface (or has been leveled) that should reduce any changes during the night.

 

But I agree, some better mirror support is needed, too.



#4 the_chemist

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Posted 11 October 2021 - 02:25 PM

Bob, that is a very generous offer, if you can just tell me where you bought them, that will be fine.  So I assume the locations of the stock pads on the supports are are correct, not something that needs tweaking with the new pads? 

 

NGC- yep, I have incorporated that into my pre- collimation procedure, and it doesn't seem to matter much whether I do or don't tip the scope a few times before I align the mirror, but thanks for the idea.



#5 a__l

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Posted 11 October 2021 - 05:31 PM

One other idea is to collimate, and then run the scope up and down to the elevation limits a couple times to sort-of shake out the collimation and get everything to settle, and then check the collimation again.  Assuming the scope is on a level surface (or has been leveled) that should reduce any changes during the night.

 

Not certainly in that way.
Before observing, you must always move the telescope towards the horizon, and then move upwards and then collimate. Since there is clearance between the mirror and any support. Otherwise, astigmatism is inevitable.



#6 a__l

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Posted 11 October 2021 - 05:41 PM

Bob, that is a very generous offer, if you can just tell me where you bought them, that will be fine.  So I assume the locations of the stock pads on the supports are are correct, not something that needs tweaking with the new pads? 

 

NGC- yep, I have incorporated that into my pre- collimation procedure, and it doesn't seem to matter much whether I do or don't tip the scope a few times before I align the mirror, but thanks for the idea.

If you are worried about felt, buy a new one at any furniture store. The felt should be with an adhesive backing. Replace it on your telescope. This will not affect your observations in any way. A question of inner feelings. The 18" Obsession mirror is thick enough.

 

Belt. It is advisable to replace it with a steel cable. But, you can try to do better.
Do you have the rest of the belt rolled into a ring? All this needs to be cut off. Reduce the belt length slightly if necessary.
Check that there is a small clearance between the belt and the eccentrics (~ 1 mm). To do this, tilt the telescope towards the horizon and press down on the mirror with your finger. It shouldn't stick. I think one can observe this further.


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#7 a__l

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Posted 11 October 2021 - 06:15 PM

This ring from the belt must be cut off.

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#8 skywolf856

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Posted 11 October 2021 - 08:04 PM

Have you looked at the Glatter Sling?


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#9 Bob4BVM

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Posted 11 October 2021 - 09:31 PM

Bob, that is a very generous offer, if you can just tell me where you bought them, that will be fine.  So I assume the locations of the stock pads on the supports are are correct, not something that needs tweaking with the new pads? 

 

NGC- yep, I have incorporated that into my pre- collimation procedure, and it doesn't seem to matter much whether I do or don't tip the scope a few times before I align the mirror, but thanks for the idea.

Well i would hope we can I assume the locations of the stock pads on the supports are correct=i'd assume the mfgr put them in the right place . Only way to know would be to run PLOP for your mirror specs, then measure the pad locations and see if they match what PLOP comes up with.

 

As for pads, you do not want felt or cork, both too much friction against a heavy mirror back

Whatever you use, make sure is is slippery against the glass

 

re. my glides, it's been so long i can't recall where i got them, but i have plenty wink.gif

CS

Bob


Edited by Bob4BVM, 11 October 2021 - 09:32 PM.


#10 a__l

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Posted 11 October 2021 - 11:47 PM

Well i would hope we can I assume the locations of the stock pads on the supports are correct=i'd assume the mfgr put them in the right place . Only way to know would be to run PLOP for your mirror specs, then measure the pad locations and see if they match what PLOP comes up with.

 

Read the book by Kriege. All sizes are listed there. They may differ slightly from what PLOP considers, but it doesn't really matter for mirror Obsession.

 

 

As for pads, you do not want felt or cork, both too much friction against a heavy mirror back

 

 

With regard to felt, this is an erroneous statement. The new felt has no worse friction than the plastic you are proposing. There is a nuance, if the mirror bounces a lot on the cell during transportation, you will no longer have this mirror. So I strongly discourage the OP to replace the felt.

 

 

And with regard to both quotes.
Perhaps this would make a difference for Lockwood thin mirrors, but then again ... most of the opticians (LOMO etc.), they publicly express are very wary of such mirrors, given the capabilities available to ATM.

As far as I know, D. Kriege adheres to the same point of view.


Edited by a__l, 11 October 2021 - 11:57 PM.


#11 Kunama

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Posted 12 October 2021 - 12:12 AM

I prefer the type with a post or pin, for the simple reason that the pin allows one to locate the pad center precisely at the PLOP- calculated location on the triangles.

I bought a bunch of these drawer glide pads with pins. Not sure of the material, but it's a very hard,, about 1/16" thick, and VERY slick surface that my mirrors glide on with complete freedom of movement. The pads are hard so do not deform under weight of mirror.

If you want, PM me and i will send you a set.  I assume you have an 18-pt cell ?

CS

Bob

 

 

I like the pads you have there Bob, wish I had found them before spending a couple of hours on the lathe making acetal pads….

I also am not a fan of felt pads, too much stiction when the scope is tilted from vertical until it’s pointing at least down around 45 degrees…

With the acetal ones it’s as if the mirror is sitting on ice…


Edited by Kunama, 12 October 2021 - 12:14 AM.


#12 a__l

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Posted 12 October 2021 - 12:24 AM

I also am not a fan of felt pads, too much stiction when the scope is tilted from vertical until it’s pointing at least down around 45 degrees…

 

To ensure that this statement is not false, please provide your evidence! "too much stiction" how much is this?
Thousands of Obsessions are in operation all over the world and most of the owners are happy with these telescopes. Is your opinion based on one of your telescopes or something else? I submit that this is a false opinion.

Friction depends on the quality of the back surface finish, which depends on the optician. For example Zambuto polishes the back :)

 

PS. Kunama, I'll give you a hint.

I have not seen on the Internet a single interferometric study of astronomical mirrors where astigmatism was associated with felt. For the thickness that Obsession uses.
Give all interested CN users this link. If you find it.


Edited by a__l, 12 October 2021 - 04:48 AM.


#13 the_chemist

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Posted 12 October 2021 - 06:22 AM

here is the primary reference that got me thinking about my situation- http://www.loptics.c...rorsupport.html. i realize this is one persons take on a complex topic, but most of what he talks about seems to make sense. 

 

i will be getting the glatter cable, so the support pads are the issue. my mirror is a pegasus, and the back is not polished. A_I, you sound mainly concerned about mirror breakage with the hard pads- that is a very valid point, have you or anyone ever heard of this happening with hard pads? Scope design is always a set of compromises depending on the goal. the design of a permanently mounted scope will be different from one that is meant to be disassembled and transported regularly, and so forth. I would not want to take the risk of breakage if that is even a remote possibility.

 

thanks again everyone



#14 a__l

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Posted 12 October 2021 - 12:54 PM

the_chemist,

I read on this link. Moreover, on CN there was a discussion with the author on this link. Where he admitted that the felt could be used (his recommendation that it should be new).

 

The link shows a Webster triangle with dust (rust?) It's hard for me to tell where it comes from. Probably from cheap furniture fittings. Obsession has never used this. Only felt pads. I use three telescopes, some for over 10 years and have never seen so much dirt under a mirror. I only use felt. Kunama is most likely using felt as well. Earlier (many years) he always wrote that his mirror shows well.

 

Further. The reason why it is recommended to replace the felt is astigmatism. The link considers a 32" mirror in which the author saw this. In fact, these are large mirrors and there are many possible reasons for astigmatism. Poorly made cell and the mirror were not supported at all points the most common reason.
In addition, in thin mirrors of this size, astigmatism from gravity cannot be completely eliminated by ATM methods. And Delrin won't help here.

 

Your mirror is 18". Have you ever seen astigmatism in it? For example at StarTest? If not, then you have nothing to worry about. If there is astigmatism, look for the cause. The fact that this is felt, I deeply doubt. Finally, replace felt with a new one. It's easy and cheap to made.

 

I can assume that the cause of astigmatism is sticking to dried dirt on the back of the mirror. But for this you have to try very hard and not wash it for many years.

 

Transport. This can happen suddenly and for reasons beyond your control.  I had a case at night when my car jumped ~ 0.5 meters. At that moment there was a mirror of Zambuto. It remained intact. Therefore, I always use felt.


Edited by a__l, 12 October 2021 - 01:08 PM.

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#15 Bob4BVM

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Posted 12 October 2021 - 02:50 PM

I like the pads you have there Bob, wish I had found them before spending a couple of hours on the lathe making acetal pads….

I also am not a fan of felt pads, too much stiction when the scope is tilted from vertical until it’s pointing at least down around 45 degrees…

With the acetal ones it’s as if the mirror is sitting on ice…

Wow, that's exactly the right description of what i see with my plastic pads, "as if the mirror is sitting on ice…" !!!

 

When i first placed my mirror on its new cell, i was struck at how freely it moved on the support points, almost scary i would say.

 

And THAT is exactly the point of a floating cell, the mirror is free to do its expansion/contraction cycles , without restriction of glass movement, without friction restricting the free & even expansion. Anything less and you change the figure. 

 

So why anyone would recommend pads of felt  is beyond me. Not to mention that hard plastic pads will maintain the same Z-dimension unlike felt which can vary with different climate conditions.  With felt how do you know the glass is evenly supported (Z) by all points ?

 

As for breakage, the key is the mirror has many support points, providing even support.  But if i was expecting to be launching my vehicle over .5 meter jumps, i would probably be carrying my primary in its own safe padded box. Actually i do that anyway, why anyone would drive up a rough mountain road with a 30# mirror hammering on its precision cell is lost to me.  I prefer not to subject either cell or glass to that kind of punishment !

 

CS

Bob


Edited by Bob4BVM, 12 October 2021 - 02:51 PM.

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#16 niallk

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Posted 12 October 2021 - 03:57 PM

I've an Obsession 15". FWIW, I noticed one of my acorn nuts was not moving freely. This caused the mirror to pivot slightly at a certain altitude. I removed the mirror, adjusted the hex nut a fraction of a turb - bingo: acorn nut moved freely, and collimation held stable vs altitude ;)

I also point to the horizon, push the mirror forward on its sling, let it settle back onto the pads, the point to a mid altitude before collimating.
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#17 Kunama

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Posted 12 October 2021 - 04:04 PM

This is what I ended up doing (this picture is from Mike Lockwood’s site)

 

Much like Bob, I try to limit jumping my van over half metre jumps, so I am not in fear of the mirror shattering in transit.

 

 

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#18 a__l

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Posted 12 October 2021 - 07:03 PM

So why anyone would recommend pads of felt  is beyond me. Not to mention that hard plastic pads will maintain the same Z-dimension unlike felt which can vary with different climate conditions. 

 

 

This is primarily recommended by D. Kriege.

With acetal, you again have a mistake, its temperature coefficient of expansion is> 25 times than Pyrex (like with any other plastic).
Felt is more glass-friendly in that it has an advantage over any rigid material at the moment of impact.
My question remained unsolved, the connection between felt and astigmtism has not been proven by anyone. In any case, in the range of sizes and thicknesses of the mirrors under discussion.

 

 

 With felt how do you know the glass is evenly supported (Z) by all points ?

 

You don't seem to understand the floating support principle.
Rigid support (plastic), for each pads there will be one random point within the pads size. Above you wrote about PLOP, so this is just that case of inconsistency :)



#19 a__l

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Posted 12 October 2021 - 07:24 PM

This is what I ended up doing (this picture is from Mike Lockwood’s site)

 

Much like Bob, I try to limit jumping my van over half metre jumps, so I am not in fear of the mirror shattering in transit.

Actually, I used to make these pads from high-strength aluminum alloy with felt on top. In the photo this is a triangle for a Zambuto 14" mirror, which most likely saved the life of this mirror smile.gif Pads and triangle are made of the same material.
Now I plan to replace it with carbon (for 24 ", triangles + pads) + felt for pads. By significantly reducing the weight, which is the main thing for me for this telescope.
The second photo shows an all-aluminum cell for my 18" telescope with titanium screws.

The astigmatism I saw on this telescope at StarTest in the last retrofit case was from the pinching of the secondary and holder. I quickly fixed it smile.gif

 

Ps. Kunama, Mr. Lockwood is not very good about the published photos from his site. At one time, for this reason, some of my posts on CN were deleted.

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  • Юстир_винт_Iz.jpg
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Edited by a__l, 12 October 2021 - 07:52 PM.

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#20 the_chemist

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Posted 13 October 2021 - 06:48 AM

I've an Obsession 15". FWIW, I noticed one of my acorn nuts was not moving freely. This caused the mirror to pivot slightly at a certain altitude. I removed the mirror, adjusted the hex nut a fraction of a turb - bingo: acorn nut moved freely, and collimation held stable vs altitude wink.gif

I also point to the horizon, push the mirror forward on its sling, let it settle back onto the pads, the point to a mid altitude before collimating.

niallk, can you explain which acorn nut you are talking about? i would like to check mine too. thanks


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#21 niallk

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Posted 13 October 2021 - 02:43 PM

niallk, can you explain which acorn nut you are talking about? i would like to check mine too. thanks


Sure: so on my 15" Obsession mirror cell, there are 3 triangles which 'float' - ie free to pivot in any direction.

I pointed horizontal and pushed the primary mirror from the rear to swing forward on the sling. Then I waggled each of the 3 triangles to check if they were freely pivoting... and one wasn't. The pivot point is near the centre of the triangles, and there's an acorn nut which is drilled and tapped if my memory is correct. Basically one of the triangles was tightened down on this central pivot point just a tad too much, and a small adjustment made it move freely as intended! It did take removing the mirror to make the adjustment but otherwise simple!

I think the weight of the mirror shifting more towards the sling from the triangles caused a small shift in the mirror position when moving from zenith to the horizon when the triangle wasn't moving freely. With the adjustment, all was good 👍
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#22 MitchAlsup

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Posted 13 October 2021 - 04:33 PM

With the acetal ones it’s as if the mirror is sitting on ice…

Using Ball transfers it IS sitting on ice !!

 

Both the 6-point back support and the 4-point (90º) edge supports are ball transfers in my 13" F/3.

 

You can grab the mirror and spin it around like a lazy Susan.

 

I used the smallest 10mm (steel) balls for this telescope.

 

I used these:: https://www.aliexpre...-8c057e45e73c-0

Essentially zero friction on the back side of the mirror (close to 20× better than HDPE plastic which is something like 100× better than felt pads)


Edited by MitchAlsup, 13 October 2021 - 04:42 PM.

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#23 Bob4BVM

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Posted 13 October 2021 - 04:47 PM

This is primarily recommended by D. Kriege.

With acetal, you again have a mistake, its temperature coefficient of expansion is> 25 times than Pyrex (like with any other plastic).
Felt is more glass-friendly in that it has an advantage over any rigid material at the moment of impact.
My question remained unsolved, the connection between felt and astigmtism has not been proven by anyone. In any case, in the range of sizes and thicknesses of the mirrors under discussion.

 

 

You don't seem to understand the floating support principle.
Rigid support (plastic), for each pads there will be one random point within the pads size. Above you wrote about PLOP, so this is just that case of inconsistency smile.gif

Differetial expansion, you are right there... thats why we use a material that provides a very slippery contact with glass, so the pad does not try to drag the mirror along when it expands, so not a 'mistake', we plan it that way.

 

BTW i actually tried sliding a felt pad vs my plastic ones on the bottom of my mirror.   I don't know what kind of magic felt you are using, but in my test there was no comparison- like Kunama said, the plastic slides like wet ice on the glass, the Felt had to be pushed. Same upward force applied on both.

 

And i cancel the 'random point within the pads' problem by surface-sanding the triangles in situ, until all pads on all triangles show flat & even surface area, so each pad has perfectly even contact to mirror back.


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#24 Bob4BVM

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Posted 13 October 2021 - 05:01 PM

Using Ball transfers it IS sitting on ice !!

 

Both the 6-point back support and the 4-point (90º) edge supports are ball transfers in my 13" F/3.

 

You can grab the mirror and spin it around like a lazy Susan.

 

I used the smallest 10mm (steel) balls for this telescope.

 

I used these:: https://www.aliexpre...-8c057e45e73c-0

Essentially zero friction on the back side of the mirror (close to 20× better than HDPE plastic which is something like 100× better than felt pads)

Ah Mitch, you might want to check on that "HDPE plastic which is something like 100× better than felt pads)" factoid. 

Obviously you have not tried the new and improved "magic felt"  smile.gif

 

Also those rollers are a slick idea for sure, wish i had seen them sooner. 

BUT... they have a little problem--- they are steel balls so you will need to cover the ball surface with a felt pad to avoid damaging your mirror in transit. 

If that is not acceptable then you are just going to have to limit any vehicle jumping to < .457 meters, according to my rough calculations.

 

smile.gif

Bob


Edited by Bob4BVM, 13 October 2021 - 05:05 PM.

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#25 a__l

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Posted 13 October 2021 - 05:38 PM

And i cancel the 'random point within the pads' problem by surface-sanding the triangles in situ, until all pads on all triangles show flat & even surface area, so each pad has perfectly even contact to mirror back.

In the real world, nothing is perfect. On solid surfaces, there will still be one random contact point.


Edited by a__l, 13 October 2021 - 05:48 PM.



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