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obsession mirror support fixes/upgrades

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#26 a__l

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Posted 13 October 2021 - 05:47 PM

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

 

Explain for what purpose to do 100 times better? Significantly complicating the design and manufacturing time. I also do not really understand the risks of transporting such a structure. There is a simple and reliable solution that has been tested for many years on mirrors for Obsession.
I won't even consider super-thin fast mirrors. They have enough other problems that inevitably lead to astigmatism.
I have recommended for MitchAlsup. Pay some money and have your telescope autocollimation test on the interferometer done. If you have a super-thin mirror, I guarantee you astigmatism from gravity (and possibly from the manufacturer) that 100 times the margin loses any sense.


Edited by a__l, 13 October 2021 - 05:54 PM.


#27 MitchAlsup

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Posted 13 October 2021 - 05:55 PM

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.

a) my mirror rests on 6 of these things and is constrained by 4 more on the edge.

b) there is nothing between the ball and the glass

c) the mirror can only move 0.004" in any direction before being 100% constrained.

 

d) Mark Cowen states that the ball is not a point source and distributes its load over a 'significant' area.

 

e) over the last 4 years, the mirror has not suffered on its 7 trips from Austin to Prude Ranch in Ft. Davis Tx and back.


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#28 a__l

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Posted 13 October 2021 - 06:01 PM

What about an interferometer? smile.gif

This should be the next stage after building. Which is what I did for myself. Since there is no autocollimation mirror in my environment, I made a bath and roughly imagine what I have. It's in my signature.


Edited by a__l, 13 October 2021 - 06:05 PM.


#29 a__l

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Posted 13 October 2021 - 06:14 PM

By the way, I looked at the link. It's steel! I would never use steel for contact with glass on my 24" (25 kg) mirror as well as on my 18" (18 kg). Why would I need a bunch of broken glass instead of optical mirrors?

 

There is a steel use case from ASA (I posted earlier), but it is only applicable for side support and there is a steel-steel sliding contact.


Edited by a__l, 13 October 2021 - 06:16 PM.


#30 Bob4BVM

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Posted 13 October 2021 - 10:30 PM

Bob said:

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.

 

a) my mirror rests on 6 of these things and is constrained by 4 more on the edge.

b) there is nothing between the ball and the glass

c) the mirror can only move 0.004" in any direction before being 100% constrained.

 

d) Mark Cowen states that the ball is not a point source and distributes its load over a 'significant' area.

 

e) over the last 4 years, the mirror has not suffered on its 7 trips from Austin to Prude Ranch in Ft. Davis Tx and back.

Sorry Mitch, my reply was 100% tongue-in-cheek, guess that doesn't come across clearly here smile.gif

Of course i was kidding about putting felt on the balls...

 

But to continue on the light side... It still makes me wonder how your mirror survived on those trips. apparently you are not taking high enough jumps with your vehicle. I think conventional wisdom here is that you need to launch at just over .5 meter to get breakage results.  Unless of course you are using the magic-felt zero friction pads on your cell  smile.gif In that case yours is safe from all harm.

According to conventional wisdom here... smile.gif


Edited by Bob4BVM, 13 October 2021 - 11:01 PM.

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#31 Kunama

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Posted 13 October 2021 - 11:59 PM

 funnypost.gif funnypost.gif funnypost.gif



#32 Redbetter

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Posted 14 October 2021 - 12:09 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

The main problem I saw with edge support was with my 20" was the original thin manila colored sling.  It suffered some abrasion on the upper portions from lots of transport, and that along with getting it positioned just right, could pose problems for edge support.  In some positions it shifted the mirror tilt and therefoer collimation when the scope was pointed low.  I ordered a new sling from Kriege and it has worked properly.  It is thicker and dark gray/green. I changed the way I transport it, dropping the sling and using pipe insulation on the three posts to keep the mirror from sliding back and forth while traveling.

 

The problem that remained was when set up on unlevel ground.  If the ground is not reasonably level, the mirror sling will swing slightly to the low side when rotating in azimuth.  (By the way, slicker pads will only make the sling more sensitive to side-to-side shift.)  This is just the nature of this type of edge support.  Of course, the answer to that is to roughly level the scope, if set up on a slope.  I have found this to work well, even though I just eyeball it.  One of my sites has more incline than others (where I have relatively flat spots to use), so I slip a flat rock under one of the three ground board contact points to compensate.  


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#33 MitchAlsup

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Posted 14 October 2021 - 12:50 PM

Sorry Mitch, my reply was 100% tongue-in-cheek, guess that doesn't come across clearly here smile.gif

Of course i was kidding about putting felt on the balls...

 

But to continue on the light side... It still makes me wonder how your mirror survived on those trips. apparently you are not taking high enough jumps with your vehicle. I think conventional wisdom here is that you need to launch at just over .5 meter to get breakage results.  Unless of course you are using the magic-felt zero friction pads on your cell  smile.gif In that case yours is safe from all harm.

I am aware of the "chances" I am taking.

After 4 years, I am finding no actual damage.

 

Perhaps this is because the bearings carry so little weight (2.2 pounds each)

Perhaps it is because the bearings have a non-point contact

Perhaps it is because I have been lucky

 

All I can really say, is that this low friction (in all directions) mirror cell works very well in practice.


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#34 ButterFly

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Posted 15 October 2021 - 01:55 AM

The main problem I saw with edge support was with my 20" was the original thin manila colored sling.  It suffered some abrasion on the upper portions from lots of transport, and that along with getting it positioned just right, could pose problems for edge support.  In some positions it shifted the mirror tilt and therefoer collimation when the scope was pointed low.  I ordered a new sling from Kriege and it has worked properly.  It is thicker and dark gray/green. I changed the way I transport it, dropping the sling and using pipe insulation on the three posts to keep the mirror from sliding back and forth while traveling.

 

The problem that remained was when set up on unlevel ground.  If the ground is not reasonably level, the mirror sling will swing slightly to the low side when rotating in azimuth.  (By the way, slicker pads will only make the sling more sensitive to side-to-side shift.)  This is just the nature of this type of edge support.  Of course, the answer to that is to roughly level the scope, if set up on a slope.  I have found this to work well, even though I just eyeball it.  One of my sites has more incline than others (where I have relatively flat spots to use), so I slip a flat rock under one of the three ground board contact points to compensate.  

It is quite important to get both sides of the cable sling at the right height, and the same height, right along the COG of the mirror.  The back of a mirror is usually a fairly flat (but not polished smooth) reference surface.  The cell should retain the angle just fine.  But if one of those cable ends is higher, it twists the mirror up on one side as it shifts.  That relieves some of the weight holding it in place on that side and there goes the tilts on the floating cell.  The secondary's position will be off from the shift, but if the tilts remain, at least the primary's optical axis is still parallel to the eyepiece's.  Same for when the sling is not at the COG, except it's a twist from top down as alt changes.  A cable sling is almost always NOT at the right place, but it is a matter of tolerances.  After a certain weight point, cell flexure will take you out of tolerance.  Lockwood's article is very helpful: A modern guide to mirror support.  If the problem persists for you, you may want to go whiffletree, but add sling checks to your maintenance schedule first.

 

For my 2" thick 15" mirror, a sling is very tolerant when set up properly.  I used to shift between mirror high and mirror low positions when putting in my filter slider.  I have stopped doing that because those ends would not be at the COG of the mirror.  Every few months, as collimation shifts accrue, I check that.  I have still kept the felt pads on my Obsession.  They do stick from time to time and I jiggle out the astigmatism.  It takes a minute or two to return to normal after a jiggle.  Maintaining the proper sling heights on both ends at the COG lets me do that without having to tweak primary collimation.  I also wheel the thing out and transport it in my trunk without having to turn screws a whole lot at all.

 

My secondary is where the brunt of the collimation occurs.  I love the offset pupil of my autcollimator.  Up down all night, my primary stays put now.  When it doesn't do that is a tip off to check the sling.  My secondary shifts with temperature and low elevation.  I'd have to replace the upper ring of my UC to hone that in, but even with my night vision train, I haven't felt the need yet.  I just tweak the secondary every two hours or so when the temperature drops by a lot over the night.


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#35 Redbetter

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Posted 15 October 2021 - 03:08 AM

I don't need to do checks per se, because I have to position the sling every time I unload and set up.  I know by feel and hundreds of repetitions/experience how and where the sling should sit, uniformly so that the primary is properly supported and doesn't tilt. 

 

But that will not fix the problem of the whole sling/mirror shifting laterally toward the low side when the mirror box turns in azimuth on a slope.  It is pretty easy to see this when looking down at the mirror and observed it has moved relative to the posts.  (It is generally noticed first when the finder is no longer centered in the eyepiece field, or if I see the collimation is off at high power.)  

 

I was told there is no whiffle tree retrofit available for the Obsessions, so I have not pursued it.  My impression is that the whiffle tree has to float with the mirror supports, but the sling system is not set up to do this.  Instead the current collimation layout runs the support cell up and down while the sling is still attached to the frame.  


Edited by Redbetter, 15 October 2021 - 03:10 AM.

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#36 sixela

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Posted 15 October 2021 - 05:19 AM

My impression is that the whiffle tree has to float with the mirror supports, 

 

If you want a no-fuss system. It's possible to put them fixed with respect to the mirror box, but then only one position on the bottom and left/right inclination of the primary in the mirror box is optimal. In practice, you collimate using the optimally placed primary playing with the secondary offset and/or focuser orientation to get the optical axis pointing straight up (which allows you to get the whiffle trees left and right close enough to the plane of the mirror's COG).

 

After this bothersome first collimation that gets the mirror on the right spot, from then on you collimate with one "fixed, do not touch this!" bolt on both the secondary and primary tilt screw sets to avoid straying too far from that optimum when doing axial collimation.

 

The good thing about a whiffle tree is that small placement errors with respect to the COG plane do not matter as much as with a sling with attachment fixed to the mirror box. With a sling, small errors in the height of the mirror with respect to the fixed attachments where the sling starts will introduce forces normal to the mirror's surface fairly rapidly (hence the need for a solution like that of the Howie Glatter sling, which uses linear bearings to auto-adjust these to the mirror's current position).

 

With a whiffle tree if two of the roller bearings are a bit lower or higher they do not directly introduce forces normal to the mirror surface (that's why they are roller bearings), just some minor variations in forces on the back support points. With a scope pointed very very very close to the horizon it may matter because if the roller bearing is too low the mirror will roll to the mirror clips on that side when you reach the horizon, but usually we're talking really close to 0° with the horizon in the FOV! And if you get in that situation it's usually acceptable to raise the mirror just a wee bit on all bolts to avoid it.

 

A couple of millimeters are usually not a problem except if you have a really thin mirror, e.g. a slumped meniscus.

It does mean that adjusting the focal plane placement by moving the mirror forward or back becomes a big no-no, though (I move my mirror up into the tube a couple of cm when I use my binoviewer on my 16" scope -- no such thing on a scope like this). And you do need to check the height of the roller bearings with respect to the mirror every time you collimated, to avoid creeping away from the optimal edge support°.

 

And you will need to number your trusses too, since large collimation adjustments on the primary will typically skew the mirror otherwise.

 

--

°One scope I debugged was damaged in transport --a cell part slightly was slightly bent as a result--, enough to make the position and orientation of the mirror do nasty things to the sling edge support -- we had to redo the initial collimation "from the primary up" in the field under a dark sky. I still weep at the lost photons.


Edited by sixela, 15 October 2021 - 05:22 AM.

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#37 ButterFly

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Posted 15 October 2021 - 05:27 AM

If you're not measuring it to place the sling in the plane of the COG, you have to deal with it.  Lateral shifts on the cell shouldn't be affecting collimation tilts so long as the weight is bearing on the cell - the secondary is flat.  The finder shift is the optical axis having moved over.  Checking collimation against a star should reveal no change in tilt.

 

Changing the location of the supports due to lateral shifts affects figure.  Any residual errors in the placement of the secondary should be taken up with focuser adjustments.  The mirror should go on the support points of the cell.  It can be that after placing the secondary under the focuser, then adjusting tilts, the reflection of the edge of primary is not concentric with the edge of the secondary.  That's a shift.  If the primary is where it needs to be on its cell, relative to the cell supports, it's the focuser angle that must change here.  If it's not severely clipping the primary, it's just vignetting, and barely noticeable visually.

 

Taking off the mirror each time and cable slings is a hard task.  If a whiffletree is out of the question, a simple guide can help a lot.  Measure once to place the sling.  Cut out an L bracket with soft material that you can felt line.  Put the bottom under the mirror and lay the cable along the top of the bracket as you slide it along the edge.  That's better than guesswork.  If you dare velcro the cable in place along the edge, alternatively, leave a channel on the mirror side to receive the cable as you lay it.


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#38 a__l

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Posted 15 October 2021 - 05:57 PM

But to continue on the light side... It still makes me wonder how your mirror survived on those trips. apparently you are not taking high enough jumps with your vehicle. I think conventional wisdom here is that you need to launch at just over .5 meter to get breakage results.  Unless of course you are using the magic-felt zero friction pads on your cell  smile.gif In that case yours is safe from all harm.

According to conventional wisdom here... smile.gif

It depends on the road services, which do not always serve the secondary roads well. When there is asphalt, but there can be a springboard (lowering of the soil due to temperature changes and other reasons). At night it is hard to see and your speed of 100 km / h causes a similar jump.
For Kunama. Actually, I drive through several thousand kilometers on the roads of Australia, there are not always good roads. The Internet connection is much worse than ours, the industry is clearly underinvested.


Edited by a__l, 15 October 2021 - 05:58 PM.


#39 a__l

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Posted 15 October 2021 - 06:29 PM

sixela,

Fix the steel cable to the COG with Velcro. For the mirror in the photo (this is Zambuto), checked telescope on the interferometer in the vertical position of the mirror Astigmatism is minimal smile.gif

I do not raise the mirror this far when using bino. You must lift the upper mirror clips along with the mirror. There is a problem with this. Usually this is an increase in the gap between them and the mirror. The consequence of this is greater acceleration of the mirror during transport jumps.
For binos it is quite possible to use it like this, see photo (my 24").

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#40 a__l

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Posted 15 October 2021 - 06:44 PM

Perhaps it is because I have been lucky

 

A few years ago, the owner of Obsession posted on CN a photo of his mirror after accidentally colliding with a side bolt. I don't know how he did it, but the consequences were deplorable. Such a glass chip, even if it does not come out on the surface of the mirror, has consequences in the form of astigmatism ...



#41 sixela

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Posted 15 October 2021 - 06:53 PM

@a__l,

 

fixing the cable to sit at the COG for most of the way using velcro (which we were already doing for ages) doesn't fix the issue of the start of the sling being out of the plane and imparting forces normal to the surface if the sling is attached to the mirror box that does not move when you collimate the mirror.

See e.g. https://web.archive....sling/sling.htm for the problem.

 

I read a few less back-of-the envelope CAE simulations somewhere but I can't track the references for the moment, but it is a real problem. One that I have seen with my own eyes, and fixed by tweaking the collimation starting from the ideal position of the primary and collimating the rest to it.

 

Of course if the sling is attached to the cell that moves with collimation then there is no problem, but that is not very common and that's definitely not how stock Obsessions are built.

 

Neither is there a problem if you install a Howie Glatter sling with ends on a linear bearing that is perpendicular to the mirror surface. Then you don't care that the posts the linear bearings ride on are on the mirror box and do not move with the mirror, the sling will.


Edited by sixela, 15 October 2021 - 06:58 PM.

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#42 a__l

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Posted 15 October 2021 - 08:43 PM

sixela,

Allow sufficient Velcro distance and do not raise the mirror 2 cm. Adjust the height of the cable support bolts correctly. The movement of the mirror with a well-made design is minimal. Don't use super thin mirrors. You still won't solve problems with them. I mentioned the interferometer above for a reason. There will be no problems. Why complicate entities ... what can be done easier and no worse than linear bearings (Glatter).
Ps. I can find a link to CN where Glatter got started with a demonstration (and justification) of his product. I never move the mirror vertically this distance!


Edited by a__l, 15 October 2021 - 08:46 PM.


#43 the_chemist

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Posted 16 October 2021 - 11:56 AM

thanks again everyone, I really feel like I'm getting an education in optimizing my set up and the pros and cons to the different approaches, and how to measure improvement and deal with the inevitable compromises all these approaches have.

 

Eric



#44 sixela

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Posted 16 October 2021 - 01:24 PM

Allow sufficient Velcro distance and do not raise the mirror 2 cm. Adjust the height of the cable support bolts correctly. The movement of the mirror with a well-made design is minimal.


That is exactly what I wrote; it’s possible to get it right but fussy.

Note that the design needs not only to be well made, but also collimated to avoid left/right skew of the mirror and to raise the mirror to the right spot (keeping it there).

And the users need to be aware of what to avoid too. I know from experience not everyone is aware…

#45 Starman1

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Posted Yesterday, 06:24 PM

If your mirror slides easily on its support points, and you have a wire sling, it is desirable to level the scope when setting up so the mirror does not swing back and forth as you slew.



#46 Starman1

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Posted Yesterday, 06:27 PM

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.

If the 2nd picture has the wire ends attached correctly either the wire should be inside the posts half way down, or those posts should be removed.

Because in a wire sling attachment, the cable needs to contact 180° of mirror from 3 o'clock around to 9 o'clock and the wire should come off each side of the mirror with the ends parallel.



#47 a__l

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Posted Yesterday, 08:17 PM

Holes are drilled in these rods for the steel cable. These rods can move up and down in a certain range, they are fixed to the frame on two nuts each.


Edited by a__l, Yesterday, 08:19 PM.



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