OOUK Primary Cell Flop

Equipment Reflectors Help Needed
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andrea tasselli avatar
andrea tasselli avatar
I recently retook use of my old 300mm f/4 from OOUK (Orion Optics UK). Originally (2008? I bought it used for a fair price) the scope had a 3-point support cell as typical for scope of certain vintage (or cheaper ones these days). I decided early on to buy the OOUK vaunted 9-point floating primary cell just to be sure that my mirror would be properly supported when used for AP. To be honest I can't recall having seen much of a treefoil with the original cell, so maybe my concerns weren't justified. The new primary cell is the one shown in the link below:

https://www.orionoptics.co.uk/mirror-cells/

At any rate, earlier in the year I reinstalled everything back together after a proper clean-up and tested the thing during the summer. It became apparent that there was some issue with focus shift over the course of a session but due to other issues (namely, not enough of night-time), it wasn't clear at the time whether it was due to the metal tube contracting (in the very hot summer of this year)  during cool-down or what else. Eventually I re-did the tests in the past few days and it has become apparent that the issue is with mirror flop. Contacted OOUK support to try and see whether there are any instructions about installing it and it very quickly turned out to be a VERY VERY BAD experience and one I'm not inclined to have in the future.

And this bring to the question I have for those having a similar cell and installed it or dealt with issues with it:

What is the right way to mount the primary mirror in a 9-point floating cell?

All tips gratefully received.
Dale Penkala avatar
Dale Penkala avatar
andrea tasselli:
I recently retook use of my old 300mm f/4 from OOUK (Orion Optics UK). Originally (2008? I bought it used for a fair price) the scope had a 3-point support cell as typical for scope of certain vintage (or cheaper ones these days). I decided early on to buy the OOUK vaunted 9-point floating primary cell just to be sure that my mirror would be properly supported when used for AP. To be honest I can't recall having seen much of a treefoil with the original cell, so maybe my concerns weren't justified. The new primary cell is the one shown in the link below:

https://www.orionoptics.co.uk/mirror-cells/

At any rate, earlier in the year I reinstalled everything back together after a proper clean-up and tested the thing during the summer. It became apparent that there was some issue with focus shift over the course of a session but due to other issues (namely, not enough of night-time), it wasn't clear at the time whether it was due to the metal tube contracting (in the very hot summer of this year)  during cool-down or what else. Eventually I re-did the tests in the past few days and it has become apparent that the issue is with mirror flop. Contacted OOUK support to try and see whether there are any instructions about installing it and it very quickly turned out to be a VERY VERY BAD experience and one I'm not inclined to have in the future.

And this bring to the question I have for those having a similar cell and installed it or dealt with issues with it:

What is the right way to mount the primary mirror in a 9-point floating cell?

All tips gratefully received.

Hello Andrea,
I’m slightly confused with what you mean “Right Way” to mount the mirror. Are you referring to the direction/orientation of the 3 pads on the triangular support or is there slop between the edge of the mirror and clip? I modified all my DBA Certified cells and I have the same 9 point floating support pads on my 12” cell. Mine is the GSO brand but still uses the same type of support system. I took a picture of the back of my mirror cell to show orientation of the 3 triangular pads. I don’t know if this will help but here you go.

Dale

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Dale Penkala avatar
Dale Penkala avatar
I noticed after I posted this that the pad at the 2 o’clock position is orientated off a bit (I’ll have to fix this) Not sure how that happened but all 3 pads should look like the other 2 in this pix. 

Dale
Matthew Proulx avatar
Matthew Proulx avatar
The right way seems to be the widest points to the outside. But in pictures oouk don’t seem to follow any standard themselves. 

I just sold my 12” ontc with oouk mirror cell and mirrors. Horrible cell design, horrible mirror shape (tapered and ribbed edge) my mirror would tip and flop around no matter how tight or loose it was. I bought a custom aurora precision mirror cell and it was even worse. Without a square edge the mirror would tip at lower altitudes. Tightening the side supports causes it to squeeze up and out against the clips, leaving the clips loose causes it to tip even more. Flipping east to west collimation was never the same.

5000usd later… 

you can find better things to do with your time and money than f around with OOUK stuff.
andrea tasselli avatar
andrea tasselli avatar
Hi Dale,

Thanks for answering. My cell's pads look very much like yours and mounted the same way I'd reckon. What I'm not sure of is whether they are of all the same height or not. Mine can also be adjusted to allow for different mirror thicknesses. Other than that I thought the would have been an easy-peasy installation but, as most often in our line of work, it turned out not to be the case. I might have overlooked something but I don't know what. Lateral support/movement, maybe?
andrea tasselli avatar
andrea tasselli avatar
Matthew Proulx:
The right way seems to be the widest points to the outside. But in pictures oouk don’t seem to follow any standard themselves. 

I just sold my 12” ontc with oouk mirror cell and mirrors. Horrible cell design, horrible mirror shape (tapered and ribbed edge) my mirror would tip and flop around no matter how tight or loose it was. I bought a custom aurora precision mirror cell and it was even worse. Without a square edge the mirror would tip at lower altitudes. Tightening the side supports causes it to squeeze up and out against the clips, leaving the clips loose causes it to tip even more. Flipping east to west collimation was never the same.

5000usd later… 

you can find better things to do with your time and money than f around with OOUK stuff.

Thanks Matthew,

Not very reassuring, alas! And having to deal with their support staff was the worse bit. Never again if I can help it.

My mirror is tapered on the flank just like yours (why on Earth did they do that?) and I feel the issue is exactly the same. The things the puzzles me the most is that with its original 3-point support it never felt and I never noticed any flop. And I used it for visual and high-res planetary imagine so not something I wouldn't have noticed. Maybe I just need to revert back to that and see how it is performing?
Dale Penkala avatar
Dale Penkala avatar
andrea tasselli:
Hi Dale,

Thanks for answering. My cell's pads look very much like yours and mounted the same way I'd reckon. What I'm not sure of is whether they are of all the same height or not. Mine can also be adjusted to allow for different mirror thicknesses. Other than that I thought the would have been an easy-peasy installation but, as most often in our line of work, it turned out not to be the case. I might have overlooked something but I don't know what. Lateral support/movement, maybe?

Hello Andrea,
In my case there is no adjustment for the pads “up or down” they just float and I position them so they are equally “square” to the round edge like to 2 in the 6 & 10 o’clock position. As I mentioned I didn’t see the one in the 2 o’clock position was off before I took the pix.

As for the lateral/sideways support/movement goes, in my case I have a round ring/cell that the mirror sits in. (See attached pix) While there is a little slop between the edge of the mirror and that ring, I still like to try to eliminate as much of that as possible without pinching/binding the mirror so if there is enough room to use a piece of cork (I bought mine from Amazon) and or thin pieces of card stock to take up the room. I try to make it so there is just enough friction between the mirror edge and the cork/card stock shims to minimize the slop/movement. With an 8” mirror it should be very easy to do. My 12” mirror is much heavier and it takes a bit more work then your 8” should.

As a side note: The pix below is a spare cell from my DBA days and is unmodified. From the factory they always used the double stick foam tape to hold the mirror in place mainly for shipping. This caused a lot of problems with pinching and added stress to the mirror which in turn gave bad images from stock mirrors. These were always removed when I modified the cells.

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Dale Penkala avatar
Dale Penkala avatar
andrea tasselli:
Matthew Proulx:
The right way seems to be the widest points to the outside. But in pictures oouk don’t seem to follow any standard themselves. 

I just sold my 12” ontc with oouk mirror cell and mirrors. Horrible cell design, horrible mirror shape (tapered and ribbed edge) my mirror would tip and flop around no matter how tight or loose it was. I bought a custom aurora precision mirror cell and it was even worse. Without a square edge the mirror would tip at lower altitudes. Tightening the side supports causes it to squeeze up and out against the clips, leaving the clips loose causes it to tip even more. Flipping east to west collimation was never the same.

5000usd later… 

you can find better things to do with your time and money than f around with OOUK stuff.

Thanks Matthew,

Not very reassuring, alas! And having to deal with their support staff was the worse bit. Never again if I can help it.

My mirror is tapered on the flank just like yours (why on Earth did they do that?) and I feel the issue is exactly the same. The things the puzzles me the most is that with its original 3-point support it never felt and I never noticed any flop. And I used it for visual and high-res planetary imagine so not something I wouldn't have noticed. Maybe I just need to revert back to that and see how it is performing?

If you can’t get this/new cell working to your liking I’d have to agree with you on that and modify the stock cell if possible.

Dale
Matthew Proulx avatar
Matthew Proulx avatar
andrea tasselli:
Matthew Proulx:
The right way seems to be the widest points to the outside. But in pictures oouk don’t seem to follow any standard themselves. 

I just sold my 12” ontc with oouk mirror cell and mirrors. Horrible cell design, horrible mirror shape (tapered and ribbed edge) my mirror would tip and flop around no matter how tight or loose it was. I bought a custom aurora precision mirror cell and it was even worse. Without a square edge the mirror would tip at lower altitudes. Tightening the side supports causes it to squeeze up and out against the clips, leaving the clips loose causes it to tip even more. Flipping east to west collimation was never the same.

5000usd later… 

you can find better things to do with your time and money than f around with OOUK stuff.

Thanks Matthew,

Not very reassuring, alas! And having to deal with their support staff was the worse bit. Never again if I can help it.

My mirror is tapered on the flank just like yours (why on Earth did they do that?) and I feel the issue is exactly the same. The things the puzzles me the most is that with its original 3-point support it never felt and I never noticed any flop. And I used it for visual and high-res planetary imagine so not something I wouldn't have noticed. Maybe I just need to revert back to that and see how it is performing?

I think you’re always going to have the issue to some degree mainly due to the mirror shape. To me this is such a huge oversight. I would have simply replaced the mirror had it not cost 1600euro. I was not willing to buy another mirror at this point so I washed my hands of it all.
atlejq avatar
atlejq avatar
I am having some annoyances with this type of cell myself. It is not trivial to tighten it enough to avoid flop but not so hard as to avoid pinching. My cell is identical to the link in the OP, however my problem is likely less severe as my setup is a 8".

As in the discussion about pinching in the Skywatcher cell, I believe the aluminium to be one of the culprits here. It has a relatively large thermal expansion/contraction. So I am starting to wonder whether one should adjust the cell at a typical operating temperature and not when the scope is at room temperature.

Are there anybody here who has experience with the similar cell from TS?

https://www.teleskop-express.de/shop/product_info.php/info/p9621_TS-Optics-ONTC-Hauptspiegelzelle-fuer-8--Newton-Spiegel.html

This one uses cork pads instead of nylon(?) studs which should in theory be an inferior material according to this post

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

The TS cell has slightly different ways of adjusting lateral and vertical tightness, however. Maybe that makes a difference. The cork pads are likely more flexible than the plastic studs. They also have a somewhat different geometry.

A general comment to this thread: If both the mirror and the cell are of potentially sub-par quality, it would maybe make sense to test the cell with a mirror known to have straight edges?
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andrea tasselli avatar
andrea tasselli avatar
I am having some annoyances with this type of cell myself. It is not trivial to tighten it enough to avoid flop but not so hard as to avoid pinching. My cell is identical to the link in the OP, however my problem is likely less severe as my setup is a 8".

As in the discussion about pinching in the Skywatcher cell, I believe the aluminium to be one of the culprits here. It has a relatively large thermal expansion/contraction. So I am starting to wonder whether one should adjust the cell at a typical operating temperature and not when the scope is at room temperature.

Are there anybody here who has experience with the similar cell from TS?

https://www.teleskop-express.de/shop/product_info.php/info/p9621_TS-Optics-ONTC-Hauptspiegelzelle-fuer-8--Newton-Spiegel.html

This one uses cork pads instead of nylon(?) studs which should in theory be an inferior material according to this post

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

The TS cell has slightly different ways of adjusting lateral and vertical tightness, however. Maybe that makes a difference. The cork pads are likely more flexible than the plastic studs. They also have a somewhat different geometry.

A general comment to this thread: If both the mirror and the cell are of potentially sub-par quality, it would maybe make sense to test the cell with a mirror known to have straight edges?

As the OP here this is what I have gathered so far:

1. Returning the mirror to the original 3-point design seems to do the trick, once the holding clips have been tightened in the right amount. I still get some residual focus shift but not anywhere near the previous iteration. Importantly, I can run for a least an hour before refocusing but this might also be due to thermal settling of the various components under irradiation losses to the night sky.

2. Analysis shows that the amount of tilt is of the order of the focusing tolerance per hour of integration, on average.

3. The mirror, in its original configuration with the old primary cell, undergoes a small rotation and a later shift as the scope pointing changes altitude, around 1' and ~0.25 mm shift, maximum (i.e., for 60 degrees of altitude change).

People with TS ONTC scopes (which have the same cell, at least now) should be able to comment but so far the feedback I have is very positive. I don't think the thin industrial-grade cork pads have much in terms of compressibility and are virtually no expansion or contraction with temperature. Nylong or hard plastic is quite so. I read the article and the issues it highlights are relevant, if ever, to very large thin primaries used in oversized dobsonians which isn't the case for (most) of us anyhow.

I'd avoid tapered edges at any cost, unless you're stuck with one same as me. I toyed with the idea of getting a better mirror cell but I don't think is worth the risk with the wrong mirror. Who on Earth would think about tapering the edge of mirror blank for no gain whatsoever?
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atlejq avatar
atlejq avatar
As the OP here this is what I have gathered so far:

1. Returning the mirror to the original 3-point design seems to do the trick, once the holding clips have been tightened in the right amount. I still get some residual focus shift but not anywhere near the previous iteration. Importantly, I can run for a least an hour before refocusing but this might also be due to thermal settling of the various components under irradiation losses to the night sky.

2. Analysis shows that the amount of tilt is of the order of the focusing tolerance per hour of integration, on average.

3. The mirror, in its original configuration with the old primary cell, undergoes a small rotation and a later shift as the scope pointing changes altitude, around 1' and ~0.25 mm shift, maximum (i.e., for 60 degrees of altitude change).

People with TS ONTC scopes (which have the same cell, at least now) should be able to comment but so far the feedback I have is very positive. I don't think the thin industrial-grade cork pads have much in terms of compressibility and are virtually no expansion or contraction with temperature. Nylong or hard plastic is quite so. I read the article and the issues it highlights are relevant, if ever, to very large thin primaries used in oversized dobsonians which isn't the case for (most) of us anyhow.

I'd avoid tapered edges at any cost, unless you're stuck with one same as me. I toyed with the idea of getting a better mirror cell but I don't think is worth the risk with the wrong mirror. Who on Earth would think about tapering the edge of mirror blank for no gain whatsoever?

Thanks for your reply. I notice that you have a F/4 setup while my mirror has diameter 199.5 mm, focal length 880.9 mm and is used with a Paracorr 2 which has a 1.15x Barlow effect, hence I am at approximately F/5.1. So my setup is probably more tolerant of small misalignments and focus shifts than yours.

When you say "tapered edges", what sort of angular errors are we talking about here?
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andrea tasselli avatar
andrea tasselli avatar
Thanks for your reply. I notice that you have a F/4 setup while my mirror has diameter 199.5 mm, focal length 880.9 mm and is used with a Paracorr 2 which has a 1.15x Barlow effect, hence I am at approximately F/5.1. So my setup is probably more tolerant of small misalignments and focus shifts than yours.

When you say "tapered edges", what sort of angular errors are we talking about here?

Not sure what you are referring to here. If it is the taper it is about 3mm shorter in radius on the bottom side of the mirror (the one facing away from the aperture) than the top side. The height of the mirror is 32mm, if I recall it right.
atlejq avatar
atlejq avatar
andrea tasselli:
Not sure what you are referring to here. If it is the taper it is about 3mm shorter in radius on the bottom side of the mirror (the one facing away from the aperture) than the top side. The height of the mirror is 32mm, if I recall it right.

This was exactly what I wondered about. In my case the mirror seems to have a slightly wider bottom than top (I will measure it precisely the next time I take it apart). It does not have a ribbed edge, the finish looks nice in that respect. So I may be dealing with a slightly different problem.
Tim Hawkes avatar
Tim Hawkes avatar
I have an F 4 300 mm VX12  (Jan 2022 vintage).  It has the 9 point mirror cell design.  I didn't notice a rib or indeed an obvious taper at the mirror edges – it may be the case though that I was unobservant and  will check next time I have the mirror out.   Apart from a slight mirror centering shift  with telescope orientation which was minimised by tightening up the nylon screws at the mirroe edges (but not too tight of course)  I have not noticed any big problem  with respect to focus or holding focus - in fact in terms of FWHM values it has produced  the sharpest images of all of my scopes but that could have been luck of course - good skies and objects nearly overhead  etc.  

Just wondering whether ribbed mirror edges  etc were some peculiarity of older mirrors and whether something has changed in OOUKs mirrors?  

Tim
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atlejq avatar
atlejq avatar
I have been investigating this issue and here is a post I found from 2017, second picture seems to show a 12" mirror with ribbed edge.
https://www.cloudynights.com/topic/585866-a-new-oouk-vx12-optics-issues-star-test/page-3#entry8079468

My 8" mirror fortunately doesn't look like this. 

andrea tasselli avatar
andrea tasselli avatar
I forgot to mention that my 2008 vintage mirror IS sligthly ribbed too, although I don't think this is an issue.
Tim Hawkes avatar
Tim Hawkes avatar
I have been investigating this issue and here is a post I found from 2017, second picture seems to show a 12" mirror with ribbed edge.
https://www.cloudynights.com/topic/585866-a-new-oouk-vx12-optics-issues-star-test/page-3#entry8079468


I think that is just the pattern of a thin tape around the mirror edge  -  not actually ribs ?
atlejq avatar
atlejq avatar
Tim Hawkes:
I think that is just the pattern of a thin tape around the mirror edge  -  not actually ribs ?

You are probably right. Seems like there is some silicone gunk on the triangular floaters also.
andrea tasselli avatar
andrea tasselli avatar
Here are few images of the cell and mirror (300mm f/4) from OOUK (note the tapered side and the waviness as well):

Tim Hawkes avatar
Tim Hawkes avatar
See what you mean Andrea!  That looks odd - my (also OOUK) mirror doesn't look anything like that at the sides ?!
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