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Help regarding primary mirror clips

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#1 Astrohoven

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Posted 06 December 2018 - 02:48 PM

Hello!

 

I just checked my Newtonian's primary mirror clips and they were really tight (from factory). I loosened them a bit (fits a business card or a bit less) but I noticed when I move the mirror from facing the ground to facing the ceiling, the mirror is moving inside the cell ( towards the ground it's touching the clips, and towards the ceiling it's not) which if I'm not mistaken could lead to tilting the mirror and losing collimation when using the scope on the field? 

Is that normal or should I tighten the clips a bit more? From what  I read, the mirror should be able to rotate inside its cell so maybe I should tighten the clips just so the mirror can still rotate but not tilt (up & down) when it moves?

Appreciate any help on the subject.

 

Cheers!

-Anthony



#2 wrnchhead

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Posted 06 December 2018 - 02:51 PM

I will be interested to know about this also. I asked this question a few months ago but never got an answer. Inquiring minds want to know! 



#3 Starman47

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Posted 06 December 2018 - 02:54 PM

It may prove helpful in crafting a response to know what telescope you have and to see a picture of the mirror cell, clips and mirror.


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#4 sg6

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Posted 06 December 2018 - 03:01 PM

Try this: http://www.loptics.c...rorsupport.html

 

It is one of those areas where there is no real right or wrong. You want the mirror held, but also no pressure to cause deformation, and I suspect you cannot have "held" and "no pressure".

 

It used to be that you tightened up until you could just slip something like a single thickness of tissue paper between mirror and clamp. Expect the principle being the mirror could move but no more then the thickness of the tissue paper. So almost nothing, or so small to be irrelevant.


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#5 havasman

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Posted 06 December 2018 - 03:04 PM

Anthony, you did the right thing. Mirror clips should never be tight. There should always be some play in the system and the mirror should me able to move.


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#6 havasman

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Posted 06 December 2018 - 03:12 PM

Try this: http://www.loptics.c...rorsupport.html

 

It is one of those areas where there is no real right or wrong. 

To quote the Lockwood article you reference, "Do not ever "pinch" a mirror in a support.  It should NOT be snug - there should always be slight clearance around it in all directions so that it can move slightly in the cell.  To verify this, make sure the mirror "rattles" in the cell."

 

That's a pretty clear declaration of what's right and wrong. I agree completely with the article and always check to be sure my mirrors can move in the cell when I collimate at the start of a session. The clips are to keep the mirror from falling out. I think Orion and others ship them tight because they think the risk of shipping damage is lessened and because they know many/most of their customers will never notice a pinched mirror.



#7 Astrohoven

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Posted 06 December 2018 - 03:13 PM

Anthony, you did the right thing. Mirror clips should never be tight. There should always be some play in the system and the mirror should me able to move.

But wouldn't that mess up the collimation? 



#8 Astrohoven

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Posted 06 December 2018 - 03:15 PM

To quote the Lockwood article you reference, "Do not ever "pinch" a mirror in a support.  It should NOT be snug - there should always be slight clearance around it in all directions so that it can move slightly in the cell.  To verify this, make sure the mirror "rattles" in the cell."

 

That's a pretty clear declaration of what's right and wrong. I agree completely with the article and always check to be sure my mirrors can move in the cell when I collimate at the start of a session. The clips are to keep the mirror from falling out. I think Orion and others ship them tight because they think the risk of shipping damage is lessened and because they know many/most of their customers will never notice a pinched 

I understand that the mirror should be able to rotate inside the cell but tilting doesn't make sense to me! I mean this will really mess up the collimation.



#9 havasman

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Posted 06 December 2018 - 03:26 PM

I noticed when I move the mirror from facing the ground to facing the ceiling, the mirror is moving inside the cell ( towards the ground it's touching the clips, and towards the ceiling it's not) which if I'm not mistaken could lead to tilting the mirror and losing collimation when using the scope on the field? 

The mirror edge should rest (depending on design) on a sling, cable, post, roller or other stop when it is in use. There is really no likely case in which the mirror should rest on the clips while in use. It would be pointing at the ground instead of the sky. The mirror edge stops are designed to locate the mirror optimally in the optical path. As above, the clips are only to keep the mirror from falling out of the cell if the mirror cell is tilted unnaturally forward for some reason or bumped hard in transit.

 

The mirror cell will be designed with rear support for the mirror and that will hold much of the weight, depending on pointing angle. That locates the mirror longitudinally in the optical path so that the focal plane remains predictable.

 

In my Newts: the 16" mirror clips are @ 5mm (3/16") clear of the mirror surface; so is the 10". Collimation is rock solid at all pointing angles throughout a session.


Edited by havasman, 06 December 2018 - 03:36 PM.

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#10 Mike G.

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Posted 06 December 2018 - 03:27 PM

in my 12" Lb, I had to add some side shims to the cell to take up the difference between the I.D. of the cell and the slightly smaller O.D. of the mirror.  you can still spin the mirror in the cell, but there is no apparent 'slop' now between the cell diameter and the mirror diameter.  I have seen others drill and tap holes in the side of the cell then use nylon screws to just touch the side of the mirror, preventing it from shifting while going from vertical to near horizontal.  2 screws, touching the mirror sides at 45 degrees from bottom dead center.  too much work for me, I just use some thin plastic shims at the same positions.  for the mirror clips, I bring them down carefully until they just touch the mirror without any pressure on the surface.  fingers only for tightening the screws, and again, not even tight!  on the LB and my 8", the clips have 2 screws each with a rubber piece that actually contacts the mirror.  I bring the screws down evenly, wobbling the rubber piece back and forth as I do.  when I can tell the rubber is contacting the surface of the mirror - but I can still move it back and forth easily, I stop.  that's enough to keep the mirror from falling forward at low altitudes but there is no pressure on the mirror.  if you get them too tight - and it is very easy to do - you will get some whacky star patterns - Pinched optics! primary mirrors need to be loose, but not sloppy.



#11 kfiscus

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Posted 06 December 2018 - 03:45 PM

You can add paper shims around the edge and fold them to add thickness.  You want to find that happy place between the mirror being loose and unable to move.


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#12 Astrohoven

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Posted 06 December 2018 - 05:10 PM

Ok so I just did a simple test. I'm using a barlowed laser collimator to collimate the primary. I collimated on a flat service perfectly and while leaving the laser on slewed the scope to about 45 deg. and the collimation just went way out. This is definitely not how it should be. I can even feel the mirror moving. The clips are almost touching so if I tighten them more I guess that would be too tight? 

 

What should I do?



#13 PETER DREW

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Posted 06 December 2018 - 05:42 PM

Packing the mirror with shims as already recommended would sort it out.

#14 Astrohoven

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Posted 06 December 2018 - 06:16 PM

I just added some shims and the mirror looked fine; was still able to rotate it. Collimated and the same thing happened. It gets out of collimation when i slew to about 45 degrees. When i slew back to its original position, it remains out of collimation until i slew downwards, the doughnut is back in the middle. There's definitely something moving. I'll check with the Cheshire collimator if it shows the same result.


Edited by Astrohoven, 06 December 2018 - 06:16 PM.


#15 Astrohoven

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Posted 06 December 2018 - 06:30 PM

If I use the laser with the barlow, the same thing happens but what's interesting is that the dot on the primary doesn't move when i move the scope which I think eliminates any slop in the focuser, laser, adapters... which means the primary mirror or the cell is the issue...


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#16 Starman47

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Posted 06 December 2018 - 06:57 PM

You may be correct about the focuser etc. But do not be too sure just yet. It is possible that the focuser or some other part is moving a very small amount,which may not be easy to see as laser dot movement in the primary. Just check that very carefully. I know that this is possible from personal experience. I found out the the focuser had small bit of movement that went away when I tightened the focuser brake/lock bolt. I discovered this in the return laser beam on the tublug. When I tightened the brake, the movement of the return laser beam stayed almost 100% still. 

 

Good luck



#17 Astrohoven

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Posted 06 December 2018 - 07:11 PM

Major Update: I removed the mirror and tightened the clips till they touched the mirror basically. Tried collimating again and the collimation didn't even budge a hair. So I guess I'll have to experiment to see how much can I loosen the clips without messing  the collimation and pray that the clips don't cause any pinching... But that makes me wonder, every single post or site mentions how the clips shouldn't be tight and shouldn't be touching the mirror and that they're there just to keep the mirror from falling but that doesn;t seem to be the case here; and I wonder how many of u guys tried to do the test I've done which is to move the scope around while collimating and check what happens. I'd be interested to hear the results.



#18 Starman47

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Posted 06 December 2018 - 08:15 PM

You may want to try a star test, if the clips are too tight, then you will probably have astigmatism showing in the star test.


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#19 havasman

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Posted 06 December 2018 - 08:45 PM

Major Update: I removed the mirror and tightened the clips till they touched the mirror basically. Tried collimating again and the collimation didn't even budge a hair. So I guess I'll have to experiment to see how much can I loosen the clips without messing  the collimation and pray that the clips don't cause any pinching... But that makes me wonder, every single post or site mentions how the clips shouldn't be tight and shouldn't be touching the mirror and that they're there just to keep the mirror from falling but that doesn;t seem to be the case here; and I wonder how many of u guys tried to do the test I've done which is to move the scope around while collimating and check what happens. I'd be interested to hear the results.

At what vertical pointing angle are you doing your initial collimation?



#20 bbbriggs

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Posted 06 December 2018 - 08:53 PM

You might want to check your spider vanes and secondary too. I have had to tighten mine on occasion.


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#21 Astrohoven

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Posted 07 December 2018 - 06:23 AM

At what vertical pointing angle are you doing your initial collimation?

Parallel to the ground.



#22 Astrohoven

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Posted 07 December 2018 - 06:25 AM

You might want to check your spider vanes and secondary too. I have had to tighten mine on occasion.

They are tight. As is said before, i highly doubt it's anything other than the primary mirror (If I use the laser with the barlow, the same thing happens but what's interesting is that the dot on the primary doesn't move when i move the scope which I think eliminates any slop in the focuser, laser, adapters... which means the primary mirror or the cell is the issue...) plus the movement stopped when I tightened the clips.



#23 spencerj

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Posted 07 December 2018 - 09:20 AM

Ideally, you would want to dial in the collimation at about 60*.  If the mirror can tilt at all in the cell, collimating with the OTA parallel to the ground is just about the worst case scenario (outside of the scope being upside down) and will not give you good results when you move to a normal viewing position where the mirror has settled into its normal position.


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#24 Astrohoven

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Posted 07 December 2018 - 09:26 AM

Ideally, you would want to dial in the collimation at about 60*.  If the mirror can tilt at all in the cell, collimating with the OTA parallel to the ground is just about the worst case scenario (outside of the scope being upside down) and will not give you good results when you move to a normal viewing position where the mirror has settled into its normal position.

Ideally the mirror shouldn't tilt in the first place imo. Even if I collimate at 60 degrees, the collimation will be ruined while balancing the scope for example or slewing to a higher or lower target. All this doesn't make sense to me. I''m using a coma corrector so the slightest miscollimation might ruin the images which is why I really take my time in collimating perfectly before an imaging session.I decided to tighten the clips a bit more. The mirror can barely rotate inside the cell so I think they're loose enough but not too loose to mess up collimation. I won't be able to find out if this will cause any astigmatism anytime soon due to the terrible weather. 



#25 spencerj

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Posted 07 December 2018 - 09:43 AM

Sure, but it is not like you are dealing with an Aurora Precision Mirror cell.  You need to work with what you have and in this case, that means choosing the compromise that works best for you.  My personal opinion is that astigmatism from the mirror clips has a more adverse effect on the final image in the scope than a bit of collimation drift. 

 

In the field, you can deal with collimation drift.  You can't do anything about astigmatism.  Gravity is your friend, let it help hold the mirror in place.  And worst-case scenario, you could always do a final touch-up on collimation once you are pointed at and tracking the target you want to image.      


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