Primary Mirror Clips

[Anthony RS]

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

[Sunshine]

The clips should be tight enough so the mirror doesn’t flop around, yet not so tight as to pinch the mirror which deforms the light path. Use your best judgment when turning the screws, tighten them incrementally until they’re all just snug enough to prevent movement under normal use.

[John]

It's said that you should be able to slide a piece of thin card between the clips and the primary mirror surface. Not to be actually tried of course but it illustrates that the clips really don't need to be tight at all.

 

[Anthony RS]

5 minutes ago, John said:

It's said that you should be able to slide a piece of thin card between the clips and the primary mirror surface. Not to be actually tried of course but it illustrates that the clips really don't need to be tight at all.

 

Ok but should they be touching the mirror? I mean if I don't tighten the clips but tighten them just enough so they barely touch the mirror to prevent it from tilting and to prevent any slop, would that be fine? 

[Merlin66]

Anthony, 

No. The clips are there to stop an unfortunate accident happening. They prevent the mirror from falling out.

The mirror should be resting on the rear supports and gently held by the edge supports - only.

(Don't turn the telescope up side down!)

 

[Anthony RS]

7 minutes ago, Merlin66 said:

Anthony, 

No. The clips are there to stop an unfortunate accident happening. They prevent the mirror from falling out.

The mirror should be resting on the rear supports and gently held by the edge supports - only.

(Don't turn the telescope up side down!)

 

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?

[Merlin66]

Sounds like the radial (edge) supports are not restraining the mirror.

Are there any adjustable stops around the edge of the mirror cell??

 

[Anthony RS]

2 minutes ago, Merlin66 said:

Sounds like the radial (edge) supports are not restraining the mirror.

Are there any adjustable stops around the edge of the mirror cell??

 

I don't know what that means ? I'll upload a couple of photos in a bit

[John]

What scope are we discussing ?

 

[Anthony RS]

2 minutes ago, Anthony RS said:

I don't know what that means ? I'll upload a couple of photos in a bit

 

[Anthony RS]

1 minute ago, John said:

What scope are we discussing ?

 

Celestron's 8 inch newtonian f5

[John]

It's the same as the Skywatcher Explorer 200P - made by the same manufacturer.

I wonder if the mirror cell is moving around rather than the primary mirror itself ?

I've owned a couple with this design and they hold their collimation well usually. I generally collimate with the scope at around a 45 degree angle.

[Anthony RS]

Just now, John said:

It's the same as the Skywatcher Explorer 200P - made by the same manufacturer.

I wonder if the mirror cell is moving around rather than the primary mirror itself ?

I've owned a couple with this design and they hold their collimation well usually. I generally collimate with the scope at around a 45 degree angle.

The cell looks sturdy enough. It's definitely the mirror moving.

[Merlin66]

Anthony,

the black blocks which have the front clips at the mirror surface, usually also gently touch the side of the mirror to prevent it sliding within the cell.

I don’t know your particular telescope, but I hope another user can comment.

 

[Anthony RS]

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...

[Anthony RS]

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.

 

[neil phillips]

On 07/12/2018 at 00:11, Anthony RS said:

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.

 

As was mentioned earlier, with that amount of movement it was highly likely the edge support wasnt working too well. The edge has to be held in place. Not just the the top. Both should be loose so you can turn the mirror in its cell. But not so loose that when you make small movements of the scope. the collimation goes way out. its a fine line. That often takes some tweaking to get right. Do a out of focus star test, and make sure the out of focus star looks circular. You should easily tell when its pinching. The out of focus star image will look deformed. In that case. something is pushing down too tightly either the clips on the top. Or the pressure from the clips on the side.

[Craig a]

To be honest I’ve always doubted not having the clip touching the mirror as the scope moves through the sky collimation would change, on my 130pds I’ve recently siliconed the mirror to its cell well not silicone but grip bond pro, it’s an adhesive but stays abit flexible for thermal expansion, I put the clips back in but not touching the mirror as a fail safe but to be honest grip bond pro shouldn’t fail, the mirror is firmly in its place, I used penny’s as spacers between the mirror and cell and took them out after it was cured, I’ve read arguments for and against this Meathod but the mirror will not move in its cell now so I’m happy 

Edited October 10, 2021 by Craig a