Primary Mirror Mounting Advice Please

[PeterBolt]

So last night I decided to take the plunge and clean my primary mirror.  I followed the instructions on the Sky at Night website and it went pretty smoothly and I am pleased with the result.

However, when I came to remove the mirror in the first place I noticed a couple of things 1) the retaining clamps were very slack, approx. 3mm between the foam pad and the mirror surface and 2) the mirror would not rotate as silicone had been used in three positions to hold the mirror in situ.

Now that I have removed the silicone and fastened the retaining clamps using the "business card" method the mirror is very slack.  Mainly in the radial direction as there is a gap between the edge of the mirror and the housing.  This allows the mirror to move from side to side by approx. 2mm.

Can anyone tell me if this is correct? or do I need to make some shims to go around the perimeter of the housing i.e. around the edge of the mirror to keep it centred?

Any help would be appreciated.

[PeterBolt]

Well I decided to make up some shims to prevent 90% of the movement.  I will see how it performs next time out ?

[AstronomyUkraine]

Posted by: @petedevon

So last night I decided to take the plunge and clean my primary mirror.  I followed the instructions on the Sky at Night website and it went pretty smoothly and I am pleased with the result.

However, when I came to remove the mirror in the first place I noticed a couple of things 1) the retaining clamps were very slack, approx. 3mm between the foam pad and the mirror surface and 2) the mirror would not rotate as silicone had been used in three positions to hold the mirror in situ.

Now that I have removed the silicone and fastened the retaining clamps using the "business card" method the mirror is very slack.  Mainly in the radial direction as there is a gap between the edge of the mirror and the housing.  This allows the mirror to move from side to side by approx. 2mm.

Can anyone tell me if this is correct? or do I need to make some shims to go around the perimeter of the housing i.e. around the edge of the mirror to keep it centred?

Any help would be appreciated.

Once the primary is collimated with the adjustment screws, the retaining screws are tightened finger tight only, but you should have no play in the mirror whatsoever. It sounds like someone collimated the scope previously, and forgot to tighten the mirror down, or the primary mirror cell is poorly designed, or not the correct one.

[PeterBolt]

Thanks for the advice.  I have had the scope from new so I guess the design wasn't great in the first place...

[AstronomyUkraine]

Posted by: @petedevon

Thanks for the advice.  I have had the scope from new so I guess the design wasn't great in the first place...

Seems the silicon was put there to stop the mirror from wobbling about. My newtonian has 3 brackets that screw into the primary mirror cell, holding down the mirror, and there is no movement at all.

[PeterBolt]

@AstronomyUkraine I have the same three brackets but I read they should not be too tight otherwise it might pinch the optics?  The movement is between the edge of the mirror and the mirror cell so I am hoping the shims I made will do the trick?

[AstronomyUkraine]

Posted by: @petedevon

@astronomyukraine I have the same three brackets but I read they should not be too tight otherwise it might pinch the optics?  The movement is between the edge of the mirror and the mirror cell so I am hoping the shims I made will do the trick?

Mine are just screwed down until there is no movement, I don't crank them down. Maybe shims will do the trick, or some thin foam, like draught excluder.

[PeterBolt]

Well I am hoping for some clear nights to give it a try ?

[paul]

My 250mm was re-silvered last year.  I was surprised how little clamping there was was when it came to removing it. It has three clip positions, the cell uses M8 nylon bolts to just clamp at each position for minimal side-ways movement. Insulating tape is applied across the back of those clips and on to the mirror's side to prevent rotating in the cell.  The top of each clips had a thin low density pad to stop the mirror flopping forward without any real compression. It does hold collimation very well so the solution works.

 

[PeterBolt]

@phar brilliant thank you I might give the insulation tape a try!