Dobsonian mirror can move in cell?

[Ratlet]

I had the mirror out of the Stellalyra so I could put some spacers between the cell and the fan (just felt it was too close to do much good) and loosen the mirror clips.

When I put it back together I noticed that there was a fair bit of play between the mirror and the cork pads on the cell in the X/Y axis.  I seem to remember that my 130pds was a snug fit so just want to check if this is anything to be concerned about or needing rectified.  Can't imagine the mirro would love much during a session and I normally use a barlowed laser at the start of a session.

[bosun21]

When you reassembled it you should place say a credit card in between the mirror clips and the mirror and tighten the clips until it starts to grip the card. Just tight enough so that you can still pull the card out but feel the friction of the clamp. Otherwise the mirror will move when slewing from a low target to one near the zenith.

[jetstream]

The mirror must not be restrained or really strange things happen like intermittent astig

[NGC 1502]

The mirror must have a tiny amount of slack.  With due respect to popular opinion, a credit card is a bit too thick for measuring how close the mirror surface is from the clips. I’d suggest something much thinner is sufficient, whilst still leaving a tiny gap.  
 

This works for me-  A piece of stiff paper.  For radial movement, gaffer tape or similar can be used to fine tune the tiny gap needed between mirror and side restraints or cell.

[Piero]

Agree with the posts above. The mirror shouldn't be squeezed or locked by its supports.

The original mirror cell of my 12" f6 dob (Lukehurst classic) did squeeze the mirror and caused severe astigmatism and spherical aberration. The telescope couldn't be used above 150x and it showed poor images below that threshold too. The back supports were also causing issues and the sling was incorrectly installed aside from being too much oversized.

 

The horror above was replaced by a mirror cell I made based on Kriege's design and now those issues are all gone. The mirror cools down much faster too.

The redesigned mirror cell above (3rd photo) was just assembled when the photo was taken. The Glatter' sling cable was shortened after the mirror was added to the cell. In the 4th photo the dark circular area above the mirror is the mirror lid which I leave it there when the telescope cools down due to cats in the area (and they love boxes..!).

BTW the structure was redesigned too.

 

6-point mirror cells are easier to make and they work as well as a correctly implemented 9-point mirror cell. If you need more points, it's better to step up from 6 to 18, skipping 9 completely. Besides, a 6 can be transformed into an 18 if really needed. My 12" f6 mirror is quite thick (37mm) and I have no issue with 6 points.

Edited April 9, 2023 by Piero

[PeterStudz]

On 09/04/2023 at 07:50, NGC 1502 said:

The mirror must have a tiny amount of slack.  With due respect to popular opinion, a credit card is a bit too thick for measuring how close the mirror surface is from the clips. I’d suggest something much thinner is sufficient, whilst still leaving a tiny gap.  
 

This works for me-  A piece of stiff paper.  For radial movement, gaffer tape or similar can be used to fine tune the tiny gap needed between mirror and side restraints or cell.

I agree with this. When I put the primary back in my Dob I initially used a credit card in order to set the distance. However, when I tried to collimate the primary was so slack that I spent at least 45 frustrating minutes chasing the donut (it was impossible to collimate) before I worked what the problem was. 

I then used some thick/stiff paper which did the trick. 

[Ratlet]

58 minutes ago, PeterStudz said:

I agree with this. When I put the primary back in my Dob I initially used a credit card in order to set the distance. However, when I tried to collimate the primary was so slack that I spent at least 45 frustrating minutes chasing the donut (it was impossible to collimate) before I worked what the problem was. 

I then used some thick/stiff paper which did the trick. 

I used card for the mirror clips.  I suspect that the popular opinion got morphed from a business card to a credit card as business cards fell out of favour.

For the supports round the side I shimmed it out slightly so that it has slightly less freedom.  I got the scope in and left it for 24 hours so the mirror was warm.  Measured the gap at about 2mm with the mirror warm.  Shimmed it down to about 0.5mm whilst warm.

[niallk]

My mirror is on a sling with a 9 point floating cell.  I think what's important is that the mirror doesn't shift in use - after collimation (and i collimate every time i set up).  I point my scope at the horizon, lightly push the mirror off the floating cell and let gravity take care of the mirror sitting centred in the sling, gently let it back on to the floating cell, then proceed to point at say 45° and collimate.  I do this after the scope has been out for cooldown, and I'm about to start observing.

In use, I move my scope smoothly and gently, and aim to set up on reasonably level ground.  My scope has 'felt' pads on the 9 contact points, so there's non-zero friction, which helps to suppress slip shift.

The actual mirror clips are like a cm from the mirror, not a fraction of a mm 🙂, and only serve to stop the mirror from falling out if the scope was on its side for transport or something...  so yep, the mirror should be loose!

Edited April 18, 2023 by niallk

[Carbon Brush]

I'm surprised no mention has been made of the temperature when checking, and the materials used in the mirror cell.

Assuming a mirror glass has zero expansion with temperature. It is only the metal cell we need think about.
Thinking about a 300mm diameter mirror, maybe 30mm thick.
If the cell is made from (expanding with heat) metal, it needs to have a gap around periphery at room temperature, so it closes to near enough zero when cold.
If the gap isn't there, the mirror is pinched when cold.

Taking an aluminium cell, it needs to be about 300.14mm to hold a 300mm mirror at 20C, then grip with zero clearance at freezing point.
In other words something like 0.14mm around the periphery indoors will be good for a cold UK night. That is 0.07mm on each clip, regardless of the number of clips.
This sort of number is business card, recycled xmas card, rather than credit card.
Obviously a smaller mirror can have lower clearance. Or if you set up the spacers when the scope is already cold, you can use a smaller clearance.

Cast iron has about 1/2 the expansion of aluminium so requires less clearance.
I haven't looked up aluminium alloys, or steel. But I don't see the numbers being hugely different.

If you have thin cork, or similar squashy material holding the mirror edges, if you lose the thermal clearance, then initially the cork/sponge squashes in preferences to the glass deforming.

As for the screws holding the mirror front to back. Being only around 30mm long, the numbers above can all be divided by 10. The pads are to prevent mirror scratching.
In other words a mirror has bit of side to side movement when checked indoors, but almost no front/back play.

Just my uneducated and ill iformed thoughts🤔

David.