Mirror Movement

[Stu1smartcookie]

Hi , the more i get into this hobby the more questions i ask . I reset my mirror in its cell , and , used cork pads without sticking them to the mirror . This has worked admirably for the last few weeks but last night i had a nightmare whilst collimating . One of the collimattion bolts was having none of it and the mirror on one side had excessive movement . I belive that maybe one of the clips have come loose , investigation will take place tonight . I am also thinking about sticking the supporting triangles to the mirror instead of leaving it "floating" , for the want of a better description . I hate the idea of this but this mirror movement has had an unsettling effect . Im thinking of just introducing double sided tape onto the cork pads . A good solution ? or is it better to leave the pads unfixed to the mirror ?

Also the locking screws are a nightmare ...you spend your time collimating only for those screws to push the mirror zzzz 

By the way , i have installed Bobs Knobs for collimation . 

Stu

[ONIKKINEN]

The mirror in my OOUK 8 inch is fixed to the support triangle with some sort of strong and "plasticy" loosely applied tape around the entire mirror. The mirror does not move on its own because of this but i can move it if i try, so no pinching anywhere. I dont see adding tape in there somewhere creating pinching unless you shim it way too tight.

Consider buying strong springs for your collimation nobs, that way you wont need the locking screws. My mirror cell only has 3 springloaded thumbscrews in it and no locking screws, holds collimation well as long as the springs are tight enough. Might not work on larger mirrors though, but worth a try at least.

[NGC 1502]

From your description you have a multi point mirror cell with triangular supports, each triangular support having 3 contact points using cork pads.

You’ve mentioned using double sided tape on the cork pads.  You can obtain double sided sticky foam tape much thicker than the regular tape. I’ve used that to good effect by not using cork pads but instead two thicknesses of sticky foam to make a thicker pad. This supports the mirror without introducing stress on the glass but holds the mirror with less chance of it shifting around and messing up collimation especially when transporting the scope.

I buy better quality outdoor foam tape rather than from Poundland or similar.

If it were me, I’d dismantle the mirror and cell to investigate. Remove all bolts and run a thread cutting tap through each hole to ensure a clean easy running movement without any snagging. Do similar with all bolts using a thread cutting die. When assembling thoroughly clean the threads and use a very small amount of grease on the threads to ensure smooth snag free movement. As already suggested get some strong springs. Many collimation springs I’ve come across have been too weak. If you can compress them at all with your fingers then they’re useless. Collimation springs can be checked for adequacy before using, it should only be possible to compress them in a vice. If the collimation springs are strong enough then locking bolts can often be dispensed with, or alternatively used just tighter than by hand to give a bit of extra support to hold collimation better.

My first scope was bought second hand in 1979. I quickly learned to hate those pesky locking bolts for the same reasons you’ve mentioned.

Hope you sort it, Ed.

 

[Stu1smartcookie]

Thanks guys , I bought some strong sticky dots to put on the cork pads today .. going to try them tonight . It’s strange but I collimate the scope and then when I check collimation after pointing the scope to the zenith the collimation has slipped ( as per the laser dot has reappeared outside the centre hole . It can only be a bit of mirror movement . I used bobs nobs from RVO and stronger springs from them but I have to say I may get some heavy duty ones from Amazon .. apparently 3D printer springs ( green colour are the ones to go for ) . 
Stu 

Edited September 11, 2021 by Stu1smartcookie

[NGC 1502]

Hi Stu.  Indeed mirror movement can affect collimation. I feel certain you’re aware that mirrors must never be clamped in place. But I feel that can be taken too far, for instance with regard to clearance between the optical surface and restraining clips I’ve often seen “credit card thickness”.  Whilst I appreciate that the intention is not to clamp the mirror, the thickness of a credit card is enormous with regard to that purpose. Perhaps the thickness of writing paper is more appropriate. Another factor is radial support. Large mirrors are often held in a sling, but somewhere around 12” diameter or smaller (in other words that’s the majority) most have radial support at each mirror clip. Again the radial supports only need a smidge clearance to avoid clamping.

All of the above means that mirrors are held without clamping, but avoiding too much clearance that allows mirrors to shift, thus losing our carefully tuned collimation.

Ed.

[Stu1smartcookie]

Yes Ed , thats a really good point about the clips . There is so much empathise put on a gap of a "card " . As you quite rightly point out , that gap is in fact abe to introduce a fairly substantial error . I will certainly re check this tomorrow morning . i also believe that paper thickness is more than adequate . 

Stu

[John]

I've forgotten what scope you have Stu ?

My 12 inch Orion Optics has the older 3-point primary cell with fairly simple mirror clips and lateral nylon screws to hold the mirror central. I keep the clips slightly off the mirror surface and the 3 nylon screws gently touching the mirror to keep it's optical axis central. The mirror is resting on nylon pads but is not stuck to the cell.

I don't use the locking screws unless taking the scope for a journey because they do impact the collimation.

Other than that, mine holds collimation pretty well.

 

 

[NGC 1502]

The primary cell on my 10” f4.8 Orion Optics Dob is also the earlier basic 3 point. And yet with such a basic support I get excellent views. I’ve always thought that Jupiter is a good test of optical quality. In decent conditions details in Jupiter’s cloud belts are a joy to see, the GRS is obvious, moon and shadow transits crisp. It does make me wonder if the later multi point cell design is really necessary. I don’t go entirely on theory, just judge by the results.

Ed.

[Stu1smartcookie]

9 hours ago, John said:

I've forgotten what scope you have Stu ?

My 12 inch Orion Optics has the older 3-point primary cell with fairly simple mirror clips and lateral nylon screws to hold the mirror central. I keep the clips slightly off the mirror surface and the 3 nylon screws gently touching the mirror to keep it's optical axis central. The mirror is resting on nylon pads but is not stuck to the cell.

I don't use the locking screws unless taking the scope for a journey because they do impact the collimation.

Other than that, mine holds collimation pretty well.

 

 

Hi John .. a revelation 12” which is basically a GSO …I’ve uploaded a picture of the layout .. a fairly standard one .. I changed the screws back to the original but will be changing them back to bobs knobs shortly . I am certainly going to follow Ed’s suggestion about the clip screws . I like the idea of not “sticking “ the  cork pads to the mirror although if it helps keep the mirror from movement then I will fix it . 

[CraigT82]

The type and arrangement of the rear and edge supports for any mirror will depend on it's thickness.

A full thickness mirror (diameter to thickness ratio of 1:6 or thicker) Will perform well with rudimentary support.

Thinner mirrors need more support in the right places. 

It's no use having people suggest what mirror cell arrangements works for them with their scopes when you have no idea how thick their mirrors are and how they compare to your own mirrors thickness.

[Pixies]

@Stu1smartcookie - how tight are your secondary mirror vanes? I thought I had some mirror slip with my 8" dob, but it turned out it was the secondary assembly flexing between horizontal and vertical positions.

If you have a laser, see whether its position in the doughnut moves as you change altitude.

[alacant]

On 08/09/2021 at 11:54, Stu1smartcookie said:

used cork pads without sticking them to the mirror

Hi

Hands on with a GSO. A few easy but essential jobs for cheap mirror cells.

• The very minimum: screw the triangular mirror supports so they bear against their support and don't float.
• Better: also use a generous blob of neutral silicone sealant on the cork on each of the now stationary supports.
• Even better: use more blobs of silicone on the side of the mirror coinciding with the position of each mirror clip.
• Icing on the cake: replace the springs with 1.6mm wire versions of the same length and fit extra springs over the -as you've discovered useless- locking screws. The factory fitted GSO springs are a joke.
• Allow the mirror to settle overnight under gravity on a flat surface. 

Now you can forget about primary mirror movement and turn your attention to the secondary.

Cheers and HTH

springs: details here

Edited September 13, 2021 by alacant

[John]

2 hours ago, Stu1smartcookie said:

Hi John .. a revelation 12” which is basically a GSO …I’ve uploaded a picture of the layout .. a fairly standard one .. I changed the screws back to the original but will be changing them back to bobs knobs shortly . I am certainly going to follow Ed’s suggestion about the clip screws . I like the idea of not “sticking “ the  cork pads to the mirror although if it helps keep the mirror from movement then I will fix it . 

It looks the same as my old Meade Lightbridge 12 inch.

The stock primary collimation springs needed upgrading on that and I found that the thin metal used for the cell construction flexed when the locking screws were tightened so I didn't use those.

I replaced the collimation and locking screws on mine which also helped in adjustment accuracy although the latter still caused cell flex if used.

I have to say that my simple Orion Optics cell (pictured in my last post) has proved a better one than the GSO type.

 

[johninderby]

Had a Lightbridge and the mirror cell was a bit light weight in construction.

The cell on the Bresser dob was a lot stronger design. 

[Stu1smartcookie]

Thanks to everyone for the replies .. I fixed the cork pads with  the sticky dots  . I replaced the collimation screws with the bobs nobs but reverted back to the original locking screws which I barely tightened . The secondary is fine @Pixiesas the laser dot doesn’t move out of the doughnut . Anyway it’s better than it was but although I have better springs than the original I am going to buy some stronger springs . 

[NGC 1502]

14 hours ago, CraigT82 said:

The type and arrangement of the rear and edge supports for any mirror will depend on it's thickness.

A full thickness mirror (diameter to thickness ratio of 1:6 or thicker) Will perform well with rudimentary support.

Thinner mirrors need more support in the right places. 

It's no use having people suggest what mirror cell arrangements works for them with their scopes when you have no idea how thick their mirrors are and how they compare to your own mirrors thickness.

Indeed that’s correct and is well worth mentioning.  Having said that it can depend on what the scope will be used for. If it’s for faint galaxy hunting then a large thin mirror could rest on heavy duty bubble wrap. Hardly good enough for general use but ok for faint fuzzies.  Sometimes called a “light bucket”.

Ed.