removing primary from cell

[sunshine185]

Gonna replace the springs and collimating screws on the 250px, what's the correct procedure for removing the mirror from the cell? I thought of this but not sure if its correct 1, remove the screws holding cell to the ota. 2, undo/ remove the clips holding the mirror in the cell. 3, undo the collimating/ locking screws and lift out the mirror. Is this the correct way or do I have it mixed up?

[Moonshane]

I'd recommend that you do the replacements one at a time if you can. this way you can leave the mirror in the cell and it will be safer. if not it's not a big issue. try to mark the side of the mirror and cell in some way to ensure you put the whole thing back in the tube as it was and the mirror in the same place in the cell itself.

it's not scary really.

the main thing is to ensure than nothing drops, especially the scope when all the weight is taken out of the OTA. be prepared for the weight, a 10" mirror and SW cell is surprisingly heavy.

[sunshine185]

I know! Took it out of the ota last night to get a feel of the thing, was very surprised how heavy it was. So its it possible to replace the springs and screws without taking the mirror out of the cell? I am hoping to raise the mirror up by about an inch to enable the binoviewers to focus without the 1.6x corrector, I just cannot find the funds for a low profile focusser.

[Moonshane]

depends on the cell really. any chance of a pic?

[sunshine185]

As you can see,I think I've gone as far up as I can with the original screws and springs

[Mark at Beaufort]

This morning I removed the mirror cell from my Skywatcher 250px Dob so that I could clean the mirror. As Shane stated above I put a colour sticky marker to mark the position on the OTA and the mirror cell. I then removed the 6 mirror retaining clips, again marking the same position of cell/clip. I then lifted out the mirror and then cleaned it.

The mirror cell was then separate from the mirror so I could have done things to it if I wanted.

My scope is now all back together again and collimated.

Sunshine185 you could do the same as I did this morning but clearly you will be changing springs and bolts.

Mark

[sunshine185]

I think I've totally misunderstood the whole thing, for some reason I thought that the collimation screws actually screwed into the back of the primary mirror, and had visions of the primary bouncing out of the cell when I took the screws out, I feel such a fool now. So in theory it should be quite easy to remove the primary and store it safely while I faff about with the cell and new springs. Many thanks for your advice, God knows what I'd be like if I didn't have sgl.

[Moonshane]

untitled.bmpin this sort of cell you just remove the adjusters which are really nuts, and attached to the ends of the long bolts from the cell.

just take them off (marking as mentioned) replace your springs and then out together again. it's easy peasy. if you remove the mirror you get access to the heads of the bolts (they will be epoxied in but can be removed with some 'persuasion') and can remove them and add longer bolts if you like. if you want to move the mirror up the tube. this sort of cell is an easy adapt. just add a bolt say 50mm longer than the ones in it and use two nyloc nuts either side of a washer (larger than the spring diameter) in the right place and you can move the mirror as far as you want within reason.

a bit of a rough and ready sketch attached which hopefully explains what I mean.

[sunshine185]

That's a great help thnx. I think I have a good idea what I'm doing now, might have a trip to b&q to see if I can rustle something up.

[Moonshane]

I am pretty sure the bolts are M6 and countersunk heads.

[sunshine185]

So if I can get an m6 countersunk headed screw, what size would you reckon to move the primary up , 60mm or a bit longer?

[Moonshane]

it just happens I have a 10" SW mirror in a cell on the shelf awaiting a project. the bolts seem to be 40mm long. I'd buy something about 90mm I think - you might have to buy 100mm. try and get stainless if you can (ebay is good for this sort of thing)

e.g. http://www.ebay.co.uk/itm/M6-Stainless-Countersunk-M6-m6-Socket-Countersunk-Bolts-Any-Length-Qty-/360437047837?pt=UK_DIY_Material_Nails_Fixing_MJ&var=&hash=item53ebb8e21d

http://www.ebay.co.uk/itm/A2-Stainless-Steel-Nyloc-Nuts-M3-M4-M5-M6-M8-M10-M12-M16-M20-M24-/260947248858?pt=UK_DIY_Material_Nails_Fixing_MJ&var=&hash=item3cc1ab06da

http://www.ebay.co.uk/itm/M6x25-Penny-Washers-Stainless-A4-Marine-Grade-Pack-of-20-/120990818778?pt=UK_CarsParts_Vehicles_BoatEquipment_Accessories_SM&hash=item1c2b9d61da

This would give you about 40mm of play (taking into account the nuts) which would hopefully be plenty? just estimate the amount of infocus you need and set up the bolts etc accordingly. you might need to trim off the ends of the bolts to suit but that's easy too. if you are struggling just pm me. I have plenty of tools and don't live that far away.

[brantuk]

When you put the mirror back in the cell be careful not to pinch it when retightning the screws or all your stars will be spiked and uneven. I place a very thin piece of paper under the mirror clips and ensure it can just be withdrawn when the screws are firmly in place. This leaves a very timy gap but the mirror will be held in place sufficiently without pinching it. This worked fine on my 300P

[sunshine185]

I can't believe I'm actually saying this, but I feel quite confident of doing this now thanks to you guys. I'm really thankful for all your help.

[Moonshane]

you'll be fine mate. just take your time and remember the old adage.....measure twice and cut once.

[atmenterprises]

I'm sorry if I missed this but was there a recommendation for the replacement spring type mentioned too?

[sunshine185]

If I can find some that are suitable I will let you know.

[earth titan]

The 1st take a scope apart it is daunting but after while it loses any fear.

So far I've had most of my apart (some completely) and it doesn't hold any concerns any more.

They are generally simple devices. The only one I've not been brave enough with so far is my SCT but it is going to get flocked soon, once I've sorted a decent collimator.

[sunshine185]

Well I've hit a snag, the collimating screws are epoxied in the back of the primary cell and I don't think there's any chance of me getting them out, if I try to unscrew them anymore I think I will end up threading the Philips head and then I'd be in big trouble, the collimating screws are only 35mm long so are pretty much useless for moving the primary up the tube, I'm back to square one fit the time being.

[atmenterprises]

Any chance of taking pliers to the screw heads to get them out? I'd read from others that they are epoxied but they managed to get them out. Not trying to get you to damage your scope. Just wondering if channel locks or something may help.

[sunshine185]

I might have to give it a go, I need to get them out.

[atmenterprises]

Keep me posted. I'm afraid I'm going to have to do the same thing to mine.

[sunshine185]

Are you doing it for photography?

[atmenterprises]

That's the plan if it won't focus.

[Moonshane]

I've sent you a PM mate. use two nuts locked together on the bolt side and then use a spanner to turn it out. this will almost certainly work but if not then I have a vice that will.