Replacing collimation screws on primary mirror?

Guys n Gals

I had a go collimating my scope a couple of weeks back - not used to it. I believe I have improved it but equally believe I could do better. I Look forward to trying again soon.

One thing I found difficult was that the adjustment screws where of phillips type screw head type. Trying to move from screw to screw and re-inserting a screw-driver in the dark is a bit of a pain.

I was thinking of replacing these with thumb-screws - something one turns directly by hand.

I don't know how a mirror is physically mounted - can I safely remove these without mirror dislodging?

There is one Allan bolt associated with each. If locking screws are secure can the adjustment screws be removed?

Any suggestions?

Mark

On both my Orion XT12i Dob and on my LB16 the collimation bolts screw into the cell and the locking bolts just push against it. If your Newt is the same, then no you can't undo all the collimation screws at the same time. You could change them one at a time however. Is the back of your newt baffled with a cover? If it is, you'll need to take this off so you can hold the collimation bolt spring in place while you unscrew the collimation screw and replace with the thumbscrew, otherwise when you undo the collimation bolt the spring might fall out and drop between the cell and mirror and ota somewhere and then you'll definately have to take the whole thing apart.

mjc wrote:

I had a go collimating my scope a couple of weeks back - not used to it. I believe I have improved it but equally believe I could do better. I Look forward to trying again soon.

I don't know how a mirror is physically mounted - can I safely remove these without mirror dislodging?

Hi Mark,

Try collimating your scope in daylight - you'll find it a lot easier - then you can star test it at night by pointing it at polaris (because it won't move on you if you don't have a motor!) and defocus under high magnification. If all the diffraction rings look nicely round and concentric then you've hit the bulle-eye.

You will need to remove the entire primary mirror cell from the tube to safely replace the collimating screws with wing nuts or similar, otherwise the mirror could conceivably part company from the rest of the cell/telescope.

Phil.

Calibos wrote:

On both my Orion XT12i Dob and on my LB16 the collimation bolts screw into the cell and the locking bolts just push against it.

Sounds like your cell is a "Push - Pull" arrangement. The cell has three spring loaded collimating screws and three locking screws that push against the back of the cell to prevent it from vibrating loose in transport. To collimate this type of cell, loosen off all three locking screws/bolts, collimate and then tighten the three locking bolts again. In theory you'll never loose collimation unless a bomb goes off next to your scope!

Phil.

I suppose your advice to remove the whole cell from Marks scope is the safest not knowing what kind of arrangement the collimation screws/locking bolts and springs are. I thought all cells on prebuilt newts were push/pull so I thought one could replace one collimation screw at a time.

Well you learn something new everyday. So its not just ATM scopes that might have different cell arrangements?

I actually use a different method with my Lightbridge. The locking bolts on a Lightbridge are 60º from the collimation bolts. In other words the collimation bolts are at 12 O'Clock, 4 and 8 for example and the Locking bolts are at 2 O'Clock 6 and 10. Accepted wisdom is that the locking bolts should be right beside the collimation bolts and indeed this is the way they are on the Orion.

However, although the Lightbridge locking bolts now act as another set of 3 collimation bolts, it can actually be used to our advantage. I use the barlowed laser technique to collimate the primary with the collimation bolts after loosening the locking bolts. Rather than then just tightening the locking bolts till they barely touch the cell, I actually tighten them beyond that point, slightly warping the cell and putting the cell under tension. Obviously this knocks the collimation out slightly. I then recollimate with the locking bolts and now one has hyperstable collimation.

Thanks for the response and advice.

I had another attempt last night (a very good night for viewing from D15). Found that I had to bring the scope in and collimating in brightness made life easier but would like to be to tweak while setup for observation for final adjustments when necessary. Will overcome that in time.

I shall not rush any decision with the thumb screw but do want to do this at some point. I need to determine what size and pitch and find somewhere to the suply these. I will probably attempt one screw at a time whe that time comes.

Mark