primary mirror collimation springs

[adman]

I have just moved my primary up the tube by putting in longer collimation bolts - so I can now get focus with my DSLR.

New bolts means new springs....does anyone know how strong these springs should be?

Adam

[erick]

Errrr.... strong enough. Check out the supply of compression springs in Bunnings. They don't do both short and heavy gauge together unfortunately. I redid my 8" reflector springs a few days ago. Bought longer heavy gauge springs and cut them in half :scared: Filed the cut end fairly flat. Seems to be working fine.

Springs too weak and the mirror will sag. Springs too strong and you'll be building up your muscles turning the collimation knobs - any maybe risking stripping a thread?

Test - Collimate with the tube horizontal. Move the tube to vertical and see that the collimation only moves fractionally, if at all. Mirror sag should be quite obvious. Some collimation movement might not be mirror sag, but a bit of focusser draw tube slop or tube flexing?

If you now have a 3-4cm gap, you don't have to fill it completely with spring. You could fit a solid spacer over the longer bolt leaving a 1.5-2cm gap in which to insert a spring.

EDIT: Of course, it also depends on the weight of your mirror. If you are talking about the 8" skywatcher, then that is not a big issue. I'll look up the dimensions of the ones I bought and post here.

[adman]

Thanks for that Erick

I have found the spring section at Bunnings, and as always it seems that they don't quite have the exact one I am looking for. I tried some that were quite hefty, and while they certainly held the mirror well, it was a two person job to get them in place - one to compress the springs and the other to get the bolts screwed in...so I got some lighter ones, but these are maybe not quite enough.

I am interested to see which ones you used.

Cheers
Adam

[erick]

My mirror is in original position - so only a small gap to fill.

Were the original springs good enough before you moved the mirror? Maybe go with the spacer idea so you can reuse those.

Or use a spacer in order to get more compression in whichever springs you want to use. Greater compression, greater force holding the mirror in place.

[erick]

That was my problem, Adam. Ones with enough gauge were too long - so out came the trusty hacksaw and file! (for the springs, not the scope!) Try holding a spring in a vice to cut it - ker-fang! sprong!

[adman]

I think thats the way I will go - cut down the heavier ones that are slightly too long. Better too strong than too weak

Thanks
Adam

[adman]

Yep - have cut down the heavier ones and they hold collimation much better than the others. In fact - no shift from tube horizontal to tube vertical :thumbsup:.

[erick]

Great Adam, that's what you want!

To complete my story - attached photo is of previous spring on the right and new spring on the left. Now that is real muscle! Left is only half a spring and I haven't filed it down to a flat surface yet.

The Bunnings springs I bought are part number C-706. Dimensions are 14.3mm dia x 34.9mm tall x 1.83mm gauge. For an 8" f5 Newtonian reflector - the gap the spring fills is 1-1.5cm.

[adman]

geez - I wonder if the old ones were doing anything?? They look a little weedy!

The new collimation bolts I put in ended up being about 50mm long - which lands the focal plane right in the middle of my drawtube travel, and still lets me use my EPs with a 50mm extension tube!

Thanks for the pictures
Adam