Manual remote primary collimation in truss dobsonian

[erick (Eric)]

Jason, as usual, you are trying to make it simple. But I want to cut and drill things and fit knobs and chains and sprockets!

I put my hot spot on in a rush and didn't orient it. The way the mirror is slung, I have to do a little work to let me rotate it, but I will do this, next time I take it out. But I did make up a drawing to show how the image moves with the two collimation bolts that I use to adjust the Primary.

[erick (Eric)]

Originally Posted by erick

......But I want to cut and drill things and fit knobs and chains and sprockets!

Well today I cut things, drilled things, fitted chains and sprockets and I have.............. manual remote primary collimation on my truss dob, hopefully from the eyepiece if my flexible controls are long enough. If not, I'll get longer ones.

Took it very slowly. Drilling holes in an SDM - yikes! Got to get them right first time - measure and check, re-measure and re-check.

My design worked. While I have added stuff, all originally collimation functionality remains and everything is neatly enclosed within the mirror box. Took me just under a year from bright idea to complete!

But as I finished, the rain came down yet again in Sydney - a heavy downpour then steady rain, after a glorious sunny day. So I didn't get to try it out. I will this week, hopefully.

[alistairsam]

Any pics Eric? Sounds very interesting.
do you use any guards for the chain and sprockets to keep dust or dirt out?

[erick (Eric)]

Pics when I am sure it is working OK, Alistair. No guards around the chains/sprockets. They are all within the mirror box.

[Calibos (Keith)]

Originally Posted by erick

Well today I cut things, drilled things, fitted chains and sprockets and I have.............. manual remote primary collimation on my truss dob, hopefully from the eyepiece if my flexible controls are long enough. If not, I'll get longer ones.

Took it very slowly. Drilling holes in an SDM - yikes! Got to get them right first time - measure and check, re-measure and re-check.

My design worked. While I have added stuff, all originally collimation functionality remains and everything is neatly enclosed within the mirror box. Took me just under a year from bright idea to complete!

But as I finished, the rain came down yet again in Sydney - a heavy downpour then steady rain, after a glorious sunny day. So I didn't get to try it out. I will this week, hopefully.

I got that beat.

My head was bursting with ideas for the 16" Lightbridge I bought in December '08. By October 2009 I was nearly complete and just needed to fit the Servocat Goto drive I had just bought. Real life got in the way and I never got to fit the servocat and still haven't nearly 3 years later!! I got the scope out once in early 2010 (Whenever the last Mars opposition was) and found half my mods done in 2009 needed to be re-done/re-thought. In the intervening time I re-thought my previous mods, know where I went wrong, know how to do them properly. I also thought of a plethora of new mods. I have a fully renewed brain bursting with modding plans.

.......Still haven't had time to start them

I am hoping that before the scopes 4th birthday I might actually have it finished and be able to start using it

[erick (Eric)]

Very Happy Jan

Finally had a chance to test the system out in daylight. Simply fantastic - just want I wanted.

Photos of the finished result are attached. From the outside all that is seen are eight nice stainless steel countersunk hex drive bolt heads on the side of the mirror box and two plastic caps on top. From the mirror cell end, the chain drives can be seen but are inside the mirror cell. Everything inside the mirror box above the mirror surface has a layer of chalkboard paint on it and none seems to have got into the bearings.

Operation is just as I wanted it. Not much backlash, though that didn't matter and I can adjust that by pulling on or releasing the bottom rod support against the two springs it sits on. (The purpose of the springs was to allow the chain to be slackened off so it can be removed without having to remove the "breakable link".) The primary tilts smoothly in the two controlled directions. I had no idea how sensitive the control would be - too much or too little. However it is just right. about half a turn on the flexible drives gives me the range of tilt I wanted - fantastic. I can insert and remove the two flexible drives and the mirror doesn't flinch at all.

Only slight downside is a bit of misalignment on one side, between the access hole and the top of the drive rod, but nothing I cannot cope with. I guess I shouldn't have expected a piece of Bunnings rod to be perfectly straight!

If all else fails (or I forget to bring the flexible drives - duh!) I can still operate from the mirror cell end as before. There is a little more force required to drive the chain and rod as the bolt is turned, but not much.

Big thanks to my helpful machinists - Alan in Melbourne and John in Sydney - who says Melbourne and Sydney don't get along!

Next post shows more photos of the setup and the remote primary collimation in action.

[erick (Eric)]

Remaining photos of the setup and photos of me adjusting the primary tilt for a return laser beam and for a Cheshire.

Next test under the stars - who knows when . I think I might even be able to leave the two flexible drives in place through the night, inside the shroud. I'll check, but I don't think they stray into the light path. A few bits of chrome to be blackened, that's all. Lift the shroud up half way to access them. They will not fall out and onto the mirror. (famous last words?)

[Louwai (Bryan)]

Hey Eric, That beast is starting to look a bit technical... You've done a great job...

[Jason D (Jason)]

WOW!!!!!

Quite impressive, Eric...

Jason