Manual remote primary collimation in truss dobsonian
Following Bryan's question in my WANTED thread:- I want a cheap option, no electricals and to be able to use the collimation bolts as normal without having to disconnect anything.
OK, here is the plan:-
Fit sprockets to the two top collimation bolts, just under the mirror cell and inside the frame. Drive these by a chain drive to smaller sprockets on two shafts that run down the inside of the mirror box (yep, paint all surfaces above the mirror matt black). The shafts terminate just under the top of the mirror box. Suitable supports top and bottom with roller bearings are connected to the side of the box - being careful not to have the screws for the lower ones protrude and hit the rocker box. Lower support on springs so I can adjust chain tightness externally. Fit two covers above holes cut through the top of the miror box that the shafts almost reach. (have already found a suitable spring loaded cover at Jaycar)
So open the spring-loaded covers, poke in flexible drives with appropriate fitting on the end - just a square socket should do, then I have two knobs within easy reach with which I can adjust two primary collimation bolts. No need for power, no motor overshoot problems. Collimation completed, remove the flexible drives and close the covers.
All that we see from the top should be as attached image.
Lots of planning and testing needed before any drilling and dismantling starts.
My first thought was some long shifters fitted to the bolt heads and a big stick to reach down and move them back and forward
But - questions, questions??? Will having a chain hanging off the upper part of the collimation bolt lead to some slight movement as the scope is moved in Alt? Perhaps I will need the chain in a degree of tension to avoid this? But might that make it difficult to turn? As some I you know, I trained as a chemist, certainly not as an engineer!
Still seaching for suitable lightweight/thin chains and sprockets. I'm off to a Radio Controlled hobby shop that is advertising chains and sockets, but won't get there until next week. I don't think industrial will do - too big and heavy. Perhaps the thinnest bicycle chains and sprockets - 3/32" I believe - but they are probably super titanium racing ones worth mega bucks?
Tomorrow morning, I stop at my friendly roller bearing supplier with my piece of Bunnings 1/4" steel shaft in hand to see what bearings I can buy.
Watch this space!
But I'm going to have to be very confident before I take the electric drill to the wooden mirror box (Please no-one tell Peter - hush now)
Eric, to prevent chain slap you could fit chain a tensioner to each chain. You can make them yourself. It is just a small nylon or rubber wheel under spring tension rolling against the chain.
Or to prevent chain slap altogether use a belt drive set-up instead of chain and sprockets.
Of course, they'll never move fast enough to "slap"! In fact, they will probably move back and forward no more that a cm in either direction when in use. In fact, I'd like them to hang fairly loose to avoid pulling on the bolts up near the mirror - I can cope with backlash.
Of course, they'll never move fast enough to "slap"! In fact, they will probably move back and forward no more that a cm in either direction when in use. In fact, I'd like them to hang fairly loose to avoid pulling on the bolts up near the mirror - I can cope with backlash.
Yeah, backlash was what I was referring to re: chain slap.
But if you don't mind a bit of backlash, then that's ok.
I'd have a word with Peter Eric. If your scope doesn't hold its collimation long enough to warrant this sort of re-build then something is wrong.
Pehaps near the horizon there may be a movement in my primary, but that takes a 2 second tweek. Other than that it remains true for hours and any differential expansion/contraction in the poles causes the secondary to need tweeking first anyway.
Got my roller bearings. Have ordered some nice M5 countersunk head S/S screws (hex socket) since these will be visible from the sides of the mirror box.
Now, to really "turbo" my scope, I have ordered the chains and sprockets from these model dirt bikes:-
I'm quite convinced that I can make these do what I want. Time will tell. Wear and tear is not an issue - the chain will probably move forward and backwards a cm or two at the most!
UPDATE:- closer view of chain and rear socket on the model at about 4:25 in this video:-
diy site tube cheshire for me in the daytime,and no longer make adjustments through the night, like i used to but the pain to gain ratio
wasn't making any noticable differance at the eyepiece.
Sorry gentlemen, for the delay. I was thinking about it.
Fundamentally the issue is both a philosophical one and the desire to meet a challenge.
Philosophically, I like to be able to continuously see the effect of any adjustment. As it is, when I am using Cheshire or Autocollimator, I need to tweak and return to eyepiece, tweak and return to eyepiece. I just don't like it. And after I worked out a likely solution, then I had this challenge in front of me to meet. OK, it's just me.
Now, my scope has usually held collimation well, between dismantling and rebuild. Just a tweak needed. But a few times it has been more than a tweak and I haven't been able to see why? I've rechecked the poles and UTA are seated correctly, and they always are. Seems the bouncing to the site had caused some movement in the secondary mirror. Now, as you will have seen elsewhere, I have had the recent scare of the scope falling over in high wind during that day. All has seemed fine since then, but I had to completely recollimate from almost scratch. I have only rebuilt the scope once since then and all was fine.
With the autocollimator, I am trying to learn how to use the "Carefully Decollimated Primary" protocol, but I find that, without a simultaneous view of what happens in the autocollimator when I adjust the primary collimation bolts, I just get lost with the reflections. So, in well known Eric style, I'll create a sledgehammer to get at this peanut!
Sorry gentlemen, for the delay. I was thinking about it.
Fundamentally the issue is both a philosophical one and the desire to meet a challenge.
Philosophically, I like to be able to continuously see the effect of any adjustment. As it is, when I am using Cheshire or Autocollimator, I need to tweak and return to eyepiece, tweak and return to eyepiece. I just don't like it. And after I worked out a likely solution, then I had this challenge in front of me to meet. OK, it's just me.
Now, my scope has usually held collimation well, between dismantling and rebuild. Just a tweak needed. But a few times it has been more than a tweak and I haven't been able to see why? I've rechecked the poles and UTA are seated correctly, and they always are. Seems the bouncing to the site had caused some movement in the secondary mirror. Now, as you will have seen elsewhere, I have had the recent scare of the scope falling over in high wind during that day. All has seemed fine since then, but I had to completely recollimate from almost scratch. I have only rebuilt the scope once since then and all was fine.
With the autocollimator, I am trying to learn how to use the "Carefully Decollimated Primary" protocol, but I find that, without a simultaneous view of what happens in the autocollimator when I adjust the primary collimation bolts, I just get lost with the reflections. So, in well known Eric style, I'll create a sledgehammer to get at this peanut!
So I don't have a logical response, sorry.
Sounds logical to me.....or maybe I'm just as mad as you
I certainly nodded in agreement and recognition reading your post anyway, especially the parts about the likely solution and then the solution becoming a challenge to meet and the bit about creating a sledgehammer to get at the peanut. From further discussion with Jason Khadder and Don Pensack over on CN, I get the impression that once rough collimation is done with my Glatter tools which could be done from the back of the scope, that the backwards and forwards tweaking for the Infinity XLK was minimal after some practice( It helps to position your catseye triangles or hotspot or re-orientate your mirror so the point at the collimation bolts) and thus I probably didn't really need to come up with a solution for adjusting the primary from the front. However I am like a dog with a bone who won't let go of an idea unless it turns out to really be more trouble than its worth.
In my case the idea isn't much trouble to implement because I already had all the parts so the balance point is still shifted towards implementing the idea despite it not really being necessary
My idea was canabalising a JMI TNT motor tracking system that I had modded to a point where it was unsellable anyway. (Have a servocat tracking system now). The azimuth motors and handcontroller were perfect for a motorised collimation system and I was also going to be able to use a defunct 6 wire DIN socket and wiring under my flocking inside the OTA. ie. I'd plug in the handcontroller to this socket on the OTA for collimation. Just the idea of utilising/re-purposing 2 previous but now defunct mods was enough for me to want to proceed with this mod whether it was needed or not, simply because it was recycling, sorta unique and a challenge in and off itself.
But yeah, talk about sledgehammers and peanuts!!
In terms of the need for the Autocollimator. For a scope above F4 without a paracorr any extra accuracy achieved over and above the cheshire or laser with barlow method is probably invisible accuracy. Howver once you go below F4 or use a paracorr for coma correction even on slower scopes, the collimation tolerances are tightened by a factor of 6. This is when the extra accuracy of the Autocollimator really comes into its own and is arguably visible in the eyepiece. Then again some of us are just a bit a*** retentive about knowing that everything on the scope is perfect whether that be thermal issues, stability issues or collimation issues. We want to know the scope is setup to deliver its 100% best and that if the views aren't great at least we know we did all we could do and mediocre views are down to the atmosphere and not the scope itself.
I laugh in the face of the K.I.S.S. principle....Muahahahaha!!
Hargreaves back in the 1960's (maybe before?) had a method of collimating the main mirror from the eyepiece - threaded rods down to the main mirror cell... I remember in 1970 using a similar system on a ol' With-Browning 9.25" f10 scope.
I can buy the "because I want to" argument. I have used it on many occasions!
Personally, I love the simple purity of the Dob. Whilst I don't regret putting servocat on mine, I still look back fondly to the time when it was just me and the scope. Less is more
My observatory scope on the other hand is the ultimate expression of "because I can"...
Do you remember the "old" days (2007 in my case ) when the only electricity was in the torch in your pocket and perhaps the battery pack operating the cooling fan for a while around sunset! And we did everything by feel - change eyepieces, add filters. The "good old days"! Well, actually not so good when the secondary and the eyepieces all dewed-up (or iced-up by 11pm in the case of July in Snake Valley! ) And the sky was so dark you couldn't see the non-illuminated cross hairs in the finderscope! When out by myself, I used to like vanishing in the dark, unseen from a few metres - black car, black coat etc. Now I sometimes look like a Christmas Tree with red and green lights (and blue - dang Bingpond dongle! ) and need a huge gel battery to power it all! I'm gradually working through the lighting to remove them or substantially reduce their intensity. I work at somewhere around 5% brightness on the Argo.
If your primary mirror has the Hotspot or the triangle stickers and you have it oriented such as each tip of the sticker is pointing to a primary knob then you can cut down on the number of trips if you follow these steps:
1- Identify each tip of the sticker. In attached animation, I labeled each tip with a character. I know which knob to twist and in what direction to move the sticker in the desired direction. Either you need to memorize or write it down on a small piece of paper.
2- Most of the time, the desired direction will not line up with any of the 3 tip directions. In this case, you need to break the direct movement into two movements along the A, B, and/or C directions. Make a mental note of the two movements.
3- NOW MOVE TO THE BACK OT THE SCOPE and execute the two movements. Over time, you will have a good feel on the amount of twist.
4- DONE
OK, real life will not be as perfect as the attached animation but you will come very close in just a single trip to the back of the scope. One or two more trips for fine tuning and you are truly done.
If identifying the tips is tough, you could mark one tip and mark the corresponding primary knob. The other two tips and knobs will be easily identified as the “right” or “left” tip/knob.
forget it it is there now you must have been posting it as I typed
(Please no-one tell Peter - hush now)
Time will tell. Wear and tear is not an issue - the chain will probably move forward and backwards a cm or two at the most! 
) And the sky was so dark you couldn't see the non-illuminated cross hairs in the finderscope! When out by myself, I used to like vanishing in the dark, unseen from a few metres - black car, black coat etc. Now I sometimes look like a Christmas Tree with red and green lights (and blue - dang Bingpond dongle! 
) and need a huge gel battery to power it all! I'm gradually working through the lighting to remove them or substantially reduce their intensity. I work at somewhere around 5% brightness on the Argo.
