16" f4 Orion Optics UK Dobsonian Project

[Astrokev]

Fantastic cell there Shane. Just one (or two!) question that maybe you can help me with. Can you recap on how you designed / built the collimation bolts? I can see they go through the square bar, but is this threaded all the way through? I presume the nut/bolt attachment to the cantilevered bar swivels to allow the collimation bolt to turn. How did you fix the nuts to the bolt either side of the cantilevered bar so that they don't come loose. Of most concern to me with the cell I'm currently designing - is there any "play" in the bolt going through the square section tubing that may cause the collimation bolt to "wiggle" at all? How did you prevent this? Finally, once you've adjusted the collimation bolts, how will you stop these from moving gradually over time with use? I can't see any "locking" bolts.

Sorry for the 20 questions, but this is the bit of the design I'm struggling with.

Cheers

Kevin

[ollypenrice]

You're going to have a ball with this telescope. 16 inch is a nice aperture, big without being bonkers.

Olly

[Moonshane]

Fantastic cell there Shane. Just one (or two!) question that maybe you can help me with. Can you recap on how you designed / built the collimation bolts? I can see they go through the square bar, but is this threaded all the way through? I presume the nut/bolt attachment to the cantilevered bar swivels to allow the collimation bolt to turn. How did you fix the nuts to the bolt either side of the cantilevered bar so that they don't come loose. Of most concern to me with the cell I'm currently designing - is there any "play" in the bolt going through the square section tubing that may cause the collimation bolt to "wiggle" at all? How did you prevent this? Finally, once you've adjusted the collimation bolts, how will you stop these from moving gradually over time with use? I can't see any "locking" bolts.

Sorry for the 20 questions, but this is the bit of the design I'm struggling with.

Cheers

Kevin

hi Kevin

I have added some pics below which will hopefully help.

I have never used locking bolts (and in fact have taken them off when they have been present in the past) as I feel they pinch the optics potentially and take the collimation back out after you have set it.

The collimation bolt set up is as follows (based on Kriege and Berry):

Furniture adjustable foot - one advantage of this is big adjustable knobs and also they are designed to have heavy furniture stand on them so you can happily stand the OTA on them at least while setting up.

Thread goes through two 'T nuts' welded either side of the square section steel. This could also have been threaded straight through both sides but I thought the T nuts would be more long lasting with a heavy mirror.

I left about 18mm below the cell but this is not critical and about 6mm above the cell.

Then comes an M8 all metal lock nut (important they are all metal) and then the cantilever bar (5mm steel at least). and then a washer and a cut down M8 all metal lock nut to reduce height. tighten these down to a fraction off the bar to allow a little tilting.

on either side of the bar goes a triangle. in the correct position a central hole is drilled and then an M4 pan head bolt goes from the top and a standard M4 nut is put under this before it goes on the bar (again with enough space to give a little tilt). through the bar and then a standard nut followed by a nyloc nut to hold.

With a 16" mirror, the weight of the mirror will prevent any movement in the bolts and with this size/speed of mirror it's always a good idea to just check the primary collimation every now and again through the night in any case. you could potentially use a wing nut (or even a compression spring) below the cell on the adjusters to act as a locking mechanism on smaller mirrors?

hope this helps. the cell is upside down in all but the first of these pics.

oh yes, Olly, I am looking forward to using it for the first time even in my light polluted garden.

[Moonshane]

oh, ps, I suspect that you could add some epoxy type glue to the threads of the collimation bolts and then turn it back and forth a few times when cured. this would provide some 'stiffness' to the bolts and prevent wiggle.

[Glen]

Looking terrific Shane, tell there's a lot of time, effort and planning gone into this project

Very professional looking indeed

[Moonshane]

cheers Glen. I tend to be a bit of a perfectionist and hence I am still not happy with the paint on the cell and next summer's job will be to get this blasted, powder coated and refitted, all of which will be done in less time than the hammerite took to cure.

if you are gonna do a job, you may as well try and ensure it's done as well as you can do it.

[Gina]

Thanks Gina, you know you wanna!!

Yes, I'd like to build a scope - I like to build things rather than buy them. Which is why one of my projects is my own home built weather station, having used a cheap commercial one for a couple of years Could well be the same with a scope Eventually!

[Astrokev]

oh, ps, I suspect that you could add some epoxy type glue to the threads of the collimation bolts and then turn it back and forth a few times when cured. this would provide some 'stiffness' to the bolts and prevent wiggle.

That's great Shane. Thanks for the clear explanation and additional pics. Just need to decide how best to do mine now. My initial build is to remake a 6" newt (as previously mentioned I think), but I've always been a bit unhappy with my 10" newt cell, so may consider making one for this in the fullness of time.

Cheers again.

[Moonshane]

the other option of course is a different approach. you could make a 9 point cell easily for the 10" and the 6" which would be more than adequate. this would need a triangle on top of each collimation bolt.

there's lots of ways to do it.

I'll try and draw something for you. what shape is your tube? (oooh matrron)

[Astrokev]

the other option of course is a different approach. you could make a 9 point cell easily for the 10" and the 6" which would be more than adequate. this would need a triangle on top of each collimation bolt.

there's lots of ways to do it.

I'll try and draw something for you. what shape is your tube? (oooh matrron)

hi Shane

my 10" is a round fibreglass tube. For the 6" I am probably going to do a simple 3 point cell. I don't think it warrants anything more fancy. As this will be a predominantly wooden affair, I am considering a bottom plate (ie. the bit that attaches to the hex tube I'll be making) of 1" mahogany, with the mirror sitting on a similar wooden plate on 3 silicone pads. My uncertainty is how to fix collimation adjustment bolts between the two. Currently considering spring-tensioned bolts with "wing-nut" style nuts on the outside of the bottom plate to provide the adjustment. Fairly traditional, but don't know whether I need to add locking bolts or just rely on the springs to keep the mirror plate in position. Hope that makes sense!

[Gina]

Would a wood based scope be stable enough for imaging? If I was making one I'd certainly be thinking of wood - so workable.

[Astrokev]

Hi Gina

My wood tube will be just a "casual" transportable Dob. Regarding imaging, I'll only be using it for solar prime focus imaging with very short exposures, so shouldn't present any problems, providing I can focus it. For my main imaging I'll be using my main equatorial scope (when my obs is eventually built).

[Gina]

Hi Gina

My wood tube will be just a "casual" transportable Dob. Regarding imaging, I'll only be using it for solar prime focus imaging with very short exposures, so shouldn't present any problems, providing I can focus it. For my main imaging I'll be using my main equatorial scope (when my obs is eventually built).

Thanks for your reply Kevin Yes, I noticed the wooden scopes were Dobs. If I build my own it will probably be a Newt - mounted on my GEM. Maybe a wooden tube would not be suitable, though might be OK for lunar, solar and planetary I guess.

[spaceboy]

Currently considering spring-tensioned bolts with "wing-nut" style nuts on the outside of the bottom plate to provide the adjustment. Fairly traditional, but don't know whether I need to add locking bolts or just rely on the springs to keep the mirror plate in position. Hope that makes sense!

I'm thinking that the springs would be placed under undue strain if there were not locking bolts helping to hold the load. They would have to be pretty hefty springs to hold a mirror cell, mirror and gravity with out compressing. But too hefty a spring could make fine collimation adjustments testing. Just my thoughts.

[Moonshane]

I'm thinking that the springs would be placed under undue strain if there were not locking bolts helping to hold the load. They would have to be pretty hefty springs to hold a mirror cell, mirror and gravity with out compressing. But too hefty a spring could make fine collimation adjustments testing. Just my thoughts.

I think Kevin's design would be fine for mirrors up to 12" and certainly for 10". I'd use the springs and wing nut version with two plates - one for the supports and with the springs in between and the nuts at the very back of the tube.

my old OOUK mirror cell had just three springs and it held collimation well for the night's observing. my current 6" is the same. the springs do take some compressing though when it comes to deconstructing the cell. the secret to easy collimation nuts is to use teflon washers it makes a work of difference. I bet OOUK would sell you some springs if you asked.

[Astrokev]

I think Kevin's design would be fine for mirrors up to 12" and certainly for 10". I'd use the springs and wing nut version with two plates - one for the supports and with the springs in between and the nuts at the very back of the tube.

my old OOUK mirror cell had just three springs and it held collimation well for the night's observing. my current 6" is the same. the springs do take some compressing though when it comes to deconstructing the cell. the secret to easy collimation nuts is to use teflon washers it makes a work of difference. I bet OOUK would sell you some springs if you asked.

Cheers Shane. I've found a supplier of stainless springs, but will probably have to use a bit of educated guesswork to select the best size and compression strength etc.

[Glen]

May i suggest rear brake springs (to hold shoes)for cars/vans might be worth a think about, you should get a repair kit for around a fiver and get four of em..

Just "sprung" to mind

Glen

TAXI

[spaceboy]

This thread has gone quiet ??? Come on Shane there is a dead line to be kept

[Moonshane]

Well, as it happens, I was too hung over on Saturday (works do) and as it has not stopped raining all weekend I did not get much done - using impact adhesive does not go down well indoors.

I waited until today as my wife was at work and I had the house to myself.

The mirror created more challenges for me as despite what the zygo says, it is 403mm in diameter not 400mm. OK I should have measured it! Believe it or not this made a difference to the fit of the mirror in the cell as there's really not much room inside the OOUK tubes - only about 15mm all round.

Anyhow, I just ground off one of the clip holders and created a new clip from 3mm ali, drilled some holes in the side of the cell and then tapped the clip so it took M6 bolts. This created about 5-6mm at the bottom edge. The mirror is still fairly evenly spaced all round so not too bad.

Today I glued all the teflon buffers in place and used Evostik impact adhesive. Don't use the solvent free version - it's rubbish. I got some from Ebay thinking it would be good but it's like a thick PVA and really rubbery (no Chinese jokes please). Got some 'proper' stuff and much better.

Before permanently installing the primary, I thought it a good idea to check the position of the centre donut. Unfortunately it was a little out (see pic). Rather than risk damaging the coatings, I just made a new version (black triangle this time) and stuck this over the top of the original one. I hope this will make collimation more accurate and I doubt it will create any problems having one over the other.

Next I put it in the cell and then the scary moment of putting the cell in the tube. This went OK and the tube was then fully loaded to allow balancing. I used my Paracorr and 26mm Nagler for the balancing point and it was still just a shade bottom heavy. Correcting this is a breeze compared with adding weight at the bottom for a top heavy scope and can be corrected by e.g. adding an extra pair of batteries in the telrad. The balance point is about 11" at the bottom of the bearings so this will make the side boards of the rocker very stable. I will probably make them about 14" high at the base of the curve and a little higher for the 'shoulders'.

This will be my next job and I can make the ground board too and glue the formica and teflon bearings in place. Then paint the woodwork Satin black.

Getting there bit by bit.......

[spaceboy]

That's well nice Shane!!!!!! Looking like you will need some sort of baffling behind that mirror though.

[swamp thing]

Sure is coming on well Shane. Love the cell buddy, great job.

15mm of clearance round the tube aint much is it?

Do you think tube currents might be an issue?

Maybe the mother of all Dob fans will sort it.

Regards Steve

[Moonshane]

That's well nice Shane!!!!!! Looking like you will need some sort of baffling behind that mirror though.

cheers matey. yes, forgot to mention that, will make a ring of maybe hardboard painted matt black and attach to the cell with velcro or similar.

[Moonshane]

Sure is coming on well Shane. Love the cell buddy, great job.

15mm of clearance round the tube aint much is it?

Do you think tube currents might be an issue?

Maybe the mother of all Dob fans will sort it.

Regards Steve

cheers Steve

maybe tube currents will be an issue but to be honest I'll be observing with this mainly at lower mags so I am hoping they won't be a major issue? either way, yes, something else I forgot to mention, I have an 80mm 12v dc fan in my drawer ready to go on, just need to make a small baffle and then wire it up. will do this when the base is done though as it will be easier to move the scope about then.

I may actually wire it with 4AAs at 6v as it can then run permanently. it only draws 0.17a so some 2500mah batteries should run it for a few nights' observing even if on all night. this way I can attach it all to the cell - I hate trailing wires.

[Astrokev]

The installed mirror looks fantastic and really makes the cell come to life. Great job.

The clearance around the primary is very tight as you say. D'you think this will affect the image using wide field eyepieces? I read on another thread (the octagonal wooden tube build if I recall correctly), that you can get vignetting if the tube is too close to the primary when viewing very wide fields. May be more of a theoretical issue than in reality, but what d'you think?

Really looking forward to seeing the base built. Keep going mate, nearly there.

[Astrokev]

cheers Steve

maybe tube currents will be an issue but to be honest I'll be observing with this mainly at lower mags so I am hoping they won't be a major issue? either way, yes, something else I forgot to mention, I have an 80mm 12v dc fan in my drawer ready to go on, just need to make a small baffle and then wire it up. will do this when the base is done though as it will be easier to move the scope about then.

I may actually wire it with 4AAs at 6v as it can then run permanently. it only draws 0.17a so some 2500mah batteries should run it for a few nights' observing even if on all night. this way I can attach it all to the cell - I hate trailing wires.

If you install a fan on the cell, how effective d'you think it will be? You've a wacking big chunk of glass in the way and there's not much clearance to create air movement - you may find it doesn't draw air through the tube at all. Could you install in the side of the tube above the primary?