16" f4 Orion Optics UK Dobsonian Project

[Astrokev]

Ah I see now. Excellent. Don't think I've seen a cell where the collimation bolts attach directly to the bars holding the triangular supports. but no reason why this shouldn't work providing there's no play in the bolt/thread.

I presume the bars are mild steel? Does look quite heavy, though obviously needs to be very strong to hold a 16".

Look forward to seeing the finished job.

[Moonshane]

Hi Kevin

They are 2.5mm x 25mm square box section mild steel. obviously there's some weight but the OOUK CNC one is about 12lbs and it's unlikely to be that heavy when completed.

the mirror cell is based on the 'bible' instructions in the Dobsonian Telescope by Kriege and Berry, one of whom is the founder of Obsession Telescopes in the USA. here's a link to their design which is simpler and based on a square housing. http://www.obsessiontelescopes.com/why_buy/index_images/mirror_cell-300px.jpg

the advantages are that the back is open and therefore air can flow etc and also you have some access to the rear of the mirror for adjustments is required.

That said in my case with about 15mm all round the mirror only you'd need thin fingers!

it's also quite light and very strong.

you can use ali (or even plywood) but you have to increase the dimensions so much for strength that this increases the weight.

[Tom How]

I'm not sure I understand the collimation bolts either. Are you saying that it will collimate by vertically raising one pair of triangles with respect to the other two pairs?

In my mind that would produce uneven support on the mirror - i thought the triangles' surfaces all had to be kept in same plane and tilted together to collimate.

Or am I completely missing the point?

[Merlin66]

No, each bar and the associated pads "float" and tilt as required. I used a similar system on an 18" mirror - no problems.

[Moonshane]

quite right, each 'branch' (two triangles and a bar) is fixed to each collimation bolt in such a way as to allow a small amount of play. this is more than enough to allow collimation and support and is the way that Obsession support mirrors up to mental sizes.

it will become clearer when it's all put together. I'll update as I go.

it's the opposite of a 'normal' cell where you have bolts coming down from a plate with pads (on which the mirror sits) and through another plate with a nut on the other side and a spring between the two plates.

with larger mirrors in particular, the open frame version allows more efficient cooling and also more support all round.

[Merlin66]

On the 18" we had three small felt pads on the face of each triangle,each triangle was balanced on a 6mm ball bearing sitting in a recess/ dimple under the triangle and into a similar in the arm. ( a dab of slicon stopped them wanting to move)

The collimating bolt had a conical nose and the "arm" had a recessed dimple to allow the bolt to locate. A lock nut on the outside of the support frame was that was needed...no springs etc etc.

After almost ten years it's still fully functional.

[Moonshane]

cool - as always there's many ways to skin a cat - poor cats.

[Tom How]

quite right, each 'branch' (two triangles and a bar) is fixed to each collimation bolt in such a way as to allow a small amount of play. this is more than enough to allow collimation and support and is the way that Obsession support mirrors up to mental sizes.

it will become clearer when it's all put together. I'll update as I go.

it's the opposite of a 'normal' cell where you have bolts coming down from a plate with pads (on which the mirror sits) and through another plate with a nut on the other side and a spring between the two plates.

with larger mirrors in particular, the open frame version allows more efficient cooling and also more support all round.

I see - I get it now

Another useful lesson!

[Moonshane]

Ah I see now. Excellent. Don't think I've seen a cell where the collimation bolts attach directly to the bars holding the triangular supports. but no reason why this shouldn't work providing there's no play in the bolt/thread.

I presume the bars are mild steel? Does look quite heavy, though obviously needs to be very strong to hold a 16".

Look forward to seeing the finished job.

just weighed the assembly plus all the associated bits and it comes in at 11 pounds so not too bad really. to be honest I'd sooner have a bottom than a top heavy scope.

I can see wheelbarrow handles with air filled tyres being very useful for this scope!

[Astrokev]

Just for interest, how thick is the mirror and d'you know the weight?

[Moonshane]

hi Kevin

it's 35mm thick and at a guess weighs about 25 pounds. could be a lot worse!

still, it will be a heavy OTA when it comes to lifting into the back of the car and best as a two man (or one man and an unwilling woman I suspect) job. I generally observe with others when not at home so will generally have a helper.

I may eventually split the tube and this will allow me to stand the mirror part up in the boot rather than laying down at a shallow angle as it will be for now - amazing it fits into the back of a Rover 214i!.

the good thing is that all the weight will be close to the alt bearings (which are, naturally, at the balance point) and it should all be liftable by me just in and out of the car anyway.

according to OOUK's website the OTA with 9 point cell is 29kg (63.8 pounds) and I reckon mine will be pretty much the same weight so not too bad really. the base will probably weight about the same I think but cut-outs will reduce this to a minimum where possible.

[Astrokev]

Makes my forthcoming 6" build feel rather petty!

[Moonshane]

hi all

today I have mostly been making.......alt bearings.

in between work appointments (or job hunting appointments) and the showers I managed to router my bearings today.

I bought an 18mm and a 12mm full sheet(s) of hardwood throughout exterior grade ply. This will eventually be painted satin black so the pattern need not be perfect but it needed to be structurally sound and is much better than the stuff I used on the 'free' mount for my mate (although that will be OK too if/when painted).

My dimensions for the base are grounded on the width of the OTA/rings/trunnion blocks (480mm) plus 30mm thick alt bearings on each side and 5mm gap each side so 550mm altogether. So my base will be 550mm I got the 8x4 sheets cut into 5500mm squares.

this might seem a bit daft in a way as I won't use it all on this project but bit's the biggest project I'll do for a while, possibly for ever, and the other scope is a small job (8.5" f8 mirror) and I should have enough wood to make a truss dob for that in the price.

also, according to K&B, the ideal scenario is that the alt and azi bearings are approximately the same radius.

therefore I cut the circles with a router to about as big as I could get out of the 550mm squares, one 12mm and one 18mm each side to make 30mm thick bearings. I thought about a half circle shape but this would have meant the tube rings being much closer together than with full circular bearings. I feel that wide rings are better for support and balance (and will probably result in less twisting too) so I went in the end for circular bearings. here they all are on the kitchen floor (my 6" OOUK dob cap for scale).

the weight of these would have been an issue so I thought about ways to reduce this. The obvious solution is a 'cartwheel' design which cut-outs reducing the weight.

So I cut out a rough template in cardboard and this seemed to work well. I then cut another circle and cut out the segments in this piece which will be used as a router template. this will ensure that all the pieces are the same which is of course essential. as the collar on the router has a diameter of 17mm I had to cut the template to account for this so the dimensions will be more like the card template than the wood template and the corners will be rounded too.

here's some pics.

next job is to sand the template totally smooth and straight and then sand the bearing faces to ensure they are smooth and identical. I can then touter the pattern out confidently and eventually bond the two pieces together so four become two.

that's really the hard bit done then. this weekend (or maybe next weekend as it's father's day) I should be able to get the rocker box and ground board made after checking the balance point of the whole completed OTA to check the appropriate height for the rocker.

I am hopeful it will be a reasonably low and stiff rocker box. I will again use cut-outs to reduce weight but it's still likely to be around 30lbs in weight I suspect.

sides and bottom will be 30mm (again two bonded pieces) and the front and back 18mm. the ground board will be 18mm.

progress.

[Astrokev]

Can't say I was able to follow all of your explanation Shane, but a picture tells a thousand words. The circles look nice and..er..circular, so that's a big Well Done!

For my Dob rebuild (when I start it, etc, etc...) I'm now considering making wooden bearings, having seen the extortionate price of pipe stops. I also agree that larger diameter bearings are better.

With this new plan in mind, I need to put my creative hat on and figure out how to cut accurate circles with my limited tool kit. Unfortunately, don't have a router, so will have to resort to the jigsaw. I'm sure I can figure something out.

Look forward to seeing the next stage of your cartwheels.

[Moonshane]

hi Kevin

if the radius is not too tight you could use a thin piece of ply for a jig. drill a hole through one end for a bolt which goes through the jig and the workpiece and then fix the jigsaw to the other end (the jig should pass to the other side of the jigsaw and with the blade through a hole through to the workpiece - you'll need a starter hole in that too of course) at 90 degrees and at an appropriate distance (the radius of your bearings).

hope this is clear but I could make something to should you tomorrow if not and take pics.

[Astrokev]

Cheers Shane. Very clear and is basically what I had in mind. Main bit I'll have to be careful about is keeping the blade vertical. Sometimes they have a habit of skewing, especially when doing bends. Another thing to add to the list! Mind you, by the time I get to do it it'll be Christmas so could always put a router on my Christmas list (along with a pillar drill, new focusser, electronic camera controller....the list is endless.)

[Moonshane]

oh yeah, Kev it goes on forever. I suppose at least once you have the tools you can do anything really so cost per project comes down. a pillar drill is my next tool purchase but thankfully I have access to two for now.

I think with the circles via the hacksaw, the results may be better if you cut most of the meat off the workpiece before you try.

[Astrokev]

Actually, another solution has just jumped into my head. My father in law has an old table bandsaw in his garage. I think I could rough cut a circle slightly bigger than desired, then make a jig to attach to the bandsaw table and rotate the wood through the blade instead of the other way round. Think that will give a really clean cut.

My current plan is to make the tube out of wood (square or hex c/s. Haven't decided yet) and then varnish the final tube, rings and mount. Has to be functional, but I fancy the idea of making it look pretty also. I've ringfenced an old mahogany cupboard for the rings and probably use 6mm ply (or thinner if I can get good quality stuff locally) for the tube. Base will be 12 or 18mm ply.

Isn't wood beautiful!

Watch this space!

[Moonshane]

sounds cracking mate. I agree the bandsaw idea might be better as they usually have a thinner blade. presumably you could put a bolt through your timber/the piece under it to allow turning and then just set the radius and turn - should work!

look forward to updates in due course

[Astrokev]

Yep, that's the idea. Not sure when I'll get round to it, but it WILL happen! I'll keep you posted.

[spaceboy]

Hi Kevin

Jigsaws are great but as you say they are notorious for skewed cuts not to mention they often leave one side of the cut rough. With the band saw you could do a few dummy cuts to get the speed of the blade just right so it minimizes any splintering and would be much easier with a fixed blade to get an accurate shape. I would though say if you have the opportunity to get a router this would be the best option as there are so many advantages with a router in that it always leaves a smooth cut assuming you get the correct bit & speed. With the choice of different router bits you can always bevel the edges to give a much tidier finish if desired. Cutting out several segments can be a breeze if jigs are knocked up and you also have the choice of how deep you want the cut should you wish to counter sink Teflon bearings.

BTW great job so far Shane. Forget looking for work set up your own carpentry shop

[Astrokev]

Thanks Spaceboy.

I agree a router would almost certainly do a better job (once I'd learned how to use the thing), the problem is justifying the cost. Not massively expensive I know, but not sure what else it would ever get used for. Plus, I can think of about a hundred other things on my shopping list that are higher priority! I'll give it more thought though. Thanks for your input.

[spaceboy]

Have you considered a hire shop. Other than large deposits they can be cheaper than buying new. Just a thought.

[Moonshane]

I got a decent Bosch one for about £40 from amazon new.

it's been good so far.

I think it's one of those tools that's worth the one job you do with it. the others are just a bonus.

[Gina]

I got a decent Bosch one for about £40 from amazon new.

Well, that's not expensive, even for me Trouble is, there are so many things I would like. If I were to keep buying odds and ends my obsy would never have the funds to get started!