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gooseholla

Rebuilding 18" dobsonian

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Hello all! 

Four years ago I set out to try and build an 18" obsession style telescope knowing little more than a saw cut and a hammer hammered. Many people kindly helped me and I finished up with a telescope that worked, if not a little rudimentary and clunky in places. Now four years on I am relatively skilled and would like to fix some of the things I messed up or just didn't have the experience to do back then. Looking at bits, it was fine to say oh that is a bit wonky, or that doesn't look great, because I wasn't very good, but now I feel I can do better. So I will do little updates and ask questions, advice in here. 

1. Can I make the lower pole split blocks from plywood? A picture in the kriege and berry book suggests someone has. The beech or ash ones never seemed to work well - they all split eventually, and in the absence of maple - even drilling the hole in beech was hard enough! - is it viable to use baltic birch ply? I was thinking the cross plys would make it stronger and harder to split? Or should I abandon wood altogether and go for something else?

Regards

John 

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The split blocks on my 16" were made with MDF and have survived for over 10 years so far. With natural wood the orientation of the grain is important as all splits will be along the grain. Ply should be better but make sure that it is not bent too much when clamped up.

Nigel

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Hiya matey :)

There is a guy that has been to the south wales star parties (Bob) and he has an 18" dob that the built himself,  and that uses plywood split blocks.

As far as I know he's never had any issue with them and they certainly seemed to work fine every time I've seen him with his scope :) 

He is a member here but I I cannot recall his user name for the life of me, hopefully he will see this thread?? 

Good luck with the rebuild buddy :icon_salut:

 

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Hi all,

Thanks for the responses. MDF for split blocks?! Guess plywood will work, and I have plenty of it so will give them a go from plywood.

Today I started the rebuilding work. I started on the Upper Tube Assembly and rocker box. Why? A few sloppy errors, general rushing so finishing work wasn't great. Lack of knowledge of certain things, and also lack of tools - I had a brace drill, hacksaw, rusty handsaw, rubbish circular saw and that was about it! So I am rectifying them to make the telescope something I would be happy to show to someone at a star party rather than worry about my workmanship! I am not rebuilding it all. The mirror box will stay as it is for now. The tailgate is being rebuilt when I get back from Cyprus in two weeks. I will be using aluminium and following the guy from New Moon Telescopes way of doing it, as I cannot weld and this mild steel -- even though painted -- is rusting something rotten. Also, I didn't convert inches to mm correctly, so some of the bits are off in the tailgate system! 


Upper Tube Assembly

Old one - not completely level because I didn't have an accurate way to keep the spade bit level to make even pole seat holes. Someone I knew routed the rings for me as I had no router or ability. This time I have routed them myself and rounded them over etc. I also forgot to draw all the lines on to accurately lay the poles and clamps out, so was kind of guess work, so poles weren't truly opposite each other, nor were upper tube clamps. 

Rocker box

Old one - not entirely square. Base of it is de-laminating as it was cheap b + q plywood. Flexes a lot. Was my first attempt at routing so the part the bearings ride in wasn't quite right, so it cannot reach the zenith! The whole thing flexes really easily. Base board was made from a left over offcut as I didn't want to spend any more at the time, so it is about half the size of the rest of the rocker box, so not very stable! 

I attach some pictures to this thread (hopefully!) 

All is being rebuilt using birch ply. The original was marine ply, which does the job fine, but birch ply is cheaper at my timber yard. 

Regards

John 

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Edited by gooseholla
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This is what it looks like at the moment, taken before the rebuild.

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So to do list:

- Split blocks to rebuild

- Build new UTA clamp system

- Tailgate and flotation system

- Glue rocker together, find laminate to install on to it

- Find new material for wrapping the UTA in as a baffle

- Sand and varnish all the bits

 

Edited by gooseholla
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Nice to see another bit of dob modding going on John.

We're here watching with interest and lots of encouragement :icon_salut:

Edited by mapstar
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Thanks Damian. 

A few questions for people to hopefully answer:

1. What laminate for the rocker box bottom. I just used some offcuts of random b + q laminate last time arranged in a kind of circle. It never really worked great. I know 'ebony star' is what is suggested, and know it isn't easy or cheap to find. Has anyone found a suitable other?
2. Kydex - not cheap or easy to find again. I was looking at 1.5mm birch ply, or even other plastics. Again, anyone suggest an alternative that works?
3. I just cable tied my telrad to a pole last time but even when tight it moves pretty freely. Should I just screw it into the pole this time? 
4. The sling. I am using an old seat belt type material in split bolts. It works okay I suppose. Any reason to spend a lot of time and money upgrading. Does it make very much visual difference to the image at the eyepiece?

Thanks

John 

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Hi all

So did a bit of work on my scope yesterday and today. Found problem with the bearings one was mounted 5mm higher than other! Still out by mm or two but i will shim the teflon pad on that side. 

Rocker box is coming along. Sides are glued and the front board. Just got to mount scope in it when dry to determine height of back piece of wood. 

Ground board has been cut and has feet installed.

Gluing up some plywood for the sit blocks now. Hopefully make some later today or weekend.

still need some options for laminate please as well as answers to my questions in previous post. 

Regards

john

 

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Hi John,

Nice work and the re-engineering is coming along well.

Just a quick note on the bearings. When I mounted the bearings I made up a Jig which could be flipped from side to side working from the bottom reference surface. It had 30mm holes cut in it which the Router guide bush fitted into. 

A few answers to your Questions 

1. The laminate is quite expensive to buy and comes in 8x4 sheets. I have some and will pm you. 

2. Polypropylene sheet something like 0.8mm to 1mm would be a viable alternative to Kydex I think? I've used this company before https://www.hindleys.com/index.php/perspex-design-materials/other-design-materials/polypropylene/polypropylene-sheet/black-polypropylene-sheet-650mm-x-1100mmI.html. They supply schools and college's. Ebay may have it too.

3. I can't see why you shouldn't just need it in the right place.

4. Slings have been used for years and are a reliable way to hold the mirror. 

Keep up the good work, we're all watching :icon_salut:

 

Edited by mapstar
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Thanks Damian.

I had a template cut to place the bearings. I placed it on both sides and clamped and drilled the holes. However, I think because the clearance holes for the screws were 8mm and I drilled 7mm holes for them to wind into in the mirror box, the drill didn't line up quite right leaving a slight discrepancy. I was toying with taking it off one side, putting dowels in the holes and re-drilling, but to my mind the easiest way is just to shim the teflon on one side - lest I make it worse! 

John 


 

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"EBONY STAR" is an American trade name for Thermosetting resin ( eg phenol/formaldehyde ) sheet used primarily for worktops. If the books that mention "Ebony Star" had been written in England they would probably have used the trade name "Formica".

So where "Ebony Star" is mentioned just use any textured worktop material for the base of your rocker box.

Nigel

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On 19/09/2018 at 08:39, mapstar said:

da

2. Polypropylene sheet something like 0.8mm to 1mm would be a viable alternative to Kydex I think? I've used this company before https://www.hindleys.com/index.php/perspex-design-materials/other-design-materials/polypropylene/polypropylene-sheet/black-polypropylene-sheet-650mm-x-1100mmI.html. They supply schools and college's. Ebay may have it too

 

Oh I ordered some plastic for the uta. the interior isn't as matt (in fact probably nearly as shiny as the outer part!) compared to the kydex I had, but I will probably flock it anyway. £12 posted for a sheet. Cheers for putting me on to it!

12 hours ago, Astrobits said:

"EBONY STAR" is an American trade name for Thermosetting resin ( eg phenol/formaldehyde ) sheet used primarily for worktops. If the books that mention "Ebony Star" had been written in England they would probably have used the trade name "Formica".

So where "Ebony Star" is mentioned just use any textured worktop material for the base of your rocker box.

Nigel

Hi thanks Nigel. Damian is supplying me with some 'correct' stuff! I did use any old B + Q laminate on my current rocker but it didn't feel too great and was definitely jerky to get going. 

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10 hours ago, gooseholla said:

Oh I ordered some plastic for the uta. the interior isn't as matt (in fact probably nearly as shiny as the outer part!) compared to the kydex I had, but I will probably flock it anyway. £12 posted for a sheet. Cheers for putting me on to it!

Could you sand the inside or use course wet and dry on it just to take the shiny surface off. 

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11 hours ago, gooseholla said:

 

Hi thanks Nigel. Damian is supplying me with some 'correct' stuff! I did use any old B + Q laminate on my current rocker but it didn't feel too great and was definitely jerky to get going. 

There are different textures to laminate surfaces, some are better than others while smooth is very poor ( indicating that coarser might be better ). I have used coarse textured laminate from a variety of sources with perfectly acceptable results. You can usually get some free off-cuts from a worktop manufacturer ( if there's one nearby). My 16" is on such a sourced laminate.

You will also get different movement characteristics by changing the size and type of plastic against which the Formica if rubbing, PTFE ( various brands available e.g. Teflon, Fluon etc. ) being the defacto norm for the type of plastic. Size you can vary to get the best performance.

Nigel

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Today was an interesting day. I initially wasn't going to do any work but then decided to heat shrink the UTA poles. Eight hours later a lot more problems arose and have hopefully been sorted!

No matter how carefully you measure and cut the heat shrink and how straight you think you are, well, it isn't ever truly perfect! But they do the job - stop my hands getting cold when moving the telescope! I took the opportunity to flip the poles and drill new mounting holes for the spider so that it doesn't need such long collimation bolts. I drilled the new spider holes using a simple hole through a block of wood with the correct size hole in the right place.

 

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Then I was going to leave it at that. But decided to mount the focuser board... this is where it all went wrong. Suddenly the poles no longer stood upright, they leant to the side. After much playing about I determined some of the bolts to mount the trusses weren't completely straight, so I widened the mounting holes and they now stand as near upright as they are going to!

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After this I cut the plastic for the cage. It is held on by double sided tape, just so I could get the size needed. One roll wasn't enough to do the hole cage in one go, so I cut two bits half the size of the cage and the joins are behind truss poles, barely noticeable. Just need to sand and varnish it now. I will go back with some polypropylene friendly glue in the future and make sure it sticks properly. 


Next job is to remove the teflon pads from the old rocker box and install on the new one, and finish varnishing that. Then build new lower split blocks and upper pole clamps. 

John 

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Edited by gooseholla
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Today I played about with the split blocks. The first one went wrong - which is pretty standard I'd say for me, but I know what to do now. Second one worked great. Barely have to move it at all for it to securely hold the pole. Before hand you had to wind the knob quite a way for it to clamp, so hopefully these won't split! 

John 

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Keep at it mate ...I may not have much useful input but I'm eagerly follow your thread!

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Hi. Thanks for all the kind words people. 

Today I played around with the split blocks. Wasnt happy with a couple so rebuilt them. 

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Basic way of doing them cut on table saw drill hole etc. No need to show how the book does and it pretty obvious. Did use a scrap and clamp to keep my hands away when cutting the slots for the clamping part though. Now they are being varnised.

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They are not perfect but they sit under the scope shroud anyway and well they may well split in the near future anyway! I have seen some parts which hold 32mm pole for another reason (not lights!) which look really cool and will try and investigate them in the future. They are aluminium so won't break, anodised in nice colours and seem perfect, although pricey for each one. I will add more in the future once I've investigated some more. 

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I also started to sand the UTA ready for varnishing. The polypropylene is still being held by the double sided tape - it is working well, even where the plastic is attached to itself at the splits!. The bond that is failing is the sticky side which holds it to the wood - which had a layer of contact adhesive added first! The slightest bit of dust on the tape and it doesn't hold well in those parts. I will cut some more strips and add them to the couple of parts that are pulling away from the wood. 

Jobs left to do (at the moment!):

- Make UTA pole clamps and attach
- Varnish UTA
- Cut plastic wrap on the poles as these seats are couple of mm deeper
- Assemble rocker 
- Put inserts into tailgate to accept the collimation bolts instead of the epoxy nuts, which are failing

I was going to rebuild the tailgate this year, but I just want it working for winter. I think next year I will rebuild the tailgate, mirror box and side bearings, so it is all a uniform look and then it can be classed as version 2!!

 

 

 

Edited by gooseholla
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So today the laminate cut by mapstar turned up so I installed it straight away! Looks great and will hopefully work very well! 

 

 

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Today I did a couple of jobs. Cut out the UTA clamps so that I can sand and finish the UTA tomorrow. They are crude but work - same system as on version 1 of the scope. Two semicircles mate together and a cam lever locks them into place. They are made from Ash, 20mm and 10mm thick. Deceptively simple to make... just take a hole saw the size you want, and then butt up the wood to the central drill part. Now clamp the wood down and drill. You'll get a nice semicircle with no drill holes in, and no cutting required to make them all the same.  I will be remaking the part attached to the poles now I have the holes in the right place. 

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I then moved onto the rocker box. I drilled various holes for all the hardware. A 32mm hole to allow clearance for a ratchet around the central bolt was first. I did this by cutting a 32mm hole on my drill press into a block of wood then drilling with a hand drill the depth of the bolt into the board. This allowed me to do it nice and straight with a hand drill. I then finished off doing the same with a 12mm bit for the central bolt.

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The ground board needed a 13mm hole for the t-nut. I pondered for a few minutes then it hit me - a router bit is 12.7mm. The router allowed me to get a straight cut and with the slightest movement once a hole was made the t-nut fitted nicely. I dirlled four holes for the prongs and hammered it in. It is recessed into the ground board so that the rocker moves on the teflon pads and not the nut - like it did on version 1.

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I then took the teflon pads off the old ground board, cleaned them up, beveled the edges a little and stuck them to the board with double sided tape and some nails from my nail gun, which fired them just below the surface. I then bolted it altogether to see if it worked smoothly, and it does and there is no play between the two boards. Yey!!

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Other boring jobs were trying to even up the heat wrap on the long poles. I had a brain wave - use the tube cutter to score round and cut the wrap off nicely. It has sat in the same place for the past three years - until I want it today. Couldn't find it! So that will have to wait till tomorrow to find.

Got a couple more coats of varnish to do on the rocker and the lower truss clamps. Then I can install the clamps to the mirror box, get the teflon pads for the bearings installed. Then I need to sand and varnish the UTA and install the secondary mirror. Hopefully it might get done this week, as I have Monday evening and all of Tuesday to dedicate to it.

John 

 

Edited by gooseholla
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This evening I have added the piece of wood across the back of the rocker box to finish it off. Sanded it and put the first layer of varnish on. Tomorrow I plan to sand the UTA and varnish that for the first time. Tuesday I plan to get two more coats of varnish on the UTA, put the teflon altitude bearings on, finish the varnish on the rocker box and then rebuild the clamps for the truss poles to attach to. 

John 

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John,

To have a very smooth azimuth action (especially with these large scopes) you need to add a central teflon pad, around that central bolt.
Even with teflon pads and formica you'll end up with a bit backlash. You also end up with a little bit to much friction.(again --> large scope).
What I do is sanding the central teflon pad a bit down, just a little bit, the thickness a of a sheet of paper, maybe even less.
Then I install all options on the scope, including eyepiece and place rocker on the base.
I try and move in azimuth. I do that while the scope is almost pointing in the zenith. If there's (to) much friction or backlash then remove scope and add some thin sheet of plastic and place it UNDER the teflon pad, try again. If there still some backlash or to much fiction, again add some thin plastic and try again. Continue doing so until the scope moves with butterly smoothness.

Purpose of this procedure is to take away ALL(!!) the weight from the outer pads. All the weight of the scope and rocker should rest on that central pad.
The outer pads should only guide that rocker and also hold it level.

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Today I did a couple of bits on the scope. Did my best to get the pole blocks to attach to the mirror box at the same angles. A couple of them need re-jigging as the screws aren't holding it right, so will take them off one day and rescrew them. The pole heights are all roughly the same - maybe a mm or two out  across them.

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Also finished painting the inside of the rocker box. It is done varnish and paint wise, just needs minor sanding here and there. 

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The UTA has had a couple of coats of varnish that will be ready this week for all the hardware to reattach to. Then it is just a case of working on the collimation system and sling in the mirror box - as well as cleaning the mirror. Cleaned it a month ago and it is filthy after being in storage! 

When it is all complete I will also have to modify the storage unit I made for it, to stop so much dust blowing in to it!

John 

 

 

 

Edited by gooseholla
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