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8 inch Dob Finished at Last


Paladin

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The construction of my home-made Dobsonian is now almost complete after a lengthy gestation period, so here is the story of its construction.

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Some years ago, when I decided to build a telescope, I ordered a copy of The Dobsonian Telescope by David Kriege & Richard Berry from the States.  After reading the book, I came to the conclusion that a simple design, as described in chapter 13 would be more appropriate for me, rather than a truss tube design which I had previously planned.  I bought a second-hand Hinds 8 inch f6 mirror via UK Astronomy Buy & Sell, and also accumulated other items, including a GSO Crayford focuser and an Amici prism diagonal for the finder.  As this was before I retired from work, I was able to spend some lunch breaks making the diagonal holder and part of the mirror cell from aluminium.

About this time, a series of articles by Simon Lang of the Camden Amateur Telescope Making Society appeared in Sky@Night describing the construction of a 6 inch Dobsonian.  I took advantage of an offer in the article to buy a kit of parts which included a 445mm dia Formica Stardust laminate disk for the azimuth bearing, shorter strips for the altitude bearings plus the Teflon pads and other hardware. 

However, the project was put on the back-burner when I came into a bit of money and decided to buy a Celestron NexStar 6SE, so I could get in some star-gazing before I became too old!  Unfortunately, due to several factors, including advancing age, failing eyesight and light pollution, the scope didn’t get as much use as it should, so I sold it earlier this year, as I couldn’t justify having a reasonably expensive scope just gathering dust.
 
I was still keen on building a dob, so I recently resurrected the “bucket list” project, and came across the excellent construction articles by Ken Slater on the Stellafane web site, which were an additional influence on the design. https://stellafane.org/tm/dob/index.html
 
Newt-Web, an up-dated version of Dale Keller’s Newt Design software was also found on the Stellafane site, and I found it invaluable when designing the layout of the OTA. https://stellafane.org/tm/newt-web/newt-web.html

The main dimensions were set out on paper, and I kept mostly to the design parameters suggested by David Kriege, in particular his formula for sizing the large side altitude bearings, as did Ken Slater whose reasoning can be found here. https://stellafane.org/tm/dob/mount/altbearings.html

I was lucky enough to find a local Ironmonger, (yes! they do still exist), who sold off-cuts of 11-ply, good quality15mm birch plywood for a few pounds each so this was used as the main construction material, along with some 12mm & 7mm ply I already had.

Apart from a cheap bench drill, I possess only basic DIY tools, and as I use them only occasionally, I buy only inexpensive ones, although I’m well aware that some people consider this to be a false economy.   I’m also a great fan of Lidl Special Buys, and have bought several of their value-for-money tools for use on this project, including an orbital sander, hole-saws, drill bits and screws: the digital angle finder and digital Vernier Gauge being particularly useful for checking the accuracy of my woodwork.  I also use Wilkinsons (wilco.com) for DIY supplies and have used their own brand of varnish and paint on this scope.

More later: Paul
 

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What an interesting back story and a lovely looking scope Paul :smiley:

Those large altitude bearings are a great design feature - lovely smooth but controlled movement. Moonshane / Shane uses that design on his dobs and on the mount he built for my 12".

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Optical Tube Assembly

As the scope is to be stored in the Summer House, (aka garden shed), I decided to make the tube solid so that it would be easier to keep any moisture out.  When my sister bought a sheet of 5mm flexible plywood to repair a curved stairway in her home, she had quite a sizable amount spare which she kindly donated to the project.  Unfortunately, as the piece wasn’t quite wide enough, the tube had to be made in 2 parts which were formed around several wooden hoops and a wooden backbone down the full length.

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The short extension of flexi-ply was added to the top of the tube, and another piece was spigoted into it to reinforce the join line and provide extra thickness for mounting the focuser.  This led to a mini-disaster when the ply split and had to be covered up with some veneer, which gave me an area to fix an egotistical name badge. 

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Veneer was also used to cover some of the other joins; and narrow bands of flexi-ply and mahogany veneer strip were used to finish off the ends of the tube.  Screws were used only on the backbone of the OTA; everything else was glued with PVA wood adhesive, resulting in a very rigid and light assembly.

The GSO 2-speed Crayford focuser was mounted on flexi-ply packing to ensure that the inner part doesn’t encroach into the light-path when focused right in.

The hexagonal diagonal holder was machined from a block of aluminium, and the three vanes were made from 1.5mm aluminium strip bolted to it.  As the 46mm diagonal is slightly bigger than the optimum, there was no point in off-setting it from the centre line of the main tube, so collimation should be slightly easier.

The main mirror holder was also constructed from aluminium, in this case 32mm x 12mm channel for the 3 legs which were welded together. This assembly was fixed to a plywood cell with the usual spring arrangement between the two, and collimation is achieved by adjusting the 3 thumbscrews sticking through the bottom of the cell.

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As previously mentioned, the scope was finished with several brushed coats of Wilco’s satin yacht varnish.  It went on well and the finish is good, but it has the disadvantage of taking ages to dry properly.  Even when fully cured, it has a slightly sticky feel to it, so I will probably use a wax polish to seal it.  Any ideas, anyone?    Had I realised this earlier, I would have settled for a normal exterior grade.
 
The interior of the tube had one coat of varnish applied before the hoops & backbone were painted with blackboard paint, and then self-adhesive Hobby Craft black flocking was fixed between them.

The right-angled, erect-image finderscope was assembled from various bits & pieces.  I had intended to use half of a 50mm binocular which had gone completely out of collimation, but was tempted by an item on AstroBoot which consisted of a similar ocular with a purpose-built threaded connector that could be fitted with a standard 1¼ inch diagonal, which I already had in stock.  An initial experiment suggests that a 32mm Plössl eyepiece, which gives a magnification of approx 5.6, might be the optimum size.  The mounting for the finder was based on another item from AstroBoot: a pair of 30-45mm dia aluminium rings with 3 adjustment screws in each, which were originally made as digital camera supports.  I cobbled up a connection to a standard plastic shoe, but it was rather flimsy, so I made one from aluminium stock which is much more substantial
 

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The Mount 

I wished to incorporate some of my own ideas in the design of the mount; particularly with regard to the cradle which clamps the OTA to the altitude bearings, as I think it can often look too “boxy” in some designs.  My solution was to construct just the bottom half of the cradle, and to clamp the tube with bands of flexi-ply using over-centre clips. The base and gussets of the cradle were made from 15mm and the sides from 7mm.  Self-adhesive felt pads were fixed to the cradle and the flexi-ply bands to give the tube a snug fit, but also allow the position of the tube to be adjusted for balance. 

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The side altitude bearings were cut from 15mm ply with a jig saw and then screwed together, in order to finish them as a matching pair.  This was achieved using a cheap belt sander laid on its side and clamped to the work-bench; a set-up that was used for finishing the edges of all the wooden components, as it gave me an accurate 90° edge.  Unfortunately, the strips of Formica supplied by the Camden Amateur Telescope Making Society weren’t long enough for the large altitude bearings, so a trip to B&Q resulted in a replacement laminate called Inari Granite which has a similar textured surface.  As it was a clearance item, it cost only £5 for a 32mm x 3 metre reel. An 800mm long piece was cut down the middle with a craft knife, and fixed to the edge of the bearings with contact adhesive before being filed & sanded flush.  The rest of the mount was also constructed from 15mm ply, and as I was aware that the sides needed to match exactly to prevent any rocking movement between the bearings, they, too were clamped together for finishing.

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A shaped reinforcement at the top of the rocker box doubles up as an eyepiece holder, a feature I am particularly proud of.

The triangular shaped ground board was constructed from 2 pieces of 12mm ply, screwed and glued together to form a rigid structure upon which were fitted the three Teflon bearings pads on the top, and three12mm ply feet with rubber cushions on the bottom. The rocker box rotates on a 10mm bolt, nylon bush & spacer secured by a Nyloc nut.  Instead of using contact adhesive on the Formica disk, I used strips of 50mm wide thin double-sided tape which is a lot more forgiving to apply.  Like the main tube, the whole mount assembly was given 4 coats of varnish.

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I am now in the process of final adjustments and refinements, so will report back with details of first light.

If you have managed to stay with me to the end of this ramble, thank you for your patience; and if you have any comments, criticisms or questions, please don’t hesitate to air them.

Paul
 

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Update

Thanks to everyone for their complimentary comments about the Paladin Dob, I really appreciate it.   Although I admit that I’m rather pleased with how the scope turned out,  I can assure you that my craftsmanship is not as good as some of you have suggested, as there are many things which I wish I had done better.  However, as the first pics were received so well, here are a few more.

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As I had decided to clean the mirrors properly before final assembly, I collimated the scope only roughly, because I was waiting for my order of distilled water to arrive.  However, on Wednesday evening, the bright Moon enticed me into the garden to try out my new toy, and I’m glad I did, because it highlighted a few issues which I need to remedy before a proper first light.

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Although the finder is not yet adjusted, the Moon was an easy enough target to find in the dob, because ironically, the internal reflections told me when it was coming into sight.  So, my first mod will be to add flocking to the internal structure of the tube, but how well it will stick to matt black paint, I’m not sure.  I’m wondering if I should gloss paint the parts first as the self-adhesive backing should stick better to a smooth surface: any ideas, anyone?  While I’m at it, I will also flock the internals of the GSO focuser.  As it’s a 2 inch, the flocking should keep well away from the light-path.  The photo also shows that several fixings need to be painted over.

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During the design stage of my dob, I realised that the large semi-circular side bearings might cause balance problems, because when the scope is moved in altitude, the centre of gravity shifts from one side to the other.  This was the main reason for adding the lightening holes to the side bearings, to help reduce the mass.  My theory proved to be correct, because, as the tube moves so effortlessly on its bearings, gravity takes over as the balance point moves.  A possible solution could be an adjustable friction brake on the bearings, which will replace the small Teflon side guides that don’t appear to be strong enough anyway, so this will solve both issues at once.

I was able to try several eyepieces to check focus travel, and found that all but one would reach focus.   I will compensate for the deficit by adding another spacer under the focuser, and adjusting the mirror cell by a few mm.

In spite of these minor issues, I was very impressed with my first look, which I think gave a much clearer view than the NexStar 6SE, or am I just being biased?

I will post more updates later: best wishes, Paul
 

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