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furrysocks

Meet my scope - 8.5" f/7.6 Dobsonian Newt.

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She needs some more paint and smoother az. bearings, but she's essentially finished. :D

post-27136-0-02241200-1364419503_thumb.j

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very original scope, looks great! larger alt bearings would possibly work more smoothly but it looks well balanced. :smiley:

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Looks fantastic, very portable, what type of laquer did you use?

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Very nice design. Good work. At 7.6, I bet she is a nice planetary 'scope.

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Nice job, looks great. How long did the build take? Were they pre-made mirrors or did you grind them yourself?

Either way, well done

Phil

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Also intrigued as to where you got the design idea from, incorporating the two sets of truss poles? I expect though at F7.6 it will be fairly long, the truss designed in this format allows it to be more compact.

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Thanks, folks!

It is "original" - that made me and my wife chuckle! It evolved...

I began by planning a dirt cheap build along the lines of Toshimi Taki's 8 inch dobsonian - a long dodecagonal tube. I was going to get the tube made from a single sheet of 3mm hardboard around some scrap mdf/laminate ribs but when I mocked up a short tube, I liked the look of it and decided to change to a truss design. I've got another 8.5" mirror too, and had been thinking "binoscope". I've since made up the six tubes for both OTAs from that one sheet of hardboard.

The truss design came from Robert Royce's 8 inch truss tube newt. The double truss configuration I believe is called Serrurier and it has the advantage that as each end sags from the middle, they maintain collimation better than single truss designs. With heavy tubes and a long scope, I thought the principle sound. The tubes came out really fragile, so I used 30gsm surface tissue and polyester resin on them all to increase their strength. The middle tube got another layer of 225gsm CSM and resin as this is taking all of the weight. I had been going to put external rings round them all both to hold them together and as something to attach the trusses to - glad I didn't - shoutout to astrobeast on CN for helping me think that one through. I'll spare the details, but after ensuring structural integrity, it's all about geometry.

The mirrors came from preloved.co.uk - a guy in the North East was selling them - two primaries and two secondaries for £100-odd. They'd been salvaged from a Charles Frank scope that had changed hands and was subsequently dropped. I've had the secondaries re-aluminized. The primaries will follow but are good enough for now.

The finish is just £1shop matt black paint. I'm on my third tin and anticipate finishing the fourth when I re-coat the mount. For the most part, I just wiped it on with a kitchen sponge. I've used an old tin of blackboard paint inside - it needed a bit of thinners and a good stir to get it going. I've got a new tin too, and may re-paint inside if it helps.

I was going to make a 2" helical Crayford focuser with some skateboard bearings and aluminium tube, but stuck with plumbing parts and an old cut-down Barlow to make a very serviceable 1.25" focuser.

The bearings are running on kitchen chopping board at the moment.

I started in December - working with glue, fibreglass and paint in sub-zero is not fun so I've used a fair bit of electric to heat up a curing cabinet. It's been evenings and weekends only, and very bitty - as I had no real plan, I needed to see results before I could move on. A very inefficient way of working - I had six weeks off the project because I was getting frustrated. It did not end up a dirt-cheap build. At every step, I reasoned that I needed to spend more money - I'm not going to add it up. I never will! I do however have pretty much all I need to finish off the next one - perhaps just a sheet of ply and the odd bit of hardware needed.

I wouldn't say it's particularly portable - the OTA is >10kg and I've not weighed the mount yet. Vibrations appear settle at 50x in about 1 second. The OTA does come apart into two sections and tomorrow morning, I'm loading it into the van along with the family and heading for darker skies.

So that's a little bit on the design and the evolution of the build. I blogged it on here while I was going, for what it's worth. It's all a bit 'stream of consciousness' but the process helped, I think.

Anyway, I can put all that behind me now and start observing - you know, as we're approaching summer - ha!

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you have certainly got the 'dobspirit', a mythical quality which ATMers have all to themselves :grin:

as it happens I work in exactly the same way when addressing a project. I don't smoke but need a few fag packets for the design process and then it's a matter of thinking what you want and then adapting things and changing things as you go based on experience and what does not work / what does.

incidentally, I honestly meant original as a compliment. I like it!

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Very nice looking scope. I'm going to rebuild my 10" at some point in the future I may well go with the serrurier truss look too. Like it :)

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any one who builds there own scope deserves a :icon_salut: well done and i bet the sense of achievement is great.

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You have have put a hell of a lot of work

into that.It is great, when you build something

your self,no matter what the cost.Impresed

Steve

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Great scope at f7-8 a good all round scope,just goes to show

you do not have to spend a fortune well done.

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Finally got a 2" focuser installed onto this scope.

newfocuser.jpg

The new focuser and the secondary sit 25mm down from before. Lots of holes to patch up - had to flip the tube over on it's alt bearings to get a fresh surface to drill, will mount the monocular-finder on the central section to reduce top end weight and keep a red dot up by the eyepiece.

The old plumbing parts focuser, depending on webcam/EP took a 1.25" shoulder from something like 60-80mm from the OD but didn't have enough in-focus for webcam+focal reducer. Minimum height of new focuser is 65mm, plus whatever for a 1.25" adapter. Has 75mm travel (wouldn't want to use it that far out).

I originally moved the secondary down by 50mm, but now it's only down by 25mm. At 50mm, I tried my 2" EP during a break in the clouds and had to rack it pretty much right out, so should be +45/50mm now. I think my webcam in a 1.25" adapter will be somewhere +20, including height of adapter, so perhaps webcam + focal reducer is still going to be a bit tight. I didn't have a chance to test a 1.25" EP or the webcam, but something about these calcs doesn't seem quite right - I'll have to wait for clear skies or daytime. If it needs moved again, I've one more surface I can perforate before I have to rotate the tube in its rings.

 

Unfortunately, after being in storage, the mirror's a bit of a state - you can't see much in this picture but you get the idea.

mirrorstate.jpg

 

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2" EP focused on the moon at around +50mm, and including 1.25" adaptor height the 18mm focussed around +55mm.

I need to try a webcam at prime focus with a 0.5x focal reducer and determine how much in-focus I'm eating up. I don't expect to be able to move the focuser back out to where it was, but if I can I will. I may check the size of my spare secondary and do some calcs, possibly replace.

May cut down the end of the top tube by a few inches once I'm settled, to reduce weight and bring the CoG back a bit.

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2 hours ago, westmarch said:

Respect! Clearly a labour of love and ingenuity. 

John

Thanks, John.

I spent more time building it than I have looking through it, even now. But the two new EPs I've got for it give it a very different feel. I'm still fond of it - spandex light socks hide the aesthetics of the double truss, which is a bit of a shame.

I just don't know how bad a mirror has to get before it's not worth trying to recoat... it's pretty bad. :/

 

Yet to get it under dark skies.

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Slow motion altitude control using a clothes line prop and a length of 16mm threaded rod.

Should be fine once I've an EP in it, or re-attach the finder, or if not can always add a bungee.

Thanks for the motivation, fiestazetecmk2! :icon_biggrin:

 DSC_0106.JPG

DSC_0107.JPG

DSC_0108.JPG

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My idea was similar but having the adjuster central with a knob to turn .and using rose joints at either end for smooth movement when adjusting.but I suppose it all adds cost.

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Couple of tweaks...

New az bearings - tap top caps... https://stargazerslounge.com/topic/286719-tap-top-cap-dob-az-bearings/

Also, a broom handle az control and some wheels... currently I can pull down on the end of the ota to rock it up onto its wheels, provided the washing line prop alt adjuster is nice and tight and I line up the three feet appropriately - broom handle is softwood and too springy and I want it facing the other way anyway. A couple of stiffer, removable pole handles and the wheels can stay in place.

wheels and broom handle.jpg

However shonky my work, it's more portable/usable now than it's ever been.

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Next up, making some crosshairs for a spare EP and mounting a 3" f/4.6 reflector as a finder...

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Well done for building your own without any plans to look at.the sense of achievement must be great.keep going you owe it to yourself to complete this dobsonian I'd be very proud to build my own.?

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