14" Double Truss Newtonian Build Thread

[coatesg]

It's about time that I got together a thread describing my rebuild of my old Orion Optics SPX350. I'd bought this a *long* time ago with a mind to doing it up to use for AP, but then house move, life, etc meant it sat around doing not a lot for a long time. When I came to use it, I got some good results, esp on planetary work, but also found that under the weight of the heavier SBIG CCD, the thin tube didn't hold collimation particularly well. Here it was:

 

So, eventually, I decided to have a rebuild. I plumped for a truss tube over a remount inside a carbon tube. Not sure whether carbon tube would have been cheaper now though to be honest! 

As the scope is mounted on an EQ mount (my Losmandy Titan), it needed to have a central brace, and so I shamelessly borrowed many ideas from Rolf Olsen's excellent scopes (see: https://www.rolfolsenastrophotography.com). 

It started with the three rings - these were routed out of 21mm Baltic Birch Ply (sourced free from a mate who works in wood sales...). Internal diameter is 390mm.  

Onto these were mounted a new Orion 9 point primary cell (to replace the naff original 3-point cell) - shown here without any connecting hardware!):

and with a bit of 1.5" aluminium tube, some drilling and making of small recesses using a spade bit, a secondary cage was constructed -- again without the final countersunk parts and connecting hardware:

 

To be continued!!

 

[coatesg]

The spider shown in the previous post was a custom build by Klaus Helmi (who I believe makes hardware also sold by Teleskop-Express). It's really a lovely piece of kit - very well engineered, and much easier to adjust than the previous spider I had on the SPX which was made by Protostar. The secondary cage is bolted together by two countersunk M10 bolts that screw into star inserts inside the 4 connecting 1.5" pipes. The star inserts were from Jetpress and can be found here: http://www.jetpress.com/Products.aspx/otelfxlw12/TubeConnectorNuts/ (it took me a while to find something like this). 

The design of the spider allows a dew heater to be placed behind the secondary with wires feeding along the vanes and down the hollow threaded stalk. The spider vanes thread through the aluminium tube - slight rework here as I realised the star nuts sat beyond the spider mount points!

 

While this was being built, I asked another friend who is a welder to make up a central brace, again, following Rolf's original and something along the lines of this drawing:

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Onto making the connectors to try to join everything up (and fast forward a year - slow progress!). A real collaboration with friends on this one - another mate who is interested in hobby machining helped me out on this one and milled up some adaptors to join trusses to right angle brackets that I hacked up. Here's a shot of a few of these connecting to the back of the secondary cage. 

Similar arrangement exists at the mirror end, as well as six smaller brackets to hold the primary mirror cell onto the rear ply ring. It would have taken me a month of Sundays to make these parts, and they would have been nowhere near as nice as how these turned out. Here's the rear end with the various aluminium brackets and connectors (mirror in place in cell!). 

 

Note to self, before mounting everything up, check the fan is blowing air the right way - as supplied, it was blowing away from the mirror which isn't a lot of good in a truss scope....

 

[NGC 1502]

 

Hello Graeme. Thanks for posting. I’ve never built a scope this large, but I’ve done several smaller, and know how time consuming it can be. Several hours in the shed can disappear far faster than expected.......

Of course it’s not a race, and when it all comes together at ‘first light’ it is worth all the hard work, head scratching and unexpected expense, so good luck, and I’ll  check back now and again to see how you’re getting on.

Cheers from Ed.

[coatesg]

24 minutes ago, NGC 1502 said:

Of course it’s not a race, and when it all comes together at ‘first light’ it is worth all the hard work, head scratching and unexpected expense, so good luck, and I’ll  check back now and again to see how you’re getting on.

Definitely takes longer than you think it will! However, can further update (since it's all done!):

The brace was built successfully, and then I had to do a bit of maths (estimation!) around the required lengths for the truss tubes, with a mind that I need to get it balanced around the central brace. I didn't go down the route of a full analysis to get it to work as a perfect Serrurier where the deformations match at either end. My thinking was that the tubes are somewhat over engineered and should resist deformation to some extent (they are well over the recommended sizes from Kreige and Berry for truss dobs). Back to the mill with friend, and after the first go, test and then shortening slightly, we had a scope that would come to focus. Here's the connections to the central brace (note that I'd flocked the trusses by this point!) :

I'd also by this point mounted the focuser which was the Moonlite CR2 that I'd previously mounted on the SPX - this is affixed to a 4mm aluminium plate, which in turn is embedded in the secondary cage rings. To affix the plate to the rings, I drilled and inserted/glued some dowel into the ply rings, which allowed me to screw into the ply from the side giving the pan head screws something to bite into without splitting the ply. You can also see the CCD on the scope here, as well as the autoguider and geared stepper motor on the focuser shaft. The secondary mirror is attached with silicon adhesive to the mirror holder - this has offset built in as well.

A view down the truss shows the scope mounted on the Titan (it didn't fall off!), and also shows the dovetail that we milled out with a dovetail cutter, and subsequently drilled and tapped for the 6 M6 bolts holding it to the central brace.

 

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Some detail from the front end - I'd reduced the weight of the front ring slightly, and also sanded and stained/varnished the ply to weatherproof. You can see the countersunk bolts here holding the secondary cage together. 

At this point, it's basically a working scope! Here's some wider pictures of it in this state.

 

EDIT: I forgot the secondary cage shrouding and flocking! This is some black plastic sheet (can't recall what type - black ABS perhaps...?) that was cut carefully to size, with relevant holes for focuser, spiders, etc. and then flocked entirely inside. It's attached to the ply rings with small stainless wood screws, doesn't flex into the light path, and is very dark indeed.

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The observant will notice the Bahtinov mask (which I got laser cut in ply from Oxford Hackspace as you can't buy them that big!) - that works excellently. Also, the light box - I've never had a lot of luck with sky flats, so I built this out of a 600mm LED lighting panel and mount, two sheets of translucent polycarb, and a couple of sheets of ND film (for stage lighting) to bring the light down to sensible levels. The original plan was to mount this onto the dome, but the angles just don't work out (it would obstruct part of the opening!), so instead I park the scope at zenith, and place the light box on top. 

 

Here's a view from inside from just above the centre spot of the primary (before I added the circular mask to the light box):

 

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To reduce all the cables, I mounted a small form PC (a Gigabyte Brix) onto the brace, along with 12VDC power distribution from HobbyKing. This powers everything except the SBIG camera including the PC, focuser and fan (and in future the dew heater), so there's only 2 cables between floor and scope (1 once my new QHY cam is on there). Wiring is all zip tied along the trusses where possible to keep it tidy. The PC communicates via wireless. 

 

The last real addition to the scope so far to bring us up to date is the shrouds - these were a present from my wife and made by Heather at Scopeshrouds - they are very well made and will be excellent at blocking out stray light. I do need to mask off the various power lights in the dome (on the power distribution, wifi router that the Gemini plugs into, the PC, etc!), but most point away from the scope at least. 

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So - that leaves the snagging.

• I need to sort the dew heater out - it's already in place, but needs the feeds soldering and sticking along the spider vanes. 
• The mount needs degreasing, regreasing and then PEC training - that's one of my next jobs really (and a good one once the darkness goes in a few days!). The PE looks fairly consistent (+/-7" PE) and should correct out nicely I hope.
• I have to rebalance - I have another 22lb weight now to go on the CW shaft there to properly balance it, replacing an 11+7lb combo; balancing in DEC is a bit harder; I may have to add smaller weights top/bottom as needed. In visual mode it's very lacking in weight at the eyepiece end...
• Could do with a mask to cover the mirror clips (see Merope image!!!)

And I'm sure there will be a load more stuff that'll fall out as it's used!

 

Other than that, it works as I'd hoped - it's very satisfying to get images through your own scope - here's some links to posts using the rebuilt newt

https://stargazerslounge.com/topic/303121-99-illuminated-moon-mosaic-in-18-frames

https://stargazerslounge.com/topic/307100-merope-lighthouse-in-the-fog

https://stargazerslounge.com/topic/310304-m106-in-canes-venatici

https://stargazerslounge.com/topic/313148-an-m96-lum-and-a-distant-quasar

 

Hope it's been an interesting read - should really have kept it going as I did it, but would have been a very slow bit of progress - took about 18 months after finally getting started, and about 3 years after first thoughts!!!

Cheers, thanks for reading this far!

Graeme

[Stu]

Great thread Graeme, looks like you’ve done an excellent job on it  

[coatesg]

Thanks! I'm not the most skilled when coming to this type of work, but I think I surprised myself here (and had the good help of more skilled friends!). Learnt a lot doing it too - enjoyable!

[Dragon_Astro]

What a great build, having near completed my own build of a 12" truss dob, I can appreciate the work gone into yours. I'm in the middle of blacking the various parts out, what did you use on the truss tubes? It looks like some sort of flocking?

[coatesg]

Yep - it's flocking (rolled onto the tube). I did initially try painting the central brace with non reflective paint (ha ha!). After that failure, I flocked the tubes and the inside of the brace section.

The other smaller parts I haven't really bothered with - they are either a long way outside of the optical path, or very small in area so probably won't make much difference. Certainly, the diffraction from the mirror clips and drawtube impingement makes more difference!

 

[Whistlin Bob]

That's a beautiful piece of engineering all round!

[Stub Mandrel]

Nice one Graeme, good to see the finished result 

Next, let's see what it images like.