A baby step towards a truss imaging newt

[HenryNZ]

I have been lurking on this forum for a while now. I have always wanted to image with a fast newt. I am thinking about getting a gso 8" F/4 newt, gut it and put the optics in a DIY truss frame. This is more a test and the idea is that if it works out then it will form basis of future larger newt, and if I fail I can put the optics back in the original tube which is still within the limits of my neq6 so nothing is lost.

Now I am a complete novice with woodworking and metalworking so I have been dragging my feet making a start. But this weekend I finally take the plunge and rout out two rings for the UTA using some hardwood board I have lying around. I told myself if I could not cut out a good set of rings it would be an omen for me to stop here and there, but they seem to come out alright...total cost so far = 0.

Now that I have the top rings made, I need to figure out how to make the centre frame and the primary mirror holder / LTA. I think for the former I will copy / imitate Rolf's idea by using a few square aluminium tubings riveted and welded together. I am not sure what I am going to do with the LTA but would like to reuse the centre circles left over from cutting the top rings. For the trusses I probably will also copy Rolf and get some carbon tubings from hobbyking to save cost, but I am a bit puzzled as to how the trusses should be attached to the hardwood / aluminium parts.

Some hand holding is much appreciated.

Add: went to hardware shop. That piece of scrap timber I routed out is no hardwood. It is just an undressed pine board. Seems hard and square but probably no good in the long run. So will need to do it again but it's a good exercise nonetheless.

[glend]

I suggest you read through my 10" Truss Imaging Newt Build thread in the ATM sub-forum here. I used GSO mirrors, and focuser, and it's a great light weight imaging scope. You'll need to be able to work with carbon fibre and epoxy to copy mine but it might give you some ideas.

[HenryNZ]

I have been toying with newt25 web apps using specification for GSO gears. I have some warnings about 75% ray vignetted - not sure if it is all relevant or important. Any advice?

[dave brock]

I strongly suggest that any wooden part be made from plywood. Those rings will be weak across the short grain no matter what type wood is used.

Dave

[glend]

The Newt design apps are primarily for visual use, and imaging newts typically have larger than normal secondaries to provide full illumination of the camera sensor.

Regarding the ring material, absolutely marine plywood is the best choice. Make sure you allow the internal diametre of the ring to accomodate the mirror, and a small increase as you move forward; allow about 2.5 degrees at the front. A Ray Diagram is essential (I found) and using bright brick layers string to simulate rays on you mocked up truss tube is a good way to work things out. Build your truss tube on a jig (or strongback as the boat builders call it) - essentially an absolutely straight backbone to setup the parts to ensure correct alignment.

When setting up your secondary placement, I found it easiest to allow the spider holder ring to be able to slide on the truss structure, this allows you to accurately determine the focuser and focal point placement.

:thumbsup:

[HenryNZ]

Thank you. I will go look for marine ply wood. Both Bunnings and mitre10 do not sell them. How thick plywood would you use for an 8 inches?

[glend]

I used 19mm on my 10", you might be able to use 13mm on an 8" and you will not save much weight going thinner as your cutting out relatively thin rings. Consider how your going to build your truss or strut structure. Struts are easier to build and you can still triangulate support bracing. Importantly you want to use a material with maximum stiffness if going with a strut, it is less important for a truss.

[ZeroID]

Henry, It says I am only about 5 km away from you. I'm in Mt Albert. I have built two Serrurier Truss type scopes so far, following Rolfs ( Skyviking) design. If you want to pm me for any help please do. I have a pretty good home workshop and some wood and engineering skills, lathe, drill press etc.

[HenryNZ]

Thank you Brent! You, Rolf and Raymond up in Whangarei are my main inspiration. I am more just at testing the water / kicking tyre stage, but would be grateful if I can call on you when such need arise!

[Rac]

I think Rolf's scope would be the best to copy, mines a bit more tricky being mostly carbon. Personally though I would put a layer or two of carbon on either side of the ply rings to make them really stiff with the rear one being most important. I needed to do this on my only ply ring(at the rear) in my scope as I had some collimation shift when moving it around the sky. This is because the truss poles have 4 points of contact on the ring but the mirror cell only has three so it needs to be very stiff.
Rolf say he doesn't get any focus shift with his alloy poles so maybe the carbon poles are over kill. Alloy ones are easy to work with to.

In the end when the scope is all made and it's stiff and holds collimation well there is still one thing most people go past and that is a really good coma corrector, without a good one all your hard work will be wasted. Bite the bullet and get a Televue paracorr type 2.

I have no problem with the idea of buying a gso 8" f4 imaging newtonian and using all the parts, that should work fine.

[ZeroID]

Both my Truss type scopes are alum poled. Serrurier design tries to balance any deflection. Biggest issue was I needed to stiffen my UTA assembly. Trying or too much weight reduction.
Buying the 8" f4 is a good idea. Probably cheapest way to get all the components required. I am unsure as to how much weight you might save and solid tube OTA's are very stiff and stable but if the plan is to test build capability then why not.
As I said before, willing to assist.

P.S. : Are you a member of Auckland Astronomical Society Henry ?

[Rac]

It was a focus change due to temp changes that I was concerned with when I made the decision to use carbon poles v alloy, Rolf didn't have his new 12.5" f4 going at that point in time.
I'm going to make a new scope at some stage using a 12.5" f3.1 mirror that I have just about completed, I'm thinking about using alloy poles for that so that should be a real acid test for them.

[HenryNZ]

Thank you for all the tips.

I am not with the Society. I have been thinking about going to some meetings but between my work and my family it is really too hard to get there. I do most of my astro stuff after kids bed time by which time it is often too late for meetings.

I have tried couple of plywood suppliers asking for off cuts of marine ply but they do not have them. New marine ply is very expensive and more than I would need. I think I will take Brent's suggestions of using structural ply from Bunnings, at least for the "proof-of-concept" project. I have also settled with aluminium tubes as the trusses for similar reasons.

I do agree that the weight saving / stiffness improvement is probably minimal between a stock 8" and a DIY truss as the stock OTA is only 8kg. The reasons why I kind of choose this is because of cost and the wish to stay under 1000 mm in focal length. I guess I could go 250 mm f/4 which will give 1000 mm focal length - but it is more expensive and if the project flops I cannot put the optics back into the stock tube and use it with my NEQ6. May be I will get the mirrors and focusers by themselves only to save cost...?

[Rod]

Another option for your wood is formply. It does not have the attractive face of some plywoods but it's water proof and very flat and cheap.

Bunnings here does sell a marine ply from Asia. I am surprised they don't sell it in NZ. Perhaps you could get it through the special orders section?

Rod

[Rac]

Up here in Whangarei we have a demolition warehouse that has lots of off cuts of all sorts of ply for cheap. Some of it looks really good and with a layer of carbon on either side you don't need to be too fussy on what ply you use.
Bunnings in NZ charges heaps for ply.

[glend]

Henry I think you will find that buying the parts like primary, secondary and focuser would not be as cost effective as buying a complete GSO scope, and you get a dovetail, and finder, and mirror cell and spider if you wish to use them.

[ZeroID]

Henry, you need to come and see me and my builds. Might give you some ideas and things to try. It's all in the detail.

[astro_nutt]

Hi Henry.
Have a look at "current project" via search this thread. Hopefully it can give you a few ideas. Cheers.

[HenryNZ]

I have been a bit distracted lately and have not done much, but is keen to start again. If anyone in New Zealand has spare or left over construction bits like carbon bits or framing bits or mirror that they do not need any more because they have upgraded I am interested to buy off from you. Sourcing material for this project is by far the most consuming exercise I found. Thanks

Ps. I like to add I am thinking about building a 10" F/4 to make it really worth the trouble despite what I said originally about doing an 8". The steel tube 8" is not that heavy and I feel the effort of building a lighter frame may not be compensated for by the weight saving.

[ZeroID]

Come up with a basic design and materials list so you know what you need. It may change as materials are found but gives you a starting point. Bunnings is best for alum and ply options. CF and other exotics will be harder to find and a lot more expensive plus difficult to work with.
I went from 35 kg to 11 kg barebones on my 10" F5 build, 17 kg with mirror and accessories. Give us a yell if you have time for a look at my two builds.

[JuniorDave]

Really new here. But learning lots
If you're looking for fibreglass of carbon, talk to nzfibreglass in auckland. They're great to deal with.
I've also just started working on a project. A 14" F3.something.

[Rac]

Are you making the mirror your self?

[JuniorDave]

I am. Stephen on here sent me the blanks. Using a rough piece of glass as the tool. Slow going as they are the same size and I'm trying to get 7mm out of the middle of the mirror. Just over 4mm at the moment.

[Rac]

Cool.

You will find it faster to hog out using a ring tool or a plaster and glass tile tool that's not full size.

[JuniorDave]

Yeah I know. But I am planning on trying to use the glass tool for fine grinding as well. So I want it the same shape as the mirror. Will see how that goes. Next one I'll probably use a ring tool.
I also figure I can't really screw up the shape much if I'm using a full size glass tool. So for my first time, I'll stick with this method, then hopefully try easier methods in the future.