I have taken the plunge and have an iStar Anastigmatic Doublet 127mm F12 R30 objective on the way for my scope build. I had originally wanted to go with the 150MM but it was not available (yet) in R30. The focal length will be 1476mm. Spot diagram is here:
My scope design will be along the lines of the iStar TCR, an open strut and ring structure, in many ways similiar to the designs that Dennis Steele of Dobstuff uses for his scopes. This design will give me a very light scope for its size. My goal weight is to keep the total weight below 7kgs and this is very achievable, given the objective only weighs 3kg.
I will not be using any aluminium or steel in the strut structure, the rings will be made of marine plywood and the struts themselves of Tassie oak dowels or battens. It will be assembled on a jig to insure alignment and I will be using boat building epoxy systems to bond it all together into a monocoque structure.
I will resort to aluminium for the back plate, and of course the focuser will be a Moonlight of similiar.
More info, photos, and updates to follow once I get started on the build proper.
Interesting build Glen. I'll be watching with interest
Somewhere on CN buried is a similar scope built by someone in Israel, if I ever find it I'll let you know.
Matt
I had an email from Mike at iStar to say my objectve was out of production and they will be testing it tomorrow. It should ship early next week provided it passes the quality tests. I believe I get the test results with the objective.
I am now trying to determine which Moonlight flange adaptor to use so I can order it with the focuser.
The rain has held up the start on the strut tube. I need to build a ladder jig/frame to setup and align the vertical stations/baffles.
So tangible progress has been made. I finished the template design, cut out all the frames from 18mm marine ply, and routed and sanded them, built the jig, and have set everything up to be checked and measured (at least twice) before locking it down for the epoxy glue up.
With a structure like this it is imperative that the front plate and rear plate be exactly aligned because they will hold the objective and the focuser. Sure both those components have collimation adjustment but that's no substitute for dimensional alignment. It does't matter too much if the intermeadiate frames are out be a mm here and there but the lense and focuser must be exact. People run into this same problem with tubes that are not cut exactly square on either end - same thing.
I will be leaving the rear panel unbonded until I get the focuser and objective and can do a focal length test. I know the focal length of the objective but it is best to test it on the structure while I still have the ability to slide the rear panel (and focuser) forward or backward as required. I will have to complete the dovetail to allow it to go onto the mount for testing.
The Moonlight dual-speed long travel focuser is on the way and I have decided (after discussing with Ron) to go with the TAL250 adaptor as this allows flat panel mounting. The iStar objective has been tested and I am waiting for Mike to give me the tracking number. The objective is coming via UPS so I should have it by this time next week.
Here are some photos, (and excuse the mess in the shed, it was raining else it would be out in the carport).
Update: The strut tube is now epoxy bonded, cured, and off the jig for finishing. My Moonlight focuser is in Australia according to the tracking and I should have it around Tuesday of this week.
The objective has shipped, and presently sitting in the Fort Myers USPS facility - it needs to get to Miami ISC to get on a plane to Aus. I am hoping it arrives before the Easter Aus Post shutdown.
You'll notice the tube has extra length on the struts and this is to allow movement of the focuser backplate if required during testing. At present the backplate is just tacked in place with easily removed daps of epoxy. I expect I will need to move the backplate in slightly to allow the use of binoviewers, and it's better to check that now than find out later it's too long.
Photos attached - keep in mind the tube needs to be cleaned up, excess epoxy ground off, sanded, some edges routed, etc. - it will look much better.
Update: Today I added the dovetail plank to the bottom of the rings. It spans three rings and adds additional stiffness to the structure.
The Moonlight focuser arrived today and it's a very nice piece of gear. The TAL250 flange was already mounted on the focuser so I only have to mark the holes on the back plate and drill them. There are two dovetail mounts on the focuser and I've checked the clearance over the top of the tube and I will be able to use one of my existing finderscopes. More photos to come.
Still waiting on the objective to move from USPS, can't test without it .
Looks good Glen. Good choice with the Moonlite, any pics? When I bought my objective last year the tracking didn't work at all, took 3 weeks from order to kitchen table to get it.
Matt
Matt, the USPS tracking (internal to the US) only works up to the departure from the regional ISC (International Sorting Centre). For iStar's US production facility based in Florida (now) the nearest ISC is Miami. The tracking shows it passed through the Miami ISC this morning, so I am hopeful it is on a plane now heading west. I should see another tracking update once it gets scanned on entry to Australia. Tracking is dependent on the type of service bought by the supplier but Priority International and Priority Express International, both have tracking right to the destination - International First Class does not provide tracking.
I should mention that it took only one week for Moonlight to get my focuser to my door from Pennsylvania (and that was via USPS Priority Express International) which is pretty impressive.
Update: The strut tube is now epoxy bonded, cured, and off the jig for finishing.
Not wanting to pour cold water on this build (far from it - I am in awe of people who have the skills to attempt something like this!), but the Structural Engineer in me can't help thinking that this design (plywood discs connected by unbraced timber struts) could have some issues with maintaining collimation in service?
Basically, when the scope is anything off-vertical (i.e. virtually all of the time), the weight of the objective (and the focuser, although this won't be as heavy as the objective, I assume) is carried back to the dovetail mount by three timber members, which are rigidly attached to each other by plywood spacer discs. A quick calculation suggests that the whole assembly could flex off-axis by something of the order of plus or minus a mm or so at each end, with the two end deflections not necessarily being equal and parallel, leading to a lack of collimation of the objective and the eyepiece. I would have thought this much deflection would be very significant for a precision refractor?
Also, timber is known to "creep" under sustained load, so that if the telescope is mounted horizontally for any length of time, it could gradually "droop", again affecting collimation.
Just wondering if there are design guidelines for accounting for this sort of behaviour, or is the set-up designed to allow for easy collimation "on the fly" - or do deflections of a mm or so at both ends not matter for a telescope of this size?
The timber members are totally encapsuated in epoxy resin and all epoxy glue joints are filleted to brace the joints. This construction is very common in boat building and produces rigid monocoque structures with minimal x,y,and z axis flex. The mount plate which the dovetail will attach to, spans the three central rings and is likewise filleted with epoxy. There are many timber fabricated telescopes and just recently I have seen one that had a strip laminated tube. Marine plywood is an engineered product built to Ausralian Standards and is structure rated. It has a very low co-efficient of expansion and is dimensionally stable. I would point to the decades of construction of dobsonian telescopes using plywood as the main material, and this used by custom builders such as Obsession and SDM.
The structure and optics will be tested, should any problems emerge with maintaining collimation I already have plans for triangulation bracing of the struts using thin Dyeenma cord attached to turnbuckes under tension. This string bracing is common on many dobsonians scopes produced by Dennis Steele at Dobstuff. Rest assured I am prepared.
As I said, I'm not wanting to pour cold water on the idea, just interested from a professional perspective to know how these factors are managed.
For the record - I am a Structural Engineer, so strength and stiffness of materials and structural forms is my "bread and butter". (I feel much happier knowing you have a contingency plan to allow for turn-buckle tension bracing! )
About 30 years ago, I worked on the structural design of the dishes for the Australia Telescope Compact Array http://www.narrabri.atnf.csiro.au/ , and while I can't recall the specifications and tolerances, I do recall that the challenges of designing a steerable dish which is exposed to the elements, and which must not deform more than a fraction of the wavelengths being observed while the dish is steered and exposed to wind loads etc, was far more onerous than anything I have worked on before or since. The thoughts of meeting those tolerances with plywood and Tassie Oak make me shudder!
(But they do say that the best way to get an Engineer to solve an intractable problem is to say: "Nobody else has managed to work this out ...", and leave it lying on their desk!)
I'm not a structural engineer but my gut feel would make me roll a tube out of paper and encase that in epoxy resin. That's what I did back in '78 or so when I made my first refractor, and it worked very well. I just can't see a thick enough rolled paper tube to flex or twist.
I have spend my life living with engineers, my father was a chemical engineer and my son is an electrical engineer - I was the practical one that actually made things.
My scope is a smaller copy of a production 150mm refractor made by iStar, the TCR, link is here:
You will be able to see there that it is identical in concept to my own, the difference being mine will be using a smaller lighter 127mm f12 objective (it weights 3kg in its collimating cell), and different materials. My lense objective is an Achromatic R30 (Anastigmatic) Hand Figured Lens in Push-Pull style cell.
That TCR has gotten very good reviews. I am sure Ales or Mike at iStar would love to chat about the structural engineering of their mechanically fastened alloy tube refractor, which attaches to the mount via a dovetail on one of the struts.
That TCR has gotten very good reviews. I am sure Ales or Mike at iStar would love to chat about the structural engineering of their mechanically fastened alloy tube refractor, which attaches to the mount via a dovetail on one of the struts.
Now THAT is a structural engineering masterpiece! (Although a telescope tube CNC-milled out of a solid block of magnesium is pretty cool too!)
Nice build Glen, I think it will work well. Good choice on using the epoxy resin, I do for my scopes and it's amazing how strong and rigid it can make them.
Finished the last of the epoxy clear coar today, it looks like a piece of furniture now which is probably bad for a scope. Trying to decide how to finish it, do I spray it with polyurethane and leave the furniture finish, or spray it with something like black bedliner to make it totally stealth? If I go for the polyurethane I will need to flock the inside face of the struts and use black baffling. If I use bedliner no flocking will be needed. It will have a light shield extending back from the obective to the second frame, and another shield forward of the backplate to cover the focuser entry. Probably hard to visualise if you have not seen a production TCR with its shields.
Glen. Leave it as is and see if reflections are a problem. It'd be a shame to ruin such a nice object with paint. Whats the weight? The Moonlite looks great. That flange .... is what I should have done..still could, would have saved me a lot of filing time!
Matt
Here are a couple of photos of the tube with ray strings attached for working out the baffling diagram (if I decide to do that later). The brickie's dayglow string really make it easy to see the light cone.
Matt, re the weight, I estimate the tube weighs around 1.2 kg, maybe a little more. The objective weighs just under 3kg, and the focuser with 2" diagonal might be 2kg, so that's 6.2 kg in total. I have the 300mm dovetail to add to the mount plank. I have yet to actually weigh it all with a scale and will do that this week. As long as it comes in around under 7kg I'll be happy.
Any light shielding, baffling, will be done with lightweight ABS or foam materials. The front ABS light shield will simply slip over the front of the objective.
The iStar Asteria production (traditional tube) model 127mm F12 R30 has an OTA weight of 12.7kg.
Glen. Leave it as is and see if reflections are a problem. It'd be a shame to ruin such a nice object with paint. Whats the weight? The Moonlite looks great. That flange 
