Build Commences - iStar 127mm R30

[glend]

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:

http://www.istar-optical.com/sd-127f12-as30.html

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.

[MattT]

Interesting build Glen. I'll be watching with interest :thumbsup:
Somewhere on CN buried is a similar scope built by someone in Israel, if I ever find it I'll let you know.
Matt

[glend]

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. :question:

[anj026]

Looking forward to your project Glen. These lenses are very good. I still have mine which is one of the first R30 lenses made. :)

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4572197/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

[glend]

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).

[glend]

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.

[glend]

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 .

[MattT]

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

[glend]

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.

I will get some more photos together today.

[julianh72]

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?

[glend]

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.

[julianh72]

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!)

[Steffen]

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.

Cheers
Steffen.

[glend]

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:

http://www.istar-optical.com/perseus-tcr-150-12.html

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.

[glend]

Matt here are some photos of the Moonlight focuser and its mount flange:

[julianh72]

Now THAT is a structural engineering masterpiece! (Although a telescope tube CNC-milled out of a solid block of magnesium is pretty cool too!)

We structural engineers like to "see" the load paths "expressed" in the structural form, so we love suspension bridges http://upload.wikimedia.org/wikipedia/commons/f/f8/GoldenGateBridge1.jpg , externally braced high-rise towers http://upload.wikimedia.org/wikipedia/commons/3/3a/John_Hancock_Center_in_mist.jpg , and open frame sports cars and bikes http://upload.wikimedia.org/wikipedia/commons/8/8a/ArielAtomGoodwood.jpg
http://upload.wikimedia.org/wikipedia/commons/5/52/2007DucatiSR4-001.jpg !)

[nebulosity.]

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.

Jo

[glend]

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.

[MattT]

:eyepop: 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 :thumbsup:.... is what I should have done..still could, would have saved me a lot of filing time!
Matt

[glend]

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.

[glend]

The objective arrived from iStar this afternoon. I was initially very happy to see it, but it was clear the box had been through the wars (see photo). I noticed that there was dirt (or a small scratch) inbetween the objective glass elements - pretty much right in the middle. I have to wonder how it could have been checked prior to shipment.

I have emailed Ales in the Euro office about this and sent him the photos.

Pretty disappointed, and this is going to throw my entire build project up in the air and I have stopped work for now. If I send it back, it's going to cost me, and add probably three weeks minimum to the build. I don't think I should have to disassemble it and try to clean it .

[anj026]

Glen that is certainly disappointing to see. I know the anxiety of waiting a long time for a fragile product to arrive. If it is dust then it can be cleaned out easily enough. Best discuss it with Istar before proceeding to dis-assemble the lens.

When I purchased mine it came very well packed in paper and a moulded foam shell inside the carton. Mine came with a test paper that looked like a test report but it is kind of a generic thing, the lens doesn't have a serial number. The lens star tests very well.

Andy

[glend]

This objective does have a serial number. I would not want to try and disassemble it as special tools seem to be required. I am just going to send it back for a replacement under warranty.

[glend]

Spoke to Ales at iStar via email, he is going to sort it out for me. I have sent the objectve back.

[MattT]

Here is the scope built along similar lines. From CN ATM post a pic of your homemade scope page 20.
A real shame about the objective, had some real rough treatment by the look of it.

[glend]

Just a quick update, I have been waiting on iStar to get back to me on the replacement objective. I received the tracking number so it is on its way now. Maybe by the end of next week if I am lucky.

In the meantime, I received the flocking material from the supplier (DC Fix black velour felt). I also have the 2mm neoprene sheet that I am planning to use for any needed baffling. Once the tube frame is finished (clear coated with spray polyurethane, I will apply the flocking material to the inside of the stuts to stop side stray light reflection into to the focuser or objective. After seeing the original objective in person, I realised I will have to fabricate a collar to mount it to the front plate; I had thought it would slip inside the front plate but it won't and I don't want to machine it (the front plate) any thinner and a collar will add strength and allow the use of the collimation screws (which for some reason iStar do not supply with the objective (guess you have to buy their front plate). I have two options for the objective collar, a solid aluminium collar cut from plate or laminate four 2.5mm sheets of MDF into a plate (this would be epoxy bonded and sealed and could be painted prior to coating to mimic the ply frame rings).
I have a 10mm plate of aluminium but its going to be much easier to cutout the mdf laminate. If I could machine the aluminium I'd probably go that way. Anyone have a recommendation on cutting 10mm aluminium plate in circles? I have seen the many drill holes technique, which is alot of work.

[nebulosity.]

I have used a router for cutting al plate, on a circle cutting jig you just take it slow and if you have got a good router bit quite nice results can be achieved.

Jo

[MattT]

I cut three sections out of 12mm plate and screwed it to the plywood cell on my scope. Worked well but have never had to use the collimation bolts as the whole thing is straight anyway! Maybe try the objective first? Could save a bit of time if you can just screw it in and leave it. You have collimation at the focuser end, is that correct?? That's as good as at the objective end but a whole lot easier to use.

[glend]

Update: The replacement objective arrived today from iStar. This one was packed better, and it is pristine - no debris in between the lenses. I will get it onto the tube frame tomorrow for the focal length test so that I can fix the backplate into position. I have made an adaptor collar for the objective to sit in. I tried the aluminium plate for the adaptor but my router was hopeless at getting through it, or maybe it was the dull bit. In the end I used good old ply again, at least it will match everthing else. Re collimation of the objective, I see it as a nice thing to have; sure I can collimate the focuser.

[glend]

Update: The objective is installed on the tube and tonight I did the preliminary test to set the backplate/focuser distance. I ran through both 1.25" and 2" diagonals with all the EPs I have (2"30mm right down to 6.7mm and the 2x barlow combinations) to make sure I could achieve focus on everthing. It was clear that I had it initally set up too far back but that's the reason for leaving the backplate loose until now. I even tried out the binoviewer, which requires fairly agressive in travel.

I can report that the R30 lense, and the long fl, really control CA. Looking at the moon tonight I could not see any fringing at all.

I need to get some imaging guys to give me some info on how long imaging trains can be, and what the impact is on the focuser setup. I imagine no diagonal is required, and that can affect fl. I won't fix the backplate until I get some info on that.

I have attached a photo of the scope under test on my Vixen Porta II mount. I strapped the scope mount plate to the dovetail bar and it worked pretty well, and allowed movement but wasn't very stable.

BTW I weighed the scope once the objective and focuser) were installed, it came in at 8.1kg total, pretty good for a 127mm long fl refractor; my estimate was 8.4kg. That's the weight without the 2" diagonal or an EP.

[omegacrux]

Cool scope
I bought a SkyView alt-az and like the Porta it was too low for refractors , so I ditched the standard legs for a surveyors tripod ($100 from tool specialist) and now the vixen clamp can be up to 2m tall !

David

[glend]

Just a few word on stray light scatter on my tube scope, I intentionally setup near the house where I would pickup some scatter, and yes there is some as expected as the internals are not flocked yet, nor did I have the front or rear light shields in place. The long struts and the mount board do seem to be the major source as they have flat shiny sides that face inward and can bounce light into the other bits. I was waiting for the first light test to determine how I would address light management, and now believe that I will probably have to spray the internal structure surfaces with a black non-reflective coating (whether Plastidip, bed armour, or just paint, is still a question). Baffling is also a possibility, and I have some thin neoprene which can be cut into baffles and attached to the frames. I need to do another light ray string exercise to work out placement. If I spray the frames (internally) then I can just glue a small baffle ring of the right aperture to the frame itself. More on this later.

[glend]

Update on light scatter remediation. The problem: Is this a telescope or a piece of furniture (my daughter thought it was a wine rack).

So to flocking and finishing, I gave the scope a preliminary coat of satin finish Capot's polyurethane, just to see how the furniture approach would go, and while it looks good,... I think I will probably go ahead with a full paint job and cover up all that timber. It seems the only reasonable way to address the stray light issue, especially if it is painted black, and I do not have to spend hours getting every little imperfection out of the epoxy fillets and joints. If painted black I can also forget about baffles (or reduce them).

My Kendricks Dew shield arrived today, the front one will be a standard dew shield and have a heater in it. The second one goes behind the objective to shield it from stray light and provides an effective baffle going down towards the focuser.

I will put up a new photo when I have finished the paint job and have the bits permanently installed.

[glend]

Have managed to get abit more done over the past couple of days. The painting is all completed, after doing an initial coat of satin polyurethane to see how it would look, I decided to ditch the whole furniture approach and put three coats of satin black on it. By painting it black I do away with any flocking requirements on the struts. I redid the ray model today and finished the baffles for the frames. The baffles are just 2mm neoprene sheet which are superglued to the frame members, very light and they already have the flocking finish on them. I put the dovetail bar on and checked the balance point.

I put it all together this evening and did a quick collimation. The focuser was easy to setup with the laser as I had tried it before and I have a mask with a centre spot from when I made the frames. The cheshire was a nightmare, as the crossed wires were in the way and flaring the light making it hard to see the reflected circles (green and yellow). I moved the objective around and got the circles close but I am not happy with it and will do it again at some point.

Took it outside tonight for a star test, and while it provided great lunar, mars and saturn views it was obvious on stars that the collimation needed some more tuning. Despite the concerns raised by the structural engineer, there is no evidence of frame flex during this evenings test. I will need to get it on the NEQ6 and start tracking from side to side at extreme angles to see if I need to brace it. I have the Dyneema cord and turnbuckles ready if I need to do it.

I have attached some photos of the baffle install.

[glend]

As indicated in the last post, the collimation was giving me troubles, mainly because I was trying to shim the heavy objective. Shiming is time consuming and in my case the shim material kept falling out when I moved the lock bolts.

If you can't afford to buy a suppliers counter cell (iStar makes one for their objectives but they are over $250), make it out of plywood like I did (and some others).

I should point out that this approach will only work if your objective is setup for Push-Pull counter cell mounting (ie has push holes in the rim that may, or may not, be tapped for set (grub) screws). If your push screw holes are not tapped it is easy to run a tap through them (they are all aluminium) and cut your own).

To turn your ply mount ring into a Push-Pull Cell try this:

1. Check your ring squareness with the rear backplate to insure that the alignment will be close. As you can see from the attached photo, I had to trim my front cell slightly to square it up with the rear plate.

2. Mount your objective using the normal bolt holes (or the pull holes). Then screw three push bolts into the objective so that they push into the ply (this is normally why ply is a bad choice for push pull cells - it gives and gets sloppy).

3. Remove the objective. you will see three distinct depressions in the ply ring where the bolts pushed against the ring. Centre drill these three depressions with a very small drill bit (no more than 4mm is all that's needed).

4. Get three stainless steel nails ( I had a pack of fibre sheet nails from Bunnings that worked fine). You want a nail head that is just the same size of larger than the push screw that you will use). Cut the head off the nail leaving a small shaft tail to go into the hole you drilled. Put some five minute epoxy into each of the little holes and tap the nails into the holes flush with the ring surface.

5. Finish off your paint and remount the objective and adjust to reach collimation as normal in push-pull setup. This is also now field adjustable if required, something that would be a nightmare with shims.

That's it you now have a push pull counter cell for next to no cost compared to a shim cell.

[julianh72]

Deleted by julianh72 for unintended offence caused to OP

[glend]

As indicated previously Julian, I have the string bracing equipment ready to go onto the frame. I will be giving it a good workout at Bretti this coming new moon period. It will be riding on my new NEQ6. BTW final weight of the OTA is 8.4kg, less than 2/3rds of the weight of the production tube based scope made by iStar.

In your calculations I hope you have allowed for the bracing effect of the epoxy fillets at each joint, which effectively extend a 90 degree brace over twice the distance of each joint. These are non-flex joints, only the oak batten can possibly flex in between the frame stations, and they work in a combination of three spaced 120 degrees around the circle frame (which cannot compress). The oak batten flex over the short distances between frames could not be induced when I stood on them. I am not saying there would not be some flex if say it was used as a see-saw over a small fulcrum however there is a 300mm dovetail bar mounted on an oak mounting board (with a lamination strip) which spans three of the frames separately to the oak struts. The objective weighs 3kg and sits only 350mm from the mounting board/frame combo. The string brace will run from the bottom of the objective ring to the top of the middle frame, paired with a similiar string brace from the rear focuser plate to the same middle frame

[julianh72]

Deleted by julianh72 for unintended offence caused to OP

[glend]

Hmm, I don't seen to recall engaging your services so certainly won't be paying. The model is nice but the devil is always in the detail, for example the dimensions of those struts, the span, and not the least of which is the epoxy composite techniques used to create a monocoque structure. The struts are, as is the entire structure, completely epoxy resin encapsulated (two coats applied wet on wet to bond - of International HT9000 boat building resin (not some cheap stuff you buy at Bunnings)). The joints are HT9000 resin using International Glue Powder mixed in the ratio required to achieve maximum strength.

This epoxy system is the same used to construct Sydney-Hobart race boats, and many long distance and private commission global roaming vessels). The epoxy encapsualtion stabilises the strut much more than a naked strut that relies only on it's material properties. Epoxy resins saturate the wood, creating not just a coat but a increase in stiffness that radiates into the fibres of the strut. I'd suggest some reading on the 'Wood Epoxy Saturation Technique' if your really interested in how composite structures work.

[julianh72]

Deleted by julianh72 for unintended offence caused to OP

[glend]

This is pretty much the final post on the build of my iStar 127mm R30 F12 open tube scope.

Even though the full moon was pretty much flooding out the sky, I had it out on the NEQ6 mount tonight for the Saturn Occulation and to test the Sky Safari Pro wifi guiding of the mount (using the Nexus-S attached to the SynScan controller).

Photo is attached.

I let the scope cool in the shed for a couple of hours, fitted the Kendricks Dew Shield, and got it balanced on the mount. It tracked Saturn well and there was no strange mount behaviour, nor weight issues. The scope with 2" diagonal, EP, and 8x50 finder had an all up weight of 9.4kg. OTA alone is 8.4kg, compared to the iStar Asteria 127mm R30 f12 tube based scope at 12.7kg for the OTA.

After watching the Saturn event, I used the opportunty to test my new Televue 5mm Nagler in the scope on Mars and found the detail impressive for the conditions. I finsihed up with a few star tests at various altitudes and they were absolutely concentric with no evidence of collimation shifts due to scope flex.

If there is one thing that I need to address it is to move the dovetail rail forward on the mount board slightly to improve balance point range.

So that's it, thanks for following the build. Now to get it out to a dark site at the next new moon.

[anj026]

Well done Glen. It looks good and will give you a lot of satisfaction observing the night sky.

[Allan_L]

Well Done Glen !
An amazing journey.
Looking forward to seeing it in use at our Dark Site, shortly.
Have you considered the best way to transport it safely over those rough roads?
Regards
Al

[MattT]

Very nice Glen.
Nothing quite like sitting behind a long FL refractor at night.
Matt

[cometcatcher]

Good work Glen!

Will you be doing another iStar build, with say an 8" objective? ;)

[glend]

Thanks for the kind comments. Would I build an 8" - I might but I think I will wait awhile, as I have a lot to learn about refractors on equatorial mounts and I want to get the Canon DSLR going on the back of the iStar.