406mm F4.5 String Truss Dob Project

[stringscope]

Well the 16" mirror arrived today all in one piece, many thanks Lester :) .

I am still considering some basic design criteria:

Nested design, ie the UTA nests inside the mirror box for storage and transport?

UTA load paths. The "obsession" type UTA's have the struts offset 45 degrees from the truss attachment points. It would seem to me it is preferable to have the struts lined up with the truss attach points. I am planning to use glued timber struts as opposed to bolted aluminium tube and glued 1.8mm ply as opposed to Kydex. On this basis I think I should be able to have straight load paths from the truss attach points through the UTA?? Also due to this more rigid type of construction I might be able to go down one size in plywood thickness with the consiquential weight reduction??

2X80mm boundary layer fans blowing across the face of the mirror. appropriate vents on the opposite side of the mirror box.

UTA ID 450mm OD 530mm and Altitude bearings 600mm diameter (teflon/ebony star).

Thats about as far as I have got to date, any thoughts???:shrug:

Cheers,

[spudrick]

Hey Ian i can`t help with suggestions as far as design and operation but it will be fantastic to watch this project through to final scope. My longer term goal is to have a 16".
Keep us up to date with progress reports. Do you have a timeline for this?
:)

[SMR]

I agree with you about the UTA/truss configuration. I've never been able to figure why Kreige chose the offset-by-45 degrees arrangement. Yes, your approach will allow you to use thinner ply for the rings.

Try to make your UTA lining as light as possible. Every gram of weight up top moves your center of gravity up, which means bigger trunnions, taller rocker box, etc, etc, etc ...

Steve.

[Starkler]

You would be much better off with the fans close to the mirror edge sucking air and blowing it out of the mirror box. This keeps the airflow laminar and will ensure that the mixed tempertaure air exits the box cleanly.

To have fans blowing will create a turbulent mixture of cool and warm air in the mirror box which wont help views at the eyepiece.

I think the ideal arrangement might be to have a couple of small low powered fans to "dig up" the boundary layer, and then a number of more powerful fans to suck that air out of the mirror box.

[AstroJunk]

If I understand what your talking about correctly, it's probably so the focusser can be mounted square on rather than at the jaunty angle of the smaller dobs.

Also, one doesn't have to counter sink the the truss fixing bolts with this arrangement.

http://www.sdmtelescopes.com.au/SDM006.html

[stringscope]

Well I can finally make a real start on this project :) .

I have decided on some basic design criteria to assist with minimising weight/complexity:

Single ring UTA similar to my 8" only larger 440mm ID/500mm OD => 1.2 degree unvignetted.
HC-2 2" focuser (200g)
Rigel QuikFinder (much lighter than Telrad)
Protostar 3.1" secondary and 4 vane spider.

4 sided string truss similar to my 8" but with screw as opposed to spring tensioning.

"standard" mirror box design ID 510mm to allow the UTA to "nest" inside the MB for storage/transport.

18 point mirror cell with sling. Front access for collimation similar to my 8".

"Standard" rocker and ground board.

Altitude and azimuth bearings both about 600mm diameter. Teflon on Ebony Star.

MB probably about 400mm high.


I am planning to cut plywood for the UTA this weekend :D .

Cheers,

[xstream]

Good luck with it Ian.
Looking forward to your progress reports. :thumbsup:

[stringscope]

Thanks John, I have just sent you a PM.

Cheers,

[stringscope]

After being distracted by some heavy work commitments and sidetracking to make a couple of observing chairs I have finally cut some plywood for this scope. The single ring UTA will comprise 2 rings of 18mm plywood glued together 440mm ID and 520mm OD.

3.1" secondary and 4 vane secondary support with integral dew heater are on order from Protostar.

Focuser shelf is a piece of 100mmX50mmX4mm Aluminium angle and I will be using a light weight KineOptics HC-2 focuser.

A couple of pics for what they are worth.

Cheers,

[spudrick]

Good to see some progress Ian. I have just ordered the Berry/Kriege Book on how to Build Dobsonian.. so i just might follow in your steps, well sort of, I want to make a Classic 16" Obsession style Dobsonian. :D

[stringscope]

Hi All,

I have been off line for a while but I am back now.

I have finally managed to get back to this project. I have been distracted by a whole bunch on non-astro matters and what asto time I have had has been devoted to putting together an 8" refractor - see other thread -. After some thought I have decided to redesign the UTA to allow for a 4 sided truss and still have the eyepiece axis horizontal. This evening I cut all the parts, trial fitted same plus a gross error check with the secondary mount. All glued up now. Tomorrow I will fabricate the secondary mount attachment brackets from 40mm aluminium angle stock.

Cheers,

[iceman]

Welcome back Ian! I love your handy-work.

[h0ughy]

looks like a lot of TLC is going into that work. Looks great, will it be ready for SPSP?

[Lester]

Hi Ian,

I have been wondering how you were getting on with this project.

Looking good.

All the best.

[Orion]

Ian, what type of plywood are you using?
By the way nice project!

[stringscope]

Thanks Mike, its great to be back.

[stringscope]

Hi Houghy,

The plan is to have it ready for SPSP. However, due to a family issue there is only a 50% chance I will be able to attend this year:(

Cheers,

[stringscope]

Hi Lester,

Yes I am back into it now. The first delay was a delay in the delivery of the secondary mount and mirror. The second delay was my distraction with the 8" refractor.

Cheers,

[stringscope]

Hi Ed,

I am just using Hoop Pine plywood from a local building supply place.

Cheers,

[stringscope]

UTA sheeted with 1.8mm ply to provide additional stiffness.
HC-2 Focuser fitted.
Secondary mount fitted.

UTA truss mount blocks currently being fabricated from 40mmX12mm Mountain Ash glued into 3 laminations.

Total weight excluding finders = 2.7kg which is about the same as the UTA for a 317mm Obsession.

[Darkplague]

Looking good Ian!

That is a very different way to mount the spider, I have never seen it done this way.

[stringscope]

Thanks Mark.

You see it done sometimes with ultralight dobs. In this case it is to save some UTA weight with the resultant relaxation of balancing issues. It also results in a lower profile UTA which makes storage and trasport a little easier.

Cheers,

[Darkplague]

I guess it would save a lot of weight.

Being that exposed, just means you have to be ultra careful when transporting it. It wouldnt take much of as bump to knock it out of alignment.

[stringscope]

UTA mechanically complete.

Next jobs are constructing a "dummy mirror" 38mmX406mmX10.5kg followed by the 18pt mirror cell.

Your are right re transport risk Mark. I will be constructing a storage/transport box for the UTA. It will not only hold the UTA but also the finders, truss cables and UTA baffle.

Cheers,

[astro_nutt]

Hi Ian..a very impressive project indeed!...just a thought on using a pair of fans to blow across the primary mirror..would you consider using an automotive pod type air filter to minimize dust?..
Cheers!

[stringscope]

Hi Astronut, No I don't think I would worry re filters, the mirror is pretty exposed anyway, I don't think it would make much difference.

18 pt mirror cell virtually complete. Note, this is a temporary cell that I will use while establishing if the mirror needs refiguring. The final cell will be a 27 pt glued one similar to Peter Bobroff's design (If Peter lets me "steal" the design that is ;) ). The collimation adjustment is accessed from the front of the telescope. Using length of 16mm aluminium tube with a slot cut in the end the 2 top collimation bolts can be adjusted while looking through the eyepiece. The bottom collimation bolt requires a 17mm spanner to adjust.

Mirror box basic structure is there, still have to fabricate lower truss attach points. The MB internal dimensions give 50mm clearance all round the mirror with is a little bigger than the Obsession style MB. However it is lower in height at 250mm. To allow transport in the boot of a small/medium car (Holder Astra) the altitude bearings will be removable, I am anticipating about 500mm radius at this stage. Current weight of MB is 20kg incl mirror.

As a risk mitigator I am using a dummy mirror during the construction work, it is fabricated from plywood and lead and replicates the dimensions and weight of the actual mirror. I didn't bother to replicate the F4.5 curvature in the front face though :P

If it is clear tomorrow night I will probably replace the "dummy" mirror with the real one, attach the UTA to the MB using a length of aluminiun C channel and clamps and determine UTA/MB distance requirements.

Cheers,

[Satchmo]

Your 18 point cell is more than adequate . Many have mounted 42mm thick 16" inch mirrors on a 6 point cell without problems. A proper 6 point flotation is much better than a dodgy 27 point one.;)

[stringscope]

Thanks Mark, I won't be in any hurry to replace the mirror cell. However, an even lower profile cell would reduce the EP height slightly. Maybe in the future :whistle:

I have made some progress. I checked focal plane position in daylight looking out of the garage door and several kms across the valley. "String" truss is now complete and estimation of balance point checked. BP is 360mm from bottom of mirror box. This was checked with all the heavy items in place (including batteries and and an EP) except the altitude bearings and some wiring. I am planning to have the altitude bearing radius aligned with the bottom centre of the mirror box (this will result in a very low profile rocker box) so it is looking like a bearing radius of 370mm (to give a very slightly tail heavy outcome and/or to allow for the weight of the altitude bearings and electrics).

The setup as seen weighs in at 30.5 kg which isn't too bad.

Couple of photos. Note the battery storage arrangement (will be fitted with front covers, front collimation access and telescoping truss tensioning tubes.

Cheers,

[Satchmo]

Looking great ! I guess there is an amount of weight saving in the string scope design. What is the massive eyepiece in the photo?

[skies2clear]

Really nice work Ian!

Just wondering how much tension you need on the strings to make the truss feel stable enough? Is it a reasonable amount or fairly light? (Probably hard to gauge or describe in words?) Is there any problem with the main struts wanting to bend/flex at all, or is the pressure on them not significant enough? I'm trying to get a "feel" for this design. Love the idea of saving weight, especially as I get older and lazier!
I'm in the process of redesigning one of my old scopes and looking at all sorts of ideas.

Cheers
Nick

[stringscope]

Mark, The weight saving is in the string truss which weighs about 50% of a "standard" truss and a slimed down UTA with a light weight focuser (HC-2). This UTA with 2 finders weighs about the same as a 12.5" Obsession UTA with a Telrad.

The scope was set up to check balance so I used my heaviest EP, a 40mm Meade SWA. Totally unsuitable for this scope of course (it is waiting for me to put the 12.5" F6 together). The only "real" EP I have that is suitable for this scope is a 12mm Type 2 Nagler, I decided not to use that for the balance check (concrete floor!!!).

Hi Nick,

I was hoping the truss tension Q wouldn't be asked :shrug: . I don't have a strain gauge so I can't give you a proper answer. For the last 2 days I have had them tensioned up so they give a deep "thrum" when "played" (quite tight). I will have to wait until I have the optics in and then see how much flex is in the OTA structure as it is moved in alt/az. At F4.5 it will be more critical than my F6 string truss scope (which operates at quite low tension), but I will just have to see. Some ATM's have been quite successful with large (up to 28") string truss scopes, others have had difficulty. The function of the aluminium struts is to tension the truss. They are in straight compression they appear to be straight but if they do bend slightly it won't matter as the UTA position is determined by the strings, not the tension strut.

Yes saving weight and reducing transportation and storage bulk were the drivers for this design. It should be quite a bit lighter than an "Obsession type" and certainly less bulky for transport. Won't look as nice though :) .

BTW, I will not be using a shroud with this scope. I will only be using it from dark sites (and not for public viewing nights), the secondary is heated and the mirror box lid will hinge upwards to form a dew shield for the primary.

Cheers,

[Lester]

Ian, you are a true craftsman.

I am keen to see how the mirror performs for you.

All the best

[skies2clear]

Thanks for your comments Ian. I could see it could become an issue with really big scopes. I can see it working nicely for a 12.5" f6 though. Keep up the good work and keep us posted how things are going too! :thumbsup:

Off topic...Lester, I just noticed your comment at "footer" of the page , re: fossil record. Finally someone who sees "true" science instead of some of that brainwashing that goes on to justify peoples beliefs or to get funding/grants. :thumbsup::thumbsup::thumbsup:

Thanks
Nick

[stringscope]

I have been working on the scope every spare minute to have it in some sort of useable form for the SPSP.

Actually put it all together for the first time this afternoon...........and then had to drop everything and spend several hours with one of the dogs at the vet, real fun :sadeyes: amazing how dogs (and kids) always seem to get sick at night on the weekend :shrug:. The dog seems happy enough now, maybe she just wanted a trip in the car :rofl:

Anyway first light tonight, amazing first look through the eyepiece and I couldn't believe it ........... actually in focus first up, didn't even touch the focuser, what a fluke :lol: . Now I have spent an hour or so with the scope the good and not so good news:

Good

Scope is balanced, maybe very slightly bottom end heavy but it holds position at all angles with and without EP's.

Bearings work well, movement forces about the same and reasonably light in both axis.

I can observe feet on the ground except for the last 10-15 degrees to vertical. I can use both finders with feet on the ground at all angles.

Scope is easy to assemble and disassemble solo.

I now have only 3 days left to get it into a state where it is dew resistant and transportable :whistle: .

Not so good

As there is no shroud some body heat thermals are noticeable in a defocused star image. I expected this.

8X50 Finder location is great for using the finder at all angles, however, I was tending to bump it when reaching for the UTA to steer the scope.

Star test primary mirror. The seeing wasn't all that good, however, the initial star test appeared to indicate significant undercorrection. I tried to convince myself it was improving as the mirror cooled down mmmmmmmmm not sure :shrug: more testing required. Certainly I couldn't obtain a completely sharp focus at any stage. Tried moving the mirror and reseating it in the cell.......made no difference. I will try and get some experts to have a look at the SPSP and get some additional opinions. Hopefully this issue will just "go away" with further use.

Work remaining

All the electrics (mirror fan, UTA heat and secondary heat).
Complete disassembly for final sanding and coating of woodwork.
Transport box for UTA.

Tried to attach a couple of pics but no go for some reason!

Cheers,

[stringscope]

worked this time!

[xstream]

Looks excellent Ian. Can't wait to see it next to Rods baby at SPSP.

[Orion]

Beautiful work of art Ian.:thumbsup:

[stringscope]

OK it's sort of useable :) . Coated with a couple of coats of polyurathane, UTA and mirror box interiors blackened and some temp wiring for mirror fan and secondary mirror heater.

I have not had the time for any fine tuning on balance or motions.

Lots of other "stuff" yet to do.

Now I have to think about packing for the SPSP :D

Cheers,

[Orion]

Looks good Ian a job well done.

[skies2clear]

Ian, hope you sort out your scopes performance issues. Maybe you just had a rotten night with seeing conditions. Re: your comments about being undercorrected, keep in mind as mirrors cool, they tend to overcorrect, so hopefully you are wrong about this!

Make sure your mirror cell is working properly (no doubt you have anyway!) :)

Let us know what happens (hopefully good news).

Clear skies

[stringscope]

Had a great time at the SPSP. Mechanicaly the scope performed quite well, better than I expected. Mark Suchting very kindly had a look at the primary mirror issue, turns out it is over corrected (I had my inside/outside focus mixed up :P ), so on Sunday, the mirror was removed and handed over to Mark for optical bench testing, polishing, re-figuring and re-coating. Should be great when I get it back :) . Oh well I shold be able to finish the electrics and make a couple of minor mods to the rocker box while I am waiting.

Unfortunately, I spent some time looking through Mark's binoscope, I wish I hadn't :eyepop: , it is just awsome :jawdrop: . I have got to build one :D

Cheers,

[skies2clear]

I can only imagine what it was like looking thru Marks binoscope. Also toying with the idea.

Good to hear you found out what was going on with the mirror. In Mark's hands, you'll be guaranteed of a premium optic. I bet you can't wait to get it all together now!

Clear skies

[Starkler]

At least when you get it back you will know it will be as good as any other mirror you could buy :) May i ask the origin of your mirror?



Unfortunately :lol:
I would love to have a look but it sounds like a plot to sell more mirrors :evil:

[stringscope]

Hi Geoff,

The mirror is an old Meade unit. I believe about 20yo. I anticipated some optical drama and didn't pay much for it.

Cheers,