406mm F4.5 String Truss Dob Project - Page 2

Darkplague · #21 06-03-2007, 11:16 PM

Looking good Ian!

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

stringscope (Ian) · #22 07-03-2007, 10:20 AM

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 · #23 07-03-2007, 06:11 PM

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 (Ian) · #24 11-03-2007, 03:17 PM

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 · #25 11-03-2007, 06:25 PM

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 (Ian) · #26 27-03-2007, 10:26 PM

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

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 · #27 28-03-2007, 09:45 AM

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 (Ian) · #28 03-04-2007, 08:59 PM

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

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 · #29 04-04-2007, 08:35 AM

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 · #30 04-04-2007, 09:43 AM

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 (Ian) · #31 04-04-2007, 11:11 PM

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

. 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 · #32 05-04-2007, 07:49 AM

Ian, you are a true craftsman.

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

All the best

skies2clear · #33 05-04-2007, 09:34 AM

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!

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.

Thanks
Nick

stringscope (Ian) · #34 08-04-2007, 11:30 PM

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

amazing how dogs (and kids) always seem to get sick at night on the weekend

. The dog seems happy enough now, maybe she just wanted a trip in the car

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

. 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

.

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

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 (Ian) · #35 08-04-2007, 11:34 PM

worked this time!

xstream (John) · #36 09-04-2007, 06:12 AM

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

Orion · #37 10-04-2007, 07:36 PM

Beautiful work of art Ian.

stringscope (Ian) · #38 11-04-2007, 10:29 PM

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

Cheers,

Orion · #39 12-04-2007, 04:32 AM

Looks good Ian a job well done.

skies2clear · #40 12-04-2007, 09:47 AM

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