the curse of diy... serrurier truss and big dob

[Dave_D]

So, mounting the stepper for the electronic focuser is proving to be a bit more problematic than i first thought. There are no off-the-shelf project boxes available that will fit under the trusses so basically 2 alternatives.

First is to have a custom box 3D printed with one side curved to fit the focuser mounting plate. This will at least have the electrical side fully enclosed but i'm not particularly impressed with the surface quality of 3D printers at the moment.

Second is to mount the stepper motor on an angled bracket, which will be a pain to fabricate to fit the curvature of the focuser mounting plate and also have the electonics exposed, requiring some method of sealing to prevent moisture getting in to the motor...

Definitely one of those 'hmmmm' moments.

[Dave_D]

DESIGN COMPLETE!!!

well that was a task and a half but finally starting to get underway...

awaiting confirmation on the first batch of parts from Graham at Astrotec, just wish i could afford to have all the parts fabricated in one go and then just put it all together but hey-ho, the plight of the poor but enthusiastic was never an easy one.

ran a few (well, quite a lot) of density/volume calcs through my spreadsheet and a reasonably accurate guestimation puts the OTA between 2.5kg and 3kg lighter than my current OTA which will really help my poor Vixen GP not have a fit when guiding lol untill i can afford an AZ EQ6-GT

finally decided to bite the bullet and go for carbon fibre cage and cell rings with the spider centre piece made out of Tufnol, which, as it's electrically non-conductive will allow for the future use of the spider arms to power a secondary dew heater.

and yes, i will be using blue anodized hex bolts

even managed to flatten the battery in my digital vernier gauge it had so much use lololol

[Dave_D]

i can almost taste the reality... this will truely be the dream scope i always wanted, and with the excellent advice and assistance from the marvelous and knowledgeable Graham at Astrotec, getting closer and closer...

http://smg.photobucket.com/user/vanityassassin/media/test_zpssme7gu79_1.mp4.html

[RAC]

Looking a bit flash! Hope the real thing will look that good.

[Dave_D]

Looking a bit flash! Hope the real thing will look that good.

it's a definite 'taking my time' project so even if a bit takes a month to get just as i want it, so be it

[Dave_D]

so more ponderings, this time, the centre box section while i'm waiting for the first set of parts for the secondary cage to arrive. i don't particularly like the thought of brazing so many sections of aluminium together so, can anyone see a problem with this possible solution of using a 495mm losmandy plate, attached to a single 25mm square centre section and bracketed to the mirror cell top plate. i was worried about the expansion rate of the losmandy with temperature change, but after plopping the figures in here:

http://www.calctool.org/CALC/eng/default/cte

namely a coefficient of linear thermal expansion of aluminium of 22.2 ppm/K

temperature change of 20 degrees C (doubt i'll be out below 0 or above 20 much in the UK)

initial length 495mm

initial volume 538065 mm³

and an average initial density of 2712kg/m³

i get an overall total linear expansion of 0.219mm which, to me, seems well within the tolerance of the overall structure. any thoughts? from anyone, their dog, grandmother or half blind great uncle bulgaria?

[Ajohn]

You asked about epoxy and aluminium early on. Looks like you have changed your mind but might be of interest. Epoxy goes a long way towards holding  aircraft together. It sticks to it rather well. With skins as in aircraft they also rivet as with most bonded joints they are weaker when peal forces are applied.

I would suggest a pocket a little deeper than the diameter of a truss if you wanted to do it that way and sufficient clearance to leave space for a thin film of adhesive. It would need jigging while it set. You could also have a pin of some sort going through the set up. In short if the load is a shear force on the joint it will be strong. Peal forces wont be so  good. I recollect that epoxies were being sold n tonnes etc and that will be in relationship to shear forces.

I run Linux too. A 3D cad package like that must be a bit addictive. 

John

-

[Dave_D]

mucking about with brazing bits of aluminium... remarkably strong all things considered.

[RAC]

When are you planning to start making this, the design looks really nice. Since December I have already completed my second truss scope and that's including making the 18" f3.3 mirror

Two telescopes by Raymond Collecutt, on Flickr

[Dave_D]

the four main components are being machined soon. The person doing them is doing the final pricing in the next few days.

He's just got one of those gazillion psi waterjet cnc machines so shouldn't be long before the proper start begins. Now, if only i had a workshop like yours...

[Dave_D]

Since December I have already completed my second truss scope and that's including making the 18" f3.3 mirror

Now that's just showing off

It's been a bit of a nightmare getting stuff moving the past couple of months... had to get a car so a big hit to the wallet and free time is an issue. Kids take up most of my weekends but i'll get there eventually. Priorities and all that.

Can you believe it took over 6 weeks to find a decent 210mm 30mm bore aluminium blade for my mitre saw... curse me for not getting the 'popular brands' lol

Still, i'm in no rush... not as if the stars are going anywhere.

[Dave_D]

so, after a couple of enquiries asking if this was just a pie-in-the-sky wishful thinking kinda thing...

 

progress at last... Graham has done test cuts with his new whiz-bang high pressure water cutter before cutting expensive aluminium plates. looks fabulous decided to go for a solid spider to simplify the design so all i need to worry about is getting the correct secondary offset.

I've started cutting the central box section in 31mm 10swg square tube, which so far seems incredibly rigid, and the secondary holder from 60mm dia tube. lots of brazing to do. must admit, cutting the tubes with a power mitre saw on the kitchen floor maybe wasn't the most sensible thing, but it sure was fun (and messy)... ahh the joys of the single life

and a new animated render of the final, final FINAL! design... because i can

http://vidmg.photobucket.com/albums/v369/vanityassassin/scope_zpsrwdoskst.mp4

[Pompey Monkey]

so more ponderings, this time, the centre box section while i'm waiting for the first set of parts for the secondary cage to arrive. i don't particularly like the thought of brazing so many sections of aluminium together so, can anyone see a problem with this possible solution of using a 495mm losmandy plate, attached to a single 25mm square centre section and bracketed to the mirror cell top plate. i was worried about the expansion rate of the losmandy with temperature change, but after plopping the figures in here:

http://www.calctool.org/CALC/eng/default/cte

namely a coefficient of linear thermal expansion of aluminium of 22.2 ppm/K

temperature change of 20 degrees C (doubt i'll be out below 0 or above 20 much in the UK)

initial length 495mm

initial volume 538065 mm³

and an average initial density of 2712kg/m³

i get an overall total linear expansion of 0.219mm which, to me, seems well within the tolerance of the overall structure. any thoughts? from anyone, their dog, grandmother or half blind great uncle bulgaria?

Dave,

I've been dipping in and out of your design process with interest over the last few months. It is fascinating and educational.

However, I can't help thinking that connecting the losmandy dovetail to both the primary cell and the centre section would negate the serrurier-truss design philosophy of equal "sag" at each end?

I thought the idea was to have each end cantilevered about the central collar?

Or am I missing something?

Paul

[Dave_D]

Hi Paul

No, you aren't missing something... i suppose it's more a hybrid than a true serrurier but as long as it takes better images, who cares

There were two considerations i took into account.

First, any inbalance in sag and any collimation issues introduced will be negligable in relation to the quality of seeing (that was coming from a guy i used to work with when i was at the Square Kilometre Array, who has designed observatory class scopes before he started on radio telescopes).

Second, in comparison to my current OTA (which is a battered old Orion Optics GX250), even with the losmandy plate attached to both, it will be significantly structurally better than my naff aluminium tube. Not trying to build a Planewave y'know, but better is better

So, either way, it's a win/win compared to what i have at the moment (not to mention about 3kg lighter)

[Pompey Monkey]

Hi Paul

No, you aren't missing something... i suppose it's more a hybrid than a true serrurier but as long as it takes better images, who cares

There were two considerations i took into account.

First, any inbalance in sag and any collimation issues introduced will be negligable in relation to the quality of seeing (that was coming from a guy i used to work with when i was at the Square Kilometre Array, who has designed observatory class scopes before he started on radio telescopes).

Second, in comparison to my current OTA (which is a battered old Orion Optics GX250), even with the losmandy plate attached to both, it will be significantly structurally better than my naff aluminium tube. Not trying to build a Planewave y'know, but better is better

So, either way, it's a win/win compared to what i have at the moment (not to mention about 3kg lighter)

I did wonder how much actual flex you would get with all those triangles and just a handful of kilos at each end! 

I have a 10" f5 Revelation dob (GSO optics, Im led to believe) that I will be looking to re-purpose as an "astrograph" on my new AZ-EQ6 some time in the future, so there is a lot I can pinch learn from your design

I am watching with interest!

[Horwig]

Dave, I've not been keeping up with this thread, WOW, so far so good, love the water jet cut piece for the secondary cage.

Not sure about the dovetail from the centre box to the mirror cell end.

If you made the centre cage wider, as in one of your earlier concepts, and at the centre of gravity of the ota, then in theory, a fairly short dovetail bar would be all that you need.

Keep up the good work

Huw

[Dave_D]

interestingly enough, just noticed these on the R.F. Royce site. definitely looks like a mounting plate between the center box and primary cell... well, if they have no problems with it.

http://www.rfroyce.com/DK%20OTAs.htm

[Dave_D]

black holes are a mystery... how socks disappear in a tumble dryer is a mystery... how bacon cures all ailments is a mystery...

but the greatest mystery of all is how, when all pieces were carefully marked and clamped together with no less than three clamps, did one of the lengths of the center box section magically become 2mm shorter than the other three when i cut the opposing 45 degree angle on my mitre saw?

[Dave_D]

test piece arrived today for me to confirm the sizes and whatnot... this is going to be awesome

[Pompey Monkey]

Morning Dave,

That looks good, but I can't rid myself of the 'gut feeling' that the spider might have some flex in it with the weight of the secondary and its holder.

Have you tried doing any mechanical finite element analysis on that part of structure? Apologies in advance if I missed something from one of your earlier posts.

Paul

[Dave_D]

Morning Dave,

Have you tried doing any mechanical finite element analysis on that part of structure? Apologies in advance if I missed something from one of your earlier posts.

Paul

hey paul

the spider vanes on the final piece are solid 8mm x 3mm so will be much more substantial than the test cut Graham sent (which is in 2mm steel, not 8mm aluminium. it's only for me to confirm dimensions) the secondary holder (shown in an earlier pic and made from 3mm x 60 dia tube) is only 70mm in length.  i'll most likely take a 10mm drill to it and perforate the tube to reduce weight even more, and allow access for a behind the mirror heater, so the secondary plus holder (about 200g) will put hardly any  stress on the spider arms.

if it does happen that there's any movement, i can always revert to a tensioned spider vane but i can't see it being necessary.

[xtreemchaos]

i wish you good luck,i sure it will turn out cool"where theres a will theres a way"make sure you post some pics when your done so we can all drool.thanks for sharing..charl.

[Pompey Monkey]

hey paul

the spider vanes on the final piece are solid 8mm x 3mm so will be much more substantial than the test cut Graham sent (which is in 2mm steel, not 8mm aluminium.

That sounds a little more substantial!  

As I said, I eventually want to build a new OTA for my 10" dob to go on my shiny new mount, so I'm watching your build with interest. Have you any idea of the expected finished weight? My OTA is currently about 16 kg with finder, but no eyepiece/camera/guidescope.

[Dave_D]

That sounds a little more substantial!  

As I said, I eventually want to build a new OTA for my 10" dob to go on my shiny new mount, so I'm watching your build with interest. Have you any idea of the expected finished weight? My OTA is currently about 16 kg with finder, but no eyepiece/camera/guidescope.

my current OTA with everything equipped is 12kg. I worked out a rough new weight of somewhere between 8.5 and 9.5kg for the new OTA. Losing the skywatcher tube rings is by far the biggest weight saving. And on my Vixen GP, 25% less load and an open truss will be very helpful

[Dave_D]

time to build a jig and fire up 'ye olde blowtorch'