8 inch f5 Truss Imaging Newt

[nebulosity.]

G'day,

I'm building another newt and I thought I'd post some pics of the progress. I have only done about two days work and am surprised how fast it is coming together.

I am up to building the secondary holder and would like to know what size secondary I need, anyone have an idea? It's an 8 inch f5 system, and will be dedicated to imaging only.

Thanks for looking, any tips or advice welcome.

Cheers
Jo

[Merlin66]

Very nice.
What size disk do you have on the secondary holder?
There's a program called I think "Newt" which calculates the secondary and the spacing....
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

[nebulosity.]

Thanks Ken, the disks for the secondary holder are 3x50mm. It is a new design I'm trying, it has only one adjustment screw! And is more sturdy and rigid than any secodaries I have made before, I'll post some more pics of it when it's done.

Thanks for the link, I put in my specs and it reckons with a 63mm mirror I'll get a 32% obstruction with a 20.68mm 100% illuminated focal plane. And with a 70mm mirror I'll get a 35% obstruction with a 29.84mm 100% illuminated focal plane.

Which one do you reckon would be best? I will be using APS-C size sensors (24mm) most of the time but might upgrade to full frame later on.

probably going slightly bigger than I need is not a bad idea.

Cheers
Jo

[alistairsam]

Looking good jo.

How did you fasten the Al tube in the upper section to the wooden ring and in the mid-section as well?
why not ply rather than mdf? wouldn't it be harder to seal the mdf from moisture?

Cheers
Alistair

[nebulosity.]

Thanks Alistair,

I drilled hole in the wooden rings and hammered the tubes in (they were a nice tight fit) I'll pull them out and glue them in when I'm ready.

I used MDF because I had some laying around and also there's less chance of chipping when cutting, I find I can get a better finish with it as well.

As to sealing, I have never had any problems, :question: With a coat or two of exterior varnish or epoxy resin you'll be right for many years.

Jo

[MrB]

Looking great Jo, nice work!

If it were me, I would fit the 70mm secondary.
The pro's outweigh the con's I reckon :thumbsup:

Looking forward to progress updates.

[FlashDrive]

That's very nice work....:thumbsup:

Col.....

[Scorpius51]

That is very impressive, Jo! Nice work.

Cheers
John

[alistairsam]

I'd suggest the larger secondary as well as otherwise you'd have vignetting.

If I understand correctly, the increase to the central obstruction would only be an issue if its for planetary work.

What is the weight so far and how are you mounting the primary?

Keeping the focuser as close as possible to the rings without the drawtube protruding too much is ideal.
so focus with a dslr could be say 5mm from fully racked in. Focal point for CCD's would be further outward as distance to body is a lot lower than dslr's.

Cheers
Alistair

[nebulosity.]

Thanks John, Col, Simon :thumbsup:

Alistair, just checked the weight on mums kitchen scales and it comes up to 2.3kg's (thats just the bare frame as seen in the pics) I'm not really worried about keeping the weight down as this is going to go on my horse shoe mount on which weight isn't an issue.

You would have a heart attack if you knew how I'm mounting the primary, so I won't mention it just yet ;)


Thanks for your help Ken, Simon, and Alistair regarding the secondary size, 70mm she is :thumbsup:

I got a little bit more done on the secondary holder this arvo after school, here is a few pics.

Cheers
Jo

[nebulosity.]

A bit more progress, installed a light shield around the upper section made from 0.5mm al sheet, attached the spider vanes, and glued together the secondary assembly.

I'm now wondering about secondary offset, is it needed? and if so how should I do it? I did some quick calcs and I think the offset is around 4.5mm, does that sound right?

The next stage is building a heavy duty focuser, any ideas on a good design?

Cheers
Jo

[ZeroID]

OOoooer ! Another Serrurier truss build ! Nice work. Interesting secondary holder construction, I gather it 'hinges' on the two trapped ball bearings while restrained by the spring and adjusted by the single cap bolt. Nice and simple. Can't wait to see how you are going to mount the primary mirror. I've redesigned and modified mine several times now for various tasks. So easy having good open access to all the connections and components.

[Merlin66]

Brent, et al,
I'm sure that Mark Serrurier is turning in his grave.

His revolutionary design concept was a "truss" design where the deflections of the top and bottom sections were balanced - this maintained the alignment of the optical axis.
The "hinged" attachments also allow for the parallel deflection of the top and bottom rings to maintain their parallel alignment.
Not many (if any) of the amateur truss designs should be called "Serrurier Truss" designs.....
(Notwithstanding all the great design work he did, he had to "compromise" his final design for the 200" due to the Coude access. I have copies of his original design paper etc. etc. Ask me about Sir Barnes Wallis' "rigid dick" telescope tube design - now that IS an original!!)

[alistairsam]

hi Ken,

Interesting point, would you have links to examples of these hinged attachments?
Most of the serrurier truss telescopes in observatories seem to have the truss nodes welded in the centre and extremities.
this was one I could find
http://en.wikipedia.org/wiki/Serrurier_truss
VLT is another example where the nodes are bolted to the centre frame
http://www.cosmicdiary.org/fmarchis/files/2009/01/pano9.jpeg . not sure if its built on the same concept.

for the parallel deflection to work, would the truss nodes need to be a material that can compress or stretch? else how would they deflect even if the hinges allow small movement of the nodes?

that "Rigid dick" telescope design sounds interesting!!. couldn't find any references though. how does it work structurally?

Cheers
Alistair

[Merlin66]

Alistair,
The "hinged" ends on the larger telescopes approximate a pin joint.
The effective section of the truss tube is significantly reduced to allow it to deflect and act as a hinge.

The "Rigid Dick" truss was a series of parallel tubes each of which could be pumped up hydraulically to change their effective strength....think about it ;-)
I believe the trials Sir Barnes Wallis did verified the capability of the system, but it was eventually rejected due to concerns about oil leakage - such is life!

[alistairsam]

Ha ha, I get it, that's pretty radical..

would love to see modelling of the hinge joints and the structure as whole. I've seen modelling of the large fork structures but not the truss.

Cheers
Alistair

[Merlin66]

I'll dig through my notes and find the Surrurier paper for you.
(I did my Hons degree with a design thesis on the 98" Isaac Newton Telescope. I had the pleasure of working at Grubb Parsons for a while and getting involved with their design guys back in 1970)

[alistairsam]

wow, that's so cool. I like the wooden ladders between the fork of the Isaac Newton scope. that's pretty massive.

on the serrurier topic, are there any real world measurements of deflection at the ends when its horizontal? I presume there would be measurements to validate the design.

the deflection on the 200inch Hale for example..
http://www.astro.caltech.edu/palomar/images/blog/P-28069.jpg

Sorry Jo, don't meant to hijack your thread...

Alistair

[Merlin66]

Any finite analysis structural program given the correct inputs will quickly give you the deflections.
It's structurally a simple arrangement...much easier than building a bridge!!

(Having said that....I saw the calc sheets - logarithms/ slide rule in those days. And in the end they actually measured the end deflections under various loads and machined a bit more of the diameter of the stubs!!!! until they got to spec.)

[nebulosity.]

Thanks Brent, your correct about how the secondary works, it is a design I saw some time ago and have wanted give it a go.

The last few scopes I have built have had solid tubes so I thought it would be fun to try a truss.

Jo

[nebulosity.]

Cool info about the Surrurer truss Ken, great to know for future builds. I wasn't trying to do anything special with the truss design on this scope, I have just been making up everything as I go along and having fun.

Amazing to hear about what you had to do with the Isaac Newton telescope construction!

And Alistair, don't worry about hijacking the thread, I'm keen to hear new ideas or gain a better understanding about stuff.

Cheers
Jo

[alistairsam]

Thanks. I don't have access to any programs so any data you have on just how much it deflected would be great to know.

talk of deflection, the Space Shuttle's "Twang" comes to mind, although not quite the same.

So they machined more of the central stub so it'd deflect more? I'm guessing to match the other half??

For us with a non structural background, its hard to imagine how the steel joints would act as a hinge and deflect, but I guess once you go big like the Hale and VLT, the deflection is to be expected.

From an amateur perspective, our pseudo serruriers just work off balance with the hub at the midpoint, in my case, I've used woven Carbon Fibre Truss nodes that are quite rigid and that's why I was keen on understanding the hinge concept as I don't know where there'd be scope for deflection due to compression and tension given our tiny load.

Being hand built with rudimentary tools, its nowhere near rigid as I'd like. I took inspiration from the Officina Stellar RC trusses.

we're all learning I guess. that's part of the fun.

Cheers
Alistair

[ZeroID]

The 'design' I copied was Rolfs (SkyViking) and his build principles were for the parallel deflection you mention. I agree my build is not a perfect and pure example of Serrurier Truss engineering but the rigid centre cage mounting and the inherent traingular elements made it extremely rigid and was able to support my weight (85kg) standing on end. Hence it is an extremely light & stable OTA requiring minimum counterbalance and retaining alignment extremely well.
Purity of Serrurier design may be of value for big scopes but in a 10" f5 probably not so critical. It acheives better than my requirements are for sure.
Also easy to modify as I did when adapting for DSLR focus.
And it was fun to build and looks cool !! ... :thumbsup:

[nebulosity.]

Do you have any pics I could see? or a link a contraction thread?

I was originally going to copy Rolf as well, but changed it to make it attach to my horse shoe mount better.

Jo

[alistairsam]

hi Jo,

About the horseshoe mount, does it have motor drive and tracking? how are you driving it? I'd be very interested to see that. pics would be great.

if you don't have autoguiding yet, let me know if you're interested in adding it with microcontrollers and quadrature encoders from wheels found in deskjet printers.
I built a wooden forkmount with steppers and finally added ST4 autoguiding in RA but haven't tested it out fully, happy to share plans and ideas..

Cheers
Alistair

[nebulosity.]

Yes the mount is driven (using roller drives) with a modified dual axis stepper motor drive upgrade kit for the old synta EQ mounts, I have added an ST4 port the the hand piece for guiding but haven't tried it yet.

It has been out of action for a few months but I will be giving it some upgrades ready for the new scope. I will certainly post some pics.

Your mount sounds sounds great! I would love to see it and find out how you’ve made it. Swapping ideas and plans sound good :D.


An update on the scope progress, I got some aluminium plate, 160x250x6mm (flat pack focuser ;)) and have cut out some bits, I’m planing on building something similar to the focuser on my refractor, but I guess we’ll see.

Adding up how much the scope has cost so far:

$22 Aluminium
$16 MDF
$49 70mm Secondary mirror

Total: $87

Not to bad, should finish it off under a hundred I hope, (already have a dodgy primary mirror)

Cheers
Jo

[alistairsam]

That Focuser looks good. Did you build that?
$100 for an 8 inch is a testament to your diy skills.

This is my fork mount, started with the idea of keeping it under $200, 2 years later, I'd rather not count what I've spent, but have learnt so much along the way.

http://www.iceinspace.com.au/forum/showthread.php?t=83912

I would try and keep the aluminum to a minimum, its easily affected by temperature and can cause issues in so many places

Cheers
Alistair

[Peter.M]

Full frame sensors have a diagonal of 43mm, and your 1100d has a diagonal of 26.6mm. I would always oversize the secondary a little bit, (my 6 inch f5 has a 63mm secondary) beware though that the corrector will probably be your limiting factor when it comes to larger sensors.

[ZeroID]

Pic as below when I completed the OTA build and first mounted it up. Differs from yours and Rolfs of course. I used a ply diaphragm to maintain stability at the cage. My Primary cell is inside the OTA rather than as Rolfs which is external.

[nebulosity.]

Awesome thread Alistair, will keep me busy for a while.

EDIT, Yes I did build the focuser.

[nebulosity.]

Looks like a great build :thumbsup: thanks for the pic.

[nebulosity.]

Thanks for the info Peter, I have received the 70mm secondary so that should be good.

What do you reckon would be a good Coma corrector? I like the idea of the Sky Watcher CC as it's actually a 0.9x reducer as well which would bring my scope down o F4.5, they seem hard to find though. The other option would be a baader MPCC I suppose.

Jo

[nebulosity.]

Have been able to do some more work on the focuser, it looks like it will do the job.

It is 100mm in diameter and 55mm high, the tube is from an old rack and pinion and has an ID of 60mm, am yet to cut it to the correct length. Not sure if I have gone a bit overboard with the number of bearings :thumbsup: but I want it to be able to solidly carry my super cooled 350D which it quite heavy.

Tomorrow I will start working on the Crayford roller system.

I am starting to have a few thoughts of motorising it, what do you reckon? I have seen some neat setups driven by Arduinos and stepper motors and am keen to give it a go but am not sure were to start.

Cheers
Jo

[MrB]

Wow, so many bearings!
Do all 12 of them roll at the same time when you move the tube? If yes.... impressive!

[Steffen]

Cool, making your own focuser – that's hardcore! :thumbsup:

Keep us posted, I'm curious how it turns out.

Cheers
Steffen.

[nebulosity.]

Yeah, went a bit overboard :lol:

Yes they all turn when you move the tube.

[nebulosity.]

Will do.

Cheers :thumbsup:

[ZeroID]

That focusser is a serious bit of engineering. You obviously have the facilities (and skills) to build any thing you need. The accuracy and alignment of that many bearings is a big ask.
Excellent and inspiring work. I always like to see someone else's ideas about problems I have considered.

[FlashDrive]

I've been following this myself .... All I can say is ... this guy's got serious talent when making ' home grown ' parts ... imagine how much he is ' saving ' in $$$$$'s from buying readily manufactured components.....

Flash.......

[nebulosity.]

Thanks Fela's :thumbsup:

The holidays have given me a bit more time to work on this so hopefully have first light at the end of the week.

Got the roller drive mechanism working on the focuser today, need to find some proper bolts for screwing on the top plate and yet to drill and tap a thread for the compression bolt but on trying it out this arvo it has nice smooth movement and can lift 7.5kg's (probably more) strait up without braking a sweat, not sure if this is that good but as my camera setup it only around 3 kg's it should work alright.

Here are a few pics of it in it's rough form, will clean it up and hopefully finish it off (make nobs ect.) tomorrow.

Cheers
Jo

[alistairsam]

Excellent work Jo.
My only concern is all that Aluminium. Cold and wet nights can throw things out of whack pretty easily.
Why not replace the Aluminium baffle with black plastic or felt you can get from spotlight or officeworks. Costs just a few dollars but is waterproof, light and isnt affected as much by temp changes.
I bought some black thin flexible folders from officeworks and cut it open and am using it as baffle.

Cheers
Alistair

[cometcatcher]

I can't believe you're making the focuser! I've built maybe a dozen scopes on a shoestring budget in years past but my homebrew focusers were two bits of PVC pipe which worked as bad as they sound lol.

Anyway good work so far. I guess this is the prototype for the 40" ? ;)

[nebulosity.]

Thanks Alistair,

I hadn't thought about aluminium being a problem :confuse3: is a cold night going to warp the scope or just move the focus? I was relying to the aluminium baffle to give some extra strength to the upper cage, but I do have some 1mm plastic sheeting that would probably be just as good and not expand as much?

Thanks for you input.

Jo

[nebulosity.]

I lost respect for the PVC focuser after my first attempt at making one, :lol: I did have a wooden one that worked alright once but I need something a bit more solid for imaging :thumbsup:

40"...? Well not so sure about that, maybe if you donate the mirror? ;)

Probably the next scope will be somewhere between 12 and 16" :thumbsup: (depending on the funds :lol:

Jo

[MattT]

Jo..wow what a focuser, and the rest of the scope :thumbsup: Terrific build. What are you using to cut the aluminium plate with? I used a jigsaw. I have a couple of old R+P focusers sitting around too....
Matt

[cometcatcher]

I wonder if Stephen would part with some of his stash? http://www.iceinspace.com.au/forum/showpost.php?p=1064836&postcount=157

Of course you would have to grind them. It takes time, but since you're already building everything else.... :D

Start with a small one and work up. Do you already have the mirror for this scope? (sorry if I missed that in a previous post) Otherwise why not grind one?

[MrB]

He would ;)
http://www.iceinspace.com.au/forum/showthread.php?t=117994

[LewisM]

Simply incredible work, and from a young guy! I can just imagine what the future will hold for a capable engineer such as yourself Jo!

[nebulosity.]

Thankyou Lewis! :)

I have got the secondary in, and the focuser mounted. The focuser mounting brackets I was going to build out of something other than aluminium (due to it's problems with temperature change) but only remembered that after I had made them :lol: :( Oh well, we will see how it go's.

I have been thinking about how to drive the focuser, originally the idea was to use the usual timing belts and pulleys but I thought that if the camera is hanging down under the focuser it would try and slide the draw tube out and the motor (if not under load) will let it happen. So that got me onto the idea of using a worm drive, which will keep the focuser continually locked unless the motor turns.

I have made a worm gear before using some threaded rod so was able to make this one this arvo pretty easily. Now I have just got to think of a way to hold the worm and motor :question:
You may recognise the gear as once being the top part of a VCR head? Now put to better use!

Cheers
Jo

[nebulosity.]

Super tempting Kevin :D, Yes I do have an old mirror for this scope but grinding one will hopefully be a project soon, I have read a lot about it and I reckon I'll jump in the deep end with a 12" or something :D will have to make myself a big dob.

[nebulosity.]

Thanks Matthew!

Yep, I used a jigsaw for cutting the plate, works quite well. You know what you need to be doing with those focusers don't you... :D


... and then you'll have to nock up some scopes for them :thumbsup:

[alistairsam]

hi Jo

This doesnt have to be the case.
if the roller pressure on the drawtube is sufficient, it would hold the weight. Id suggest testing this.

worm is a good idea but I think the mounting hardware to hold the worm in mesh might get bulky although I won't be surprised if you come up with an elegant solution.

Have a look at Steve's excellent clutch idea for his motorized focuser.
http://www.iceinspace.com.au/forum/showthread.php?t=51301&highlight=stepper+clutch

I think a clutch is a very useful addition to the focuser.
you'd also need to think of the motor and controller.

will it be a motor with a gearbox, if so reduction would have to be quite high to keep the output revolution rate very low to allow fine focusing.
you'd have to use pwm to control the speed of the motor but may not have much range.
you could use a modded servo that has a gearbox built in, is very small and slow enough with an external worm or timing reduction, but quite noisy.

ideal option is a stepper either with a gearbox like the mclennan ones, or a 1.8deg stepper with your own reduction. you could use 1/16 microstepping to get a fine resolution but will require a reasonably sized motor to provide sufficient torque with microstepping.
printers have steppers that are ideal. look for bipolar ones.

finally, will this controller be ascom compatible for autofocusing or simple with just forward, reverse, fast, slow.

as for the focuser mounting, I'd strongly suggest to keep the profile as low as possible. you've got the focuser on the square brackets. that adds about 10 or 12mm to the distance to the secondary. the further you push the focal point, more the chances of vignetting. you'll also need a larger secondary.
keeping the focal point closer to the secondary allows for a smaller secondary, reduces loss of light at the edges.

Is there any way you could mount the focuser base behind the square tubes? that'll keep your focal point close.

Cheers
Alistair

[ZeroID]

Geez, wish you'd made this out of copper and brass. It would just look so awesome retro and steam punked. Very impressive work BTW.

[cometcatcher]

One thing about those spider vanes, if the opposite sides are not in perfect alignment, you will end up with dual diffraction spikes, giving 8 in total!

[nebulosity.]

I thought I might post a few pics of the scope on the mount, will have to finish it someday.

Looking forward to some imaging tonight.

Cheers
Jo

[strongmanmike]

Wow, that is an excellent piece of handy work Jo, I note you only have 3 Serrurier truss pairs, will that keep the optical elements aligned at all sky angles? and...is that you in the picture next to the scope..?

Mike

[nebulosity.]

Thanks a lot Mike!

Not sure what a three truss system is like, that's why I'm giving it a go. :D
So far haven't noticed any flexure to give concern.

Yeah, that's me near the scope.

Cheers
Jo

[ZeroID]

DIY at it's best !!! :)
One awesome piece of work there Jo, Congratulations !!
Driven Fork Mount and all !! Outstanding... :eyepop:

You are just plain dangerous !! :thumbsup:

[multiweb]

Awesome work. You're a real craftsman. :thumbsup:

[strongmanmike]

Cool..you remind me of myself when I was your age, I too spent most of my time building astrogear and photographing (as it was called then) the sky..wonderous times, keep it up :thumbsup:

Mike

[Paul Haese]

Well done Jo. A great project with a really nice looking scope. I imagine we will be seeing a side line business for you in years to come. I hope the scope performs as well as it looks.

[cometcatcher]

Blimey, the worm gear on the focuser is likely better than the one in my mount.

Are you going to use a shroud on the tube? I'm just wondering how the mirrors will go with dew.

[TimberLand]

Great work Joe, there is real satisfaction knowing you made the beast that captured those photons. I would imagine you are getting good results tracking with the RA drive having a final reduction gear almost 1 metre across.

Keep the ATM world going mate that is most new ideas start, just look at Dobsonian.

Horse Shoe mounts rule. :thumbsup::thumbsup:

[FlashDrive]

Just an outstanding achievement and with such dedication and craftsmanship.

Go to the ' Top of the Class ' Joe. :thumbsup:

Col

[sharpiel]

That's beautiful and ingenious Jo.

I built a horseshoe mount once. They are so appealing due to their great stability. And so hard to build to be really orthogonal. I love the way you have offset the declination axis away from the polar axial line to counterweight the bottom of the horseshoe.