12" F4 OTA design suggestions

[bmitchell82 (Brendan)]

I have them but i have found a few pit falls in them. with the 3 points of contact on the OTA tends to flex the tube.

Get a solid ally disk made that will help you center and strengthen the rear end

[alistairsam]

Hi Brendan,

I was thinking of similar T or L ends for the CF trusses.
http://www.trivalleystargazers.org/g...ssembly_2s.jpg
was this what you were referring to?
atm rc build page here http://www.trivalleystargazers.org/g...nik/rc_10.html
will check pics of your secondary spider vanes.

did you mean fix the cell onto a solid plate and fix the plate to the rear assembly similar to the AG rear ends?
Am drawing up a few approaches in cad. we'll see how it goes.

edit: if aluminium is used for lateral sections, would expansion have much of an effect as its lateral, not axial.
Doomsayer's Vixen rebuild is with milled Al plates.

have you seen the need for resonant frequency calculations at all for OTA's though its more for mounts?
that would influence settling down periods.

[bmitchell82 (Brendan)]

Dont get to frisky on the tolorances and natural frequency business.... remember your limiting factor will be the seeing which at best gets no better than 1.1 seconds of arc and only on phenomenal nights will you drop below that but its once in a life time event!

Lateral expansion will not change the distance of the primary to secondary hence the reason why its not a issue and in most cases ally will be used for the lateral components and carbon for the length.

As for the primary yes thats what i would loosely base my mirror cell on. This will give you the best structrual rigidity and keep your collimation stable thats in my view.

If you have a look though the two areas on my site Equipment (also look at the top under current projects and in the Image gallery theres another one of my stuff and workings youll see what im using/doing

[alistairsam]

Thanks for the info Brendan.

I will post updates as I start finalizing the design and start the build.
For the primary mirror would anyone know if the Orion optics standard grade mirrors are comparable, better, worse than the GSO's?
any other recommendations for sources of decent primary mirrors for imaging? I'm not looking high end and don't have a massive budget, but something that will yield good results.
My objective is to keep this portable, so weight savings anywhere would be beneficial.
I remember coming across some hollow cell mirrors. not sure how good they are.

[SkyViking (Rolf)]

Hi Alistair, I'd highly recommend the Serrurier truss, it was the best thing I ever did. It's extremely rigid and light.
Also, a wire spider is awesome as it keeps the light within the Airy disc where it belongs. I'm using 0.08mm steel wire and that's surprisingly rigid. It's much more stable than my old traditional 4 vane spider.

Regarding accuracy, you don't have to worry about that at all. As long as you can adjust the position and angle of your primary and secondary mirrors then the rest is irrelevant.

All the pics from my build are here, just in case: http://www.pbase.com/rolfolsen/10_in...tube_newtonian

Regards,
Rolf

[alistairsam]

Hi Rolf,

Thanks for the suggestion. yours is a work of art and I still haven't ruled it out. it all depends on the weight of my upper and lower sections and where my pivot point ends up. it'll depend a lot on choice of materials.

I've got a hybrid design in mind, just finalizing the numbers.
the primary mirror from Orion optics is 5.6Kgs, so I have to keep weight down where possible.
with the forkmount, i'll need sliding type counterweights along two axes, have to factor that in as well.
with the string spider, have you ever noticed slack or is there sufficient tension to compensate with temperature changes?
how long did it take to build it from start to finish, and do you think there would be any benefit if the three central square tubes were spaced out a bit so the dovetail is fixed over a wider distance axially?

[ZeroID (Brent)]

Originally Posted by alistairsam

Thanks Brent, did you use bearings in the rotating UTA guide?
do the rotating and stationary wooden rings sit directly on each other or is there a spacer/washer in between?
how do you lock it in place?
with your dslr in the focuser, did you have the need to add a counterweight at the opposite side of the UTA?

Rotating UTA just sits on non tear plastic sheet (like teflon) glued to lower ring, top ring is just bare ( varnished and sanded smooth ). It is quite firm to turn on purpose, only moved when I need to get a better visual access. At present firm enough to not need locking but woud not be hard to arrange. A simple clamping setup possibly.

Again I emphasise the fact that construction and assembly was accurate enough to keep the rings etc all extremely flat so it just slid smoothly with no undulations in either surface. A bit of silicon spray has helped as well. Also keep it as simple as possible, the more bits involved the greater the chance of error. I'm an engineer by trade and a dedicated DIY guy, I create solutions.

Now held down with three small steel clips 'borrowed' off some chucked out office equipment. They literally just hang over the upper surface of the lowest rotating ring and are notched and fixed into the lower non rotating ring to run against the edge. Compression packers underneath to allow some adjustment and slight movement. Not an elegant solution but very effective and light. Again I don't want this thing to spin round like a carousel.

Currently the scope is bottom heavy on purpose and the camera will put it pretty close to balanced. Easy enough to add a sliding counter weight if required. Currently there is some extra lead attached centrally on the side to compensate for the slightly heavier lower assembly. It's on the side struts because it works evenly over the range of dec movement.

Any extras, finder\guide scope, RDF etc all mount to the central cage so have minimal effect on the balance and put no extra strain on the two extremity assemblies. Once I build the SkySlab Observatory it will be almost permanently on the EQ6 and I can refine balance and other options.

If you want some closeup pics of any particular areas let me know

[ZeroID (Brent)]

Originally Posted by alistairsam

Hi Rolf,

Thanks for the suggestion. yours is a work of art and I still haven't ruled it out. it all depends on the weight of my upper and lower sections and where my pivot point ends up. it'll depend a lot on choice of materials.

That's the big bonus with Rolf's design. You put the central cage as near as you can to the balance point. Minimises the need for extra weight to balance things elsewhere. The mirror is always the single heaviest components. My truss length ratios 5:3.8, upper vs lower reflect that. My added weight is only about 300 gms at present for DOB balance and on the mount I use the long dovetail (300mm) and slide it till the scope is nearly balanced. A bit of weight bias in one direction keeps the drive slightly loaded for best accuracy.

I'll have a go at Rolf's wire spider one day, when my guitar finally packs up. Just looks and works so cool. Another advantage with my rotator system, I can just pop the whole UTA off and work on it on the bench, or build a new option. Might do that with a mounted camera at focus, no diag. Must add that to the projects list ....


Hmm, love these build discussions, always find new ideas to play with

[SkyViking (Rolf)]

Originally Posted by alistairsam

Hi Rolf,

Thanks for the suggestion. yours is a work of art and I still haven't ruled it out. it all depends on the weight of my upper and lower sections and where my pivot point ends up. it'll depend a lot on choice of materials.

I've got a hybrid design in mind, just finalizing the numbers.
the primary mirror from Orion optics is 5.6Kgs, so I have to keep weight down where possible.
with the forkmount, i'll need sliding type counterweights along two axes, have to factor that in as well.
with the string spider, have you ever noticed slack or is there sufficient tension to compensate with temperature changes?
how long did it take to build it from start to finish, and do you think there would be any benefit if the three central square tubes were spaced out a bit so the dovetail is fixed over a wider distance axially?

Hi Alistair, thanks for your kind words. I built the scope during evenings over a period of a couple of weeks. I think I might have spent some 10-12 evenings on this in total. A lot of that time went with researching and trying out different things, so to replicate it would probably take less than half the time now.

I have never had any slack in the spider. It is not wire tension that keeps it in place, the wires are actually less tensioned than on a guitar, it's quite surprising. The design is simply such that the secondary holder cannot flex or move in any direction without directly pulling on at least 2 wires. This makes it much more rigid than a traditional vane spider which easily flexes around it's own axis etc. For example, I have not had to adjust collimation since I finished the build in April - it is still spot on.

Regarding the weight, I think my OTA, without optics, was 7.85kgs. The 10" optics added 4kgs on top of that. So if you instead make a 12" you could expect maybe 13-14kgs in total I assume.

Yes I think you could easily make the central cage wider if you are concerned about the dovetail. As it is, the dovetail is very sturdy though. I actually made mine inspired from another design which used only two central square tubes, so I had the same concerns and then added one. Maybe it wasn't necessary.

Regards,
Rolf

[ZeroID (Brent)]

Ditto here re the added depth of my cage for the dovetail mount. I added some right angled brackets to widen them plus it made it easier to bolt through. Current width is about 140mm.

[alistairsam]

Thanks for the info guys.
Brent, you mentioned the ratio of your truss lengths is 5:3.8.
would it really matter what the ratio was as long as the weight on both sides was the same so sag would be closely similar?

[bmitchell82 (Brendan)]

Like i said mate easy calculation

You can presicely evaluate the balance point with a bit of algebra and solving the equation to = zero ill write it up later for you and post.

[alistairsam]

Originally Posted by bmitchell82

Like i said mate easy calculation

You can presicely evaluate the balance point with a bit of algebra and solving the equation to = zero ill write it up later for you and post.

thanks, but wouldn't you need estimated weight of the upper and lower sections as it can vary a lot depending on design and materials?

any recommendations on the primary mirror?
the standard grade from orion optics works out to around $1,000 aud for the primary, secondary and shipping.
the gso primary and secondary is cheaper but i'm not sure if they have F4 mirrors by themselves.
else, i could get the 12" F4 ota at Andrews, get rid of the tube and use the focuser, mirrors and secondary holder, but that's quite wasteful.
not sure where else I could get 12" F4 mirrors for under 1k.

[Moon (James)]

else, i could get the 12" F4 ota at Andrews, get rid of the tube and use the focuser, mirrors and secondary holder,

Since you're in Melbourne, you might as well drive down to Bintel and grab one. For $999, it's great value, no?
Then you can concentrate on your mount.
James

[bmitchell82 (Brendan)]

a bit of thought and it wouldn't be that hard, what ever materials you have just google the density, you will figure it out to within a 5 cm zone depending on how accurate and exact you want to be. takes the guess work out of it.

or just do a primary mirror plus a bit and the secondary cage plus a bit. will bring you within 15 cm... once again how accurate you want to be

[SkyViking (Rolf)]

The most accurate and simple way of finding the balance point is to simply cut a broomstick or board to the same length as your planned OTA, and then just hang your stuff from it: Primary mirror on one end, and focuser, secondary etc on the other. Then hang the whole thing on a nail and find the balance point. Then you can measure the distances correctly.
The point of the Serrurier design is that the two halves have to be balanced. There is no particular ratio of truss lengths, it only depends on the weight of your optics etc in the two halves of the OTA. I'm not sure what my ratio is but I'm pretty certain it's different from Brents' since it's very individual.

[bmitchell82 (Brendan)]

hehehe thats the practical version of the maths rolf

[alistairsam]

Hi Rolf,

what I do is similar, place the objects on a flat, and place a tube underneath the flat.
just move the tube to the rough centre point and it shouldn't lean or stay down on either side, like a see saw. gives me a rough idea.
Since I'm using sliding counterweights, I will have some tolerance.
Now to get the segments designed.

I'd ideally like to use a single secondary ring rather than two like Doomsayers RC as that would save a lot of weight.
might also end up with the conical spiders as in some RC's.
challenge would be to ensure the focuser plane stays rigid and parallel to the optical axis with no flexure. I could do that by extending the lateral width of the ring.
if it doesn't work, then i'll have to go with two rings.

Also wanted to use the collapsible connected trusses like the ultralight obsession Dobs as that'd make assembly very easy.
just options at this stage, will have to work out what's practically feasible and efficient.

will post some cad drawings once I'm back from snake valley.

[ZeroID (Brent)]

The practical way is best. But be aware things will change when you add other equipment. Mine is purposely tail heavy because the DSLR will go up top of course in some configurations. And the dovetail slides so I can adjust if needed. It will never be perfectly balanced and in fact it's not a good idea because the drive will 'rock' between pushing and holding back. A kg inbalance or so at either end is not going to be critical and its impossible to get a perfect situation regardless.

[clive milne]

Originally Posted by bmitchell82

As a general rule of thumb you want at least four triangles up and four down.

As for the secondary steer well clear of wire bad joo joos for AP, if it was good RCOS and the like would use it.... for visual its fine though because a bit of movement isn't noticeable!

If your not a big fan of diffraction spikes use something around the 0.8mm stainless

Hi Brendon, if I may, I would like to discuss a couple of points you raised here^

On the subject of trusses, contrary to popular belief, kilogram for kilogram a truss that uses three triangles is actually stronger than the traditional 4 sided design. If you dig through the old atm archives you should find a discussion between myself, Tom Krajci, Richard Schwartz (and a few others) including FEA results. I think the reason that this is so is that for any given direction of applied force, you will have 2/3rds of the truss elements resisting deformation. With a traditional truss, you only have 1/2 the elements working when you apply a force to any given side.
I think in the real world though, this is splitting hairs because extension and compression of the truss elements is so minor in the scheme of things. The vast majority of the flexure is a result of other factors such as the truss connectors and deformation in the element where the dec axis joins the OTA. I have always thought that the single best change you could make to the traditional design is to have the dec axis join the OTA at a node (or nodes) of the truss instead of half way between them. That way, all elements are acting under compression or tension.

On the subject of wire spiders, they can be made to be sufficiently rigid if you apply the offset design which in effect translates the forces into 1st order instead of second order... a very important engineering principle.

As far as spider diffraction goes, there is in fact two principles at play here:
The first one is obviously that diffraction is proportional to the thickness of the vanes, the secondary effect that most people are not aware of is that the spider vanes are generally a couple of degrees below ambient temperature due to radiation of thermal energy into the (relatively cooler) night sky. In this situation, you will have a boundary layer of air surrounding the spider vanes that has a variation of the refractive index (of the air) Wire spiders have an advantage in as much as air jacket is smaller and has a lower delt T than a traditional solid vane.
In practice, the contribution to the diffraction spikes is actually dominated by the boundary layer, and not the vane width, believe it or not.

The one advantage I see in traditional solid vane spiders is that they are cheap as chips and easy to employ.

Regards,
~clive.