12" F4 OTA design suggestions

alistairsam · #1 08-11-2011, 03:53 PM

Hi,

I'm thinking of getting a 12" F4 mirror from GSO and constructing the OTA myself. I'd be using it for both imaging as well as a visual scope on a Dob Mount.
F4 as it'd help with imaging as well as keep the OTA length manageable for portability and as a wide field scope.

The mount would be an equatorial fork mount.
I'll be using a 2" 10:1 crayford, and a wire spider.
haven't thought of the mirror cell as yet, but I can get aluminium parts machined at a local machine shop.
I have most of the tools required for woodwork though.

I'm not an expert but am handy with building things and am not targeting at building a high end scope.

any suggestions on the OTA design or what factors I should consider before thinking of the design?

bmitchell82 (Brendan) · #2 08-11-2011, 04:02 PM

Ide stear clear of a fork mount for a newtonian design, they arnt really made for it. I use my 10" 4.7 for visual and astro, so it has the dovetail and rings and also the dob circles on the side, I actually use the dob base for storage as well so after a evening of astro i just sit it back in the cradle.

Do yourself a favor and download a small dos style program called NEWT it will help you design the OTA Secondary sizings and a few other bits and pieces. its very detailed and youll need to put stuff in and measure other things.

Hope that helps mate.

alistairsam · #3 08-11-2011, 05:25 PM

Hi Brendan,

I'm building the fork mount myself with a 50mm RA shaft and suitable bearings, I somehow just can't work with GEM's. Just a personal preference.

I've already got the NEWT program for dimensions, but my question was more on what the OTA design would be, a truss newt, serrier truss, cylindrical tube, not sure if there are many other types.

a few examples
http://stargazerslounge.com/1710199-post21.html
http://www4.informatik.tu-muenchen.d...ravelnewt.html

bmitchell82 (Brendan) · #4 08-11-2011, 05:34 PM

straight up that poor dude would have had a nightmare with his collimation for one. no center spot on the mirror........

2 if your doing astrophotography the light will kill you 100% so open truss designs are not that good unless you wrap them up with a light proof medium.

3. for visual you can go a open truss, but i have used open truss designs and if you have any light around it detracts from the image.

a combination of CF truss's and then cover it in with a thick black material will help alot.

The only reason why i was suggesting the Gem was to do with the fact that you will have field rotation unless you have a polar wedge further to that Newts are quite long compared to the catadioptic bretheren eg RC SCTs, but if you like the fork mounts

run with it because a well built one is good just not portable.

Brendan

koputai (Jason) · #5 08-11-2011, 06:02 PM

Brendan, I think he means a proper EQ fork mount, not an alt-az consumer variety one.

Newt's on GEM's are just as bad for visual as Newts on EQ forks, same bad eyepiece
positioning, but if you are able to rotate the tube easily then it's not so bad.

I reckon a Surrurier Truss on an EQ fork would be fab, I've been thinking about that for
my 12", but I just haven't had the spare time.

Interesting project Alistair, we want pictures!

Cheers,
Jason.

alistairsam · #6 08-11-2011, 06:04 PM

Thanks.
I will look up some of the designs here. The Serrurier truss looks very attractive, will be using baffles/shrouds for sure.
are there sources of reasonably priced CF truss tubes, mirror cells within the country? I don't think I can get the mirror cell built unless I have some specs with tolerances, so don't want to re-invent things too much.

koputai (Jason) · #7 08-11-2011, 06:06 PM

Alistair, just noticed your signature mentions an 8" on an EQ fork, do you have any pictures?

Cheers,
Jason.

alistairsam · #8 08-11-2011, 06:24 PM

Quote:

Originally Posted by koputai

Alistair, just noticed your signature mentions an 8" on an EQ fork, do you have any pictures?

Cheers,
Jason.

Hi Jason,

The 8" is still a work in progress and is a rather crude one as it is made from wood, but I'll be completing it in 2 weeks and will post some pics.

For the 12", I'm leaning toward aluminium as there is potential for reducing weight and is easily machinable.
One such design was this, but featured a very long OTA.
http://www.vega-sky-center.com/pages...nt_Photos.html

I'm working toward a portable design for the 12".
I was told there are different grades of aluminium with different tensile properties, so will need to look that up.

I'm also leaning toward friction drives or timing belts rather than worms. still contemplating a few combinations.

I have the electronics with autoguiding and wireless control sorted out though. All home built. I'm adapting what I learnt with the 8" into the 12".

bmitchell82 (Brendan) · #9 08-11-2011, 06:44 PM

Im not talking about visual use as if your doing photography that is the limiting or critical case

Use PLOP for the mirror cell design, its simple really, you need a solid base on which sits the tiltable cell Ill be making one for my newt very soon.

As for the Serrurier truss. Read why it is and how it works as i think youll have to have a very strong grasp on structural engineering to do it properly the way it is ment to work aka both ends flex the same amount to keep the optical axis on par. Also stay away from aluminium for this as youll get shot on expansion. if it was me i would get 2 rings and lock them in with two bars like a RCOS RC then carbon truss upto the top with similar cage as the bottom that way it will allow your balancing to be in the approximate right spot for your forks and also allows you to effectively baffle and block light

alistairsam · #10 08-11-2011, 08:28 PM

hmm, the dreaded thermal expansion. thanks, will look at alternative materials.

The RC designs do look deceptively simple, but the weight of the Mirror section and the secondary area needs to be adjusted correctly for the balance point to be close to the mirror, as forks are sensitive to balancing errors. one upside is they require lighter counterweights.

Are CF truss tubes commercially available?

as for the mirror cell, I'll have a play around making one, but I might be taking on too much if I go down that route as I have all the rest to design and fabricate. I have had a look at plop.

Reason I brought this query up is because I may be able to get a 12" F4 mirror set from Orion optics for Christmas or as an alternative, from GSO as they're high on my wish list and that wish might just be granted!

As for the Serrurier designs, I understand the concept, but is sag really that noticeable if trusses are CF or are very stiff?

wouldn't axial sag on either sides of the pivot point be pronounced with cylindrical OTA's as well?

ZeroID (Brent) · #11 08-11-2011, 08:37 PM

I'll vote for the Serrurier truss, I am very happy with my build which is derived from Rolfs (Skyviking) design. Mine was built quite simply without fancy fittings, just ply rings, alum round and square tube. The critical part is to be accurate with the fabrication of al the components. Advantages are it is very light for it's size and very robust Mine does duty on an EQ6 and a DOB base. Also has the advantage of being easy to balance and it cools extremely quickly. You do have to be careful about light intrusion though. Light glow inside the UTA from nearby sources have to be watched for. My Mirror is buried down inside a lightweight black lined paper bucket lightsheild. THe whole thing is a one hand carry at about 11 kg.

I haven't noticed any sag and mines a 10" mirror. The short tube lengths make the structure very rigid.

alistairsam · #12 08-11-2011, 09:10 PM

Hi Brent,

I do admire your build, a few questions.

have you noticed expansion with long imaging runs?
how did you ensure all truss tubes have their holes perfectly aligned as the distance between upper and lower holes across all tubes would need to be precisely identical to ensure alignment along the optical axis.
finally, how did you ensure the rotating upper cage does not shift around the optical axis when rotated?

11kg is far better than the 20 plus kgs of the gso 12" F4 OTA.

edit: Read Rolf's thread again, got answers to a lot of my questions.

bmitchell82 (Brendan) · #13 08-11-2011, 11:29 PM

here is an Australian dealer for Carbon fibre Tube/rod/cloth/tape/epoxy etc etc

Here

Visual and Photography are two totally different beasts! just remember that.

as for expansion or Axial shortening/lengthening Steel has a lower COTE (Co-efficient of thermal expansion) than Aluminium and depending on the temperature differential you will get varying rates of contration.

To give you a idea ill loose a good 1 FWHM in the space of a hour in the first part of the evening after about midnight ill loose about .25 - .5 FWHM. and the difference in 1 FWHM is very noticeable when your CFZ or (Critical focus zone) is about 50 micron.

You shouldn't get much deflection over that distance but once again depends on how accurate you want it. if you want it strong and accurate for photography. spend the dollars make it right, look at Doomsayers efforts. done right

works very well!

If you triangulate the frame properly and minimize the slop in the connections you should end up with a good setup be sure to like i said enclose the OTA or Light will kill you, My canon 40d used to suffer light leak though the view finder and hence it got covered. My newt is open in the back end and hence it has what i call the shower cap on because light came past the primary and made its way to the imaging sensor (look at my website youll see my efforts for that).

If it was me ide be looking at a full tube for AP but its all about the weight sometimes and ill leave that upto you.

Brendan

alistairsam · #14 09-11-2011, 07:02 AM

Thanks for the info Brendan,

Doomsayers work is amazing.
How are the tube ends fastened to the CF tube though?

yes, weight is important for me as that'll improve the efficiency and accuracy of my gear train whichever one I choose, reduce fork deflection and ensure portability.

Moon (James) · #15 09-11-2011, 11:03 AM

I'm a bit like Brendan - I look at it from an imaging perspective. Building an OTA as well as a mount for deep sky imaging will be quite a challenge.
Here's my 2 cents worth:
- I'd go for the regular secondary support - not wire.
- A truss starts to make sense with a 300mm primary, but you are not totally beyond the realm of a tube. The other nice option would be a light but strong composite tube. You could do both of these yourself, I'm guessing the truss would be the cheaper option - so maybe that's the way to go.
- Plan in advance the full image train (camera, coma corrector, focuser and auto guiding etc) - this will determine the distance from the secondary to the focal plane and also the secondary size. Then it's easy to work back and figure out all the other dimensions from that.

If you're looking to do it on the cheap, you shouldn't have any problem picking up lots of useful parts second hand - either here or on astromart. Lots of people have replaced GSO focusers, secondaries, spiders and even primaries. Maybe you could even pick up an old-school mount and refurbish it?
James

alistairsam · #16 09-11-2011, 12:22 PM

Hi James,

yep, an imaging ota and mount would be quite a challenge. I am designing it from an imaging point of view with tight tolerances.

However I've covered most of the mount and fork requirements, it was just the OTA that I had to design.

I will definitely be using carbon fibre truss tubes, I've been looking at different joint designs.
If I manage to build the secondary section light enough, I could get away with an RC truss design rather than a serrurier truss as I could move the pivot point closer to the primary.

Final truss lengths would be after I've measured the back focus required.
I'm in two minds about the mirror cell though. it looks simple enough, but it requires reasonable precision. so not sure if I'm to build it or buy one.
haven't seen many ads for mirror cells.

Is there a min and max for the angle between trusses? from what I understand, axial rigidity improves with the angle.
Even with an F4 and well spaced primary rings, the truss tubes would be fairly long and angle small.
Will draw it up and see.

ZeroID (Brent) · #17 09-11-2011, 12:43 PM

Quote:

Originally Posted by alistairsam

Hi Brent,

I do admire your build, a few questions.

have you noticed expansion with long imaging runs?
how did you ensure all truss tubes have their holes perfectly aligned as the distance between upper and lower holes across all tubes would need to be precisely identical to ensure alignment along the optical axis.
finally, how did you ensure the rotating upper cage does not shift around the optical axis when rotated?

11kg is far better than the 20 plus kgs of the gso 12" F4 OTA.

edit: Read Rolf's thread again, got answers to a lot of my questions.

Rolf is the guru with all the technical stuff for sure.

I haven't imaged yet, still building the rest of the system so can't answer that.
The build is more tolerant of minor differences than you think. I measured and cut each set of trusses ( top & bottom ) then checked and filed them to within a half mil or so.
I then crushed the ends and drilled the holes on the drill press with an end stop centring jig, bit of alum channel bolted to the drill press table.
Ditto with jigging all the little brackets. The old adage, measure twice cut once is useful. Only loused up one length.

Got it all together but it was too long so pulled all the bottom trusses and cut another 50 mm off and redrilled to acheive the sweet spot.
The very minor differences all balance out in the randomness of assembly and the mirror cell and diag take care of the rest.

Rotating UTA was a bit of a fluke. I got two almost perfect ply circles with the router which required almost no sanding and it went from there. I was extremely careful with the spacer tubes length etc to ensure the Rotator assembly stayed extremely flat, no strain. Also the diag is generously sized and can take some amount of deviation in centring. Still refining the guides, just replaced them with much lower profile clips but again can't emphasise the importance of being able to make accurately consistent components and the drill press and jigs are the answer there.

NZ temperatures don't fluctuate wildly enough to show any length problems in expansion but they would be extremely minor and the design means they would be along the axis anyway, please adjust your focus.

As I said it very robust and stable not to mention light. With my DSLR, finder and knick nacks attached it still comes in under 15 kg.

If I ever build something bigger it will be a serrurier design, the shorter balanced axis makes it much more user friendly than the long single unit truss designs and more rigid. Hmmm, a 16" Serrurier .....

alistairsam · #18 09-11-2011, 01:19 PM

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?

alistairsam · #19 09-11-2011, 01:29 PM

Hi,
are similar mirror cells available locally?
http://www.orionoptics.co.uk/OPTICS/...irrorcell.html
anyone bought these mirror cells from orion?

bmitchell82 (Brendan) · #20 09-11-2011, 01:50 PM

from a structrual point of view 45 degree angle is the strongest you can get when you build a truss system. the OTA is no different, but its not practical or buildable. As a general rule of thumb you want at least four triangles up and four down. Tonight before i leave work ill try and model it to see what kind of flexure you will be looking at.

As for the connections try and get them made to fit the end of your carbon fibre tubes and epoxy them in if they have a T fin plate at the end that can be bolted to a similar detail on the OTA then you will have a very strong fitting. This is something that I was looking at with the designs I have done so far.

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 this will give you a good vane and try to make it no deeper than 20mm or this could give you odd defraction artifacts on off axis light paths. My 254 i built the secondary and it runs 1.5mm Stainless vanes, its built like a ox and doesn't move!

Simple calculations will give you how much weight you will require to balance free body diagrams and equilibrium conditions will do it