Looking for a new OTA - any advice?

[SkyViking]

Hi All, I'm actually thinking of getting a new OTA to put on my Losmandy G11 mount. As I'm also saving up for a serious CCD cam I'm currently considering the cheapest options for a new OTA.

I currently have a 10" Newtonian f/5 and am happy with that, but would like to maybe get a 12" or even 14", depending on the resulting load on the G11.
I'm mostly doing photography but I enjoy the odd visual session now and then too.

It has to be said that I'm not after any state-of-the-art system. My current OTA is a self assembled kit that I bought 15 years ago and I have since installed a motorised Crayford focuser on it. It consists of some no-name optics which has never been re-coated in 15 years (!), a very basic mirror cell and a paper tube which is rather unstable and absorbs moisture... So really anything will be an improvement!

I'm fairly handy and could probably build one myself (and re-use the focuser). In that case the immediate question would be where to get the optics from?

Alternatively, I also looked at available Newtonian OTA's and noticed especially the truss Sky-Watcher dobs.
Optcorp sells a 12" Sky-Watcher for US$995: http://www.optcorp.com/product.aspx?pid=1-600-1396-1456-12122
The Sky-Watcher's tube seems to extend up to and including the balance point - which would indicate I could just mount it in rings and place it on the G11.
Has anybody tried this?
Does the SkyWatcher have decent optics?
What's the build quality like?
Or should I rather consider something else?

Hope you guys would like to share your wisdom :)

Regards,
Rolf

[TheAstroGuy]

Have you considered maybe a Takahashi Epsilon? they are such a gorgeous looking thing.

Pure photographic genius and now with the dollar so strong really worth a shot, i am going to purchase one shortly.

Super fast Focal ratio and decent size maybe a 180 would be good.

Kind regards

Shane

[Alchemy]

I wouldn't be using a truss tube for imaging, IMO you would have too much flexure for imaging, as for a 12 inch solid tube like the gso offering, I found the tube to be not rigid enough for long exposure imaging, I could consistently get 5-7 minute images on a G 11, I ended up selling the 12 inch tube as it didnt meet my expectations.

For simplicity I went back to a refractor, so I could do deeper images, whilst I would like a 6 inch or bigger the budget only went to a 5 inch apo. I have yet to go past it's capabilities, am currently looking at a better camera myself as the scope can get better resolution than the camera delivers ( using an OSC with 7.8 micron pixels)

There are plenty of scopes that will work really well on a g11 so you have lots to choose from.

[mental4astro]

Hi Rolf,

I agree with Clive that the cheaper end of commercailly made truss/flex-tube newtonians lack the necessary rigidity for use as a photographic rig. They are not designed for this use. Some solid tubes may also be under engineered.

Getting a purpose made photo-rig is one option.

But if you are not affraid of flaying the hammer about, there are options. Have a look at this link. The owner of this newtonian made this newtonian rig to be able to rotate in its cradle so that its eyepiece can be left in a more favourable position for visual use. Very last entry in this link:

http://www.observatory.org/install.htm

For a truss scope to be used as a photo-rig, the truss component needs to be specifically designed to take the changes in the position of the scope. This next link shows another way to deal with this flexural problem. It is a little dated in appearance, but it is simplicity incarnate. If nothing more, it serves as inspiration for your own design:

http://www.rfroyce.com/mynewt.htm

Alex.

[SkyViking]

Thanks guys for your replies :)
I can't help but think that almost all of the large professional telescopes are truss designs and so it must merely be a matter of construction quality to achieve the required ridigity?

I think I would like to make a true Serrurier Truss (http://en.wikipedia.org/wiki/Serrurier_truss) Newtonian which will be mounted on the G11 via the central box construction. I found an excellent example here: http://www.aao.gov.au/local/www/sl/sl-tels.html#ni-tel There is a great photo of the truss design here: http://www.aao.gov.au/local/www/sl/pictures/30cm-new1.jpg
Alex/Clive, do you know if such an approach would be stable enough for imaging? Anything in particular to watch out for?

My local Bunnings sells 1" alu tubes for $10 pr. metre, both square and round. I don't have the equipment and skill to try welding alu, but I figure I can just bolt/screw it together and cut some thick plywood rings for the top end and make a mirror cell.

For the optics I have long been thinking about R.F. Royce mirrors, I checked yesterday and a 10" conical sells for US$850 which I think is not bad. One of the many advantages of these mirrors are that they simply mount with a bolt through the middle which further simplifies the design of the mirror cell.
I do dream of a 12" or larger though, but the 12" Royce is US$1700 which is a bit steep I think, compared to the 10". I would like to hear if anybody knows if it's worth it, or if I would be better off with an average run-of-the-mill 12" or larger? I do know that these optics certainly yield great results for planetary imaging, but I'm mainly thinking of long exposure deep sky imaging here. Perhaps the blurring due to seeing effects during long exposures may negate the advantages of the Royce optics to a degree?

By the way, I've attached some photos of my current mirror, as you can see it hasn't been resurfaced in 15 years:help:I must have lost some 30-40% of the reflectivity already when combined with the secondary...

[dannat]

if its a good mirror why not just re-coat?

[el_draco]

If you want a good combo, get a C11 and try for hyperstar. Going that way myself

[mental4astro]

Hi Rolf,

That truss design is brilliant for imaging (what the heck became of "photography"). It is incredibly strong and stable.

With the picture you given the link to, nuts & bolts is fine. It will be most forgiving if the OTA needs to be altered- no welds to cut.

The central brace should be welded for optimal strength. The welding can either be done by a dedicated welder, but you can also get a decent weld done by many ship wrights or other boating repairers as they regularly need to repair aluminium.

The good thing about a strong welded central brace is that it can best deal with the flexural forces. It will also provide the most secure platform to secure the OTA to the mount.

It shouldn't be that expensive either. You can also make some welding jigs for the joints to be welded. That makes the accuracy your responsability. I'd rather that fall on my shoulders than get crook with someone else about it. I can give you some ideas on how to make these if you like through PMs or email as it is beyond the scope of this thread. Other fellows here on IIS have incredible experience with welding. Just ask.

Clever desiging can give you a rotating secondary cage too for optimal visual access.

Nothing like making your own.

[Alchemy]

Late reply, the truss design on the pic looks good, weight then becomes a factor, 10 inch you could get away with it perhaps. 12 inch .... Not on a g11.

But doesn't it look good:thumbsup:

[SkyViking]

Again thank you for the replies.


I'm not sure how good my mirror is, it works fine but I've never tried anything else. It was fairly cheap, part of a kitset with sonotube and no-name plastic focuser which I later replaced with a proper Crayford. So I don't have particularly high expectations as to the mirror's quality ;)


Yeah the C11 is very nice and I like that it is so compact. But I figure I can build a scope at least as good, and then I can fiddle with it as I please. I also prefer only having the 2 optical surfaces of the Newton rather than the C11's 4.



It looks great yes :) I'm currently trying to not be deceived by it's looks... and you are probably right about the weight.

And thanks for your advice re welding Alex, I was thinking about getting someone to do a few welds on it for me.
Overall I definitely like the truss tube design and think I could build one without too much hassle. However, now I'm wondering about how to attach the truss tube to the mount? The central brace is rather narrow...
I already have my current OTA mounted in 12" tube rings from Parallax, sitting on a long dovetail. It seems to me this type of connection would be more stable than the narrow brace of the truss tube?

Another issue I thought of was how to keep the spiders out of the truss tube. They are instantly going to string their webs when they see those trusses :D Does anybody have experience with this?

So another option is a solid aluminium tube, I noticed Parallax sell them by the foot for a reasonable price.
Is alu tubing a good option?
Is there a supplier closer to here?

I kind of like the simplicity of a solid tube, and it would keep the bugs out too :)

[mental4astro]

Bugs- Well, the mirrors you would cover when not in use and a sheet over the truss assembly will help with dust and spiders. I'd be covering my scope, and do, regardless of what type and mount it is. And with the mirrors covered, a quick sweep of the webs if any form I don't have trouble with. You're never going to beat spiders, as you know.

To couple the scope to the mount requires you to think of this at the design stage. Just about all commercial scopes utilize a dove tail arrangement to couple the scope to the mount. You can do this too, as you can then incorporate this dovetail to the central brace. All in the planning, and this includes the 'dove tail assembly'. If you still stuggle with the coupling, you can always make the central brace a shallow 'box', and the dovetail can be attached it. This won't compromise the flex issue either, and in the pics you provide, notice how 'that' central brace is acutally TWO square frames welded together. A shallow box would actually be stronger too.

[SkyViking]

Thanks Alex, yes a cover might be the best solution. and I could make special fitted covers for the primary and secondary - I suppose they would then be even better protected than when sitting in a solid tube! Maybe the coating will even last longer.

When you say shallow box I suppose you mean making the central brace wider? I did notice the two square braces welded together, so I could for example instead keep some space between them and screw/weld a rectangular piece of alu sheet to each of the four sides, thus creating a solid box that is wider than just the two braces alone. And then simply bolt the dovetail to the box through the square braces.

Looks like the truss design is the winner so far :)
Again thanks for the advice.

[GrahamL]

The maker is still active here maybe you could chew his ear a little:thumbsup:

http://www.iceinspace.com.au/forum/showthread.php?t=27412

[SkyViking]

I just wanted to say thanks again to you all for your advice and suggestions. I have recently completed my Serrurier truss tube Newtonian, and I have posted a new thread with lots of details about the project here: http://www.iceinspace.com.au/forum/showthread.php?t=74690