8" f/4 travel truss dob - arrgh not good enough!

[janoskiss]

I made a wobbly dob base for my truss converted 8" f/4 newt. I'm getting pretty sick of this project. I've been working on it few hours every night this week. End result is a wobbly base that will be useless for observing. :mad2:

The A frame sides are sturdy and the altitude bearings work very well too. But the vinyl LPs (2 glued together) and aluminium base are way too flexible, making the mount very wobbly. :sad:

I'll try again with plywood. I did not want to use wood, because of all the restrictions on bringing plant based stuff into the country. :rolleyes:

[EzyStyles]

The scope might be abit too heavy for LP's to spin? still a fine piece of work you did Steve.

[Starkler]

Perhaps a little bit more trussing on the base and an aluminium plate over the LP's will get you over the line?

Interesting design :)

[EzyStyles]

or WD40 on the LP's? :P

[Miaplacidus]

Or a lazy susan???

Seriously, Steve, it looks better than I imagined. I reckon you must be very close. What are the bearings made out of?

(Still reckon you could have used a Portaball model.)

Cheers,

Brian.

[janoskiss]

The bearings are not the issue. They are very nice and smooth. Vinyl bumpers on PVC flange for altitude, and furniture sliders on vinyl LP for azimuth. These work great. It's flex in the base that is the problem. It supports a lot of vibration with little damping.

I tried replacing just the base plate with 20mm plywood. A little better, but the vibrations are still way too much. The vinyl flexes very easily, so that needs to sit on something solid as well.

Portaball would not pack up too well and would need a lot of counterweight to balance, that's why I didn't go that way.

I decided to chuck the whole thing and go all plywood (12mm). This will eliminate the wobble in the base, and using plywood for the sidewalls as well will let me get away from the straight edges, so I'll be able to drop the scope right down and still brace the mount properly. atm the scope is sitting 5-6 high to leave room for the bracing elements.

[gaa_ian]

You are a DIY ATM legend Steve !
I have seen a a few "fold up scopes", yours looks very interesting !

[janoskiss]

I forgot to say thank you for all the encouragement, guys. Much appreciated! It gives me motivation to keep at it. :)

[davidpretorius]

a 2.5mm x 300mm aluminium disc would not cost much? local fabricators?

[ving]

pretty sweet loking diy truss steve! congrats! :)

you going to do a write-up on how you did it for the site?

[janoskiss]

I guess you don't like reading posts David, just looking at photos. :rofl:
In a nutshell: The mount is a dud. :(

[asimov]

Hi Steve, my compliments on a job well done.

I'm thinking thin high tensile strength wires/cables & small turn buckles to tension the frame up to stop the vibes, but looking at the overall design I'm not sure where these could be placed to work effectively.

[astro_nutt]

Hi janoskiss,
I've made quite a few mounts for my scopes..EQ's and Alt-az..more duds than goodies..and it is a learning experience..at first you don't succeed..etc
But at least you tried to build something bold and diffrent!!.so full marks for that!!

[ving]

i did mean to do it once you have it up and running satisfactorally :rolleyes:

[janoskiss]

Sorry about my smartypants comment there... ;)
I don't want to complicate it more with cables, more braces etc. I wanted to keep it simple, so it's fairly quick to disassemble/reassemble. Mark 2 will be all plywood.

[Starkler]

Just to throw another idea your way ;)
I have also contemplated doing this with a base consisting of a number of flat panels secured to each other with dowel pins and spring loaded snap catches.

[janoskiss]

That sounds very sophisticated Geoff. I wouldn't even know where to look for the parts needed.

Anyhoo, plywood + L-bracket idea was a disaster. The execution was anyway. Probably could make it work - if I bought stiffer (thicker) plywood and thought more about how to stiffen the joins between panels too.

So I went back to my Meccano Dob after all. Added plywood to the swivel base, and plywood for ground board. Big difference. Rubber feet in pic had got to go - they are a major source of vibrations. I replaced them with steel bolts - much better. Bigger ground board might be good idea for better AZ stability when near zenith. I'll also add ALT friction control tomorrow. Otherwise it's not looking too bad. Might even get to try it out tomorrow night. :prey:

[danielsun]

Well done Steve!! Looks pretty good :thumbsup:

[stringscope]

Improvise, Adapt, Overcome! :thumbsup:

Its looking good Steve. What sort of case are you going to transport it in?

Cheers,

[ving]

water... ducks back, need i say more? :P
quite used to your smartypants comments :)

[janoskiss]

First light tonight was a disappointment. I stiffened the frame and the baseplate, but the thing still vibrates like crazy. It's okay for low powers but at 100x or more it is very painful. It took me back to my old wobbly Dirk Smith EQ mounted 4.5" Newt. I don't really know where to from here. :sadeyes:

[gaa_ian]

Ah, the pain of doing R&D !
What about a fold up plywood dob base ?

[janoskiss]

I started making one, but the plywood I chose was too thin. But even if it wasn't I'm not sure the pull-apart design would be good enough either... And next w/e is new moon. I'd rather be observing than building more bloody dob mounts. :mad2:

[janoskiss]

I might drop in at Bintel and try out vibration suppression pads under the feet (1/4" bolt heads). :shrug:

[janoskiss]

Just back from Bintel. Tried out the meade "vibration isolation pads" - no good. Roger had some good tips. Trouble is they all require a lot more work..

[Miaplacidus]

Come on, Steve, you can do it. (You probably don't realize it, but there's a sweep going to see who gets their gear up and running first, you or Ken.)

Cheers and Beers,

Brian.

[ving]

it seems to me that you are trying to build it too light?

[Merlin66]

I once had a lightweight 6" f8, where the tube would vibrate like crazy. The solution was to run a cable from the front of the tube down to the base, under a roller, up to another roller held under a tension spring, one wrap around, down to the third roller then up to the mirror cell of the tube. Acted like a triangular stiffner which still allowed freedom of movement. Very simple ; very effective.

[astro_nutt]

I'm just wondering, have you tried a torsion test on optical tube and the mount/base?
Try placing the optical frame only it standing upright on a concrete floor and attach a laser..then place a mirror on the ceiling so the laser bounces off and reflects on the floor..With the laser on, try and adjust the focuser..etc and see how much play there is...then support the tube by the hubs using some drink crates or whatever and test again..next is the mount by itself..giving it a firm rap at various places...then the whole lot together..this is a way to find out where the most likely cause of movement.
I hope I don't sound too critical...but your scope looks cool and has great potential!!
I know it will work out for you!!

[janoskiss]

Thanks for the comments and suggestions. I have checked out the vibration of the various components separately. The OTA is good and sturdy. The base is okay now too. It's the A-frame sides that flex too much. Going to go back to the plywood route. I feel like I'm in a prequel to the sequel to a bad movie. :rolleyes:

[Roger Davis]

Ahh Alian Gremlins! The Number of the Beast, you could call it 666 or Be'alzevuv (the original spelling!).
I think your problem was in the way in which the A frame was held together. Relying on aluminium grab instead of nuts ain't the best. You'd be better off redoing them with large pop rivets, that way you could put in three or four p-r's to reinforce the frame. Looked like an aluminium version of Burr Castle, but without the speculum mirror!

[janoskiss]

Here is the new model: http://www.iceinspace.com.au/forum/showthread.php?t=11358
No first light yet. But there was a zeroth light before I strengthened the side walls by adding the lighter coloured outside panels you see in the pics, and it was already quite usable, viewing Jupiter at 200x. I also repositioned the sliders on the groundboard so I think this one will do fine. :)

[ballaratdragons]

I think Steve won. I only finished mine today.

Well, it was finished about a week ago but rain has prevented me from actually going outside and mounting mine until today. :lol: