My "Pseudo" Serrurier truss MKII

[coldlegs]

Well, The rivets seemed a good idea at the time but a year on and experience has taught me that the center support of a “pseudo” Serrurier truss has to be rock solid. There was a small visible movement in mine as the rivets were not solid enough and that plays havoc with the collimation so it was time for a “Rolf special”. A local machine shop cut and welded two aluminium octagons together for the center support which is now rock solid. The plywood main mirror holder had been butchered a bit over time so was replaced with an chopped down aluminium job. Can't remember who made it but I had it lying around so in it went. Decided it was time to fix the secondary screws. Every time I moved a screw to collimate the secondary would rotate. Found an old post on cloudy nights that suggested using axle cap nuts with a hole drilled through them for the screw to push against and it works well. No more rotation. You will find the “Improving Secondary Collimation Adjustment “ blog in the archive section of cloudy nights.

So children, the next time you build a truss scope make sure the center support is built absolutely rock solid.

Cheers
Stephen

Original design
http://www.iceinspace.com.au/forum/showthread.php?t=107896

[GrahamL]

The best of ATM !,,if something dosn't work out go back and do it again :)

[ZeroID]

Yes, that solid centre cage is the key to it all. Mine is a ply diaphragm, alum square tube, bolted construction but it is rigid in all directions. I only used rivets in a few non critical attachments.

The beauty with the Serrurier design is the DIY options, I've only modified mine about 5 x now.... so far :P

Nice work .. :thumbsup:

[alistairsam]

The centre section looks very stable Stephen, I noticed in your other pics that the truss nodes don't meet at the dovetail.

I think you might improve rigidity further if you moved the dovetail to the side where the trusses meet as then the load transfer isn't via the lateral part of the mid section but direct to the dovetail and mount?
like this.
https://flic.kr/p/o6hNnU

Cheers
Alistair

[ZeroID]

Mine doesn't either. Dovetail is midsection (square cage) between truss nodes. I don't think it makes any difference at the sizes we're working.

[coldlegs]

Alistair
Had to chew on that for a bit and came to the (backyard engineer) conclusion that the bottom truss would push the most on the strongest point (the dovetail) but the top truss would pull the most on the weakest point (the top) which is also the most likely to bend. So you would win some and lose some which I guess brings us back to the point of making the central support as strong as possible. I think mine could be used as a car jack it's now that solid. Don't know if it would improve rigidity but would be interested to hear arguments for and against.

Cheers
Stephen

[alistairsam]

hmm, I thought it would make a difference.
in my case, i have almost 3 Kg's of camera gear at the top and 2 kg mirror and a 1Kg counterweight at the bottom.

I can't speak in engg terms, just lay man terms,

In Stephen's OTA's, If you keep the scope horizontal on a table with the dovetail at the bottom, the downward force at either ends is transferred via the truss to the mid section at a point that's higher than the dovetail. since the two halves of the OTA are balanced, the whole load is supported by the mid section and then the dovetail, so the bit of the mid section from the dovetail to the truss connector is free to flex, from the pictures, it looks like a 2 to 3 inches.

if you picture the Al to be soft, you could rock it as the suspension of the whole OTA depends on how rigidly the mid section meets the dovetail.

alternately, if you have the truss nodes terminating at the dovetail, that's the lowest point and the whole front and back halves of the OTA are supported from there, not the mid section. if you look at my truss you'll see what I mean.

This is just my understanding and I could be wrong, but it makes sense to me because when horizontal, the trusses form an open V with the dovetail at the bottom of the V and the load at the top. whereas the other way, its like an ^ when you look at it from the side.

when the OTA is at different orientations, I pictured how the whole weight is supported and that's at the dovetail, so made sense for me to terminate the trusses at the dovetail so no matter which orientation, it would be well supported.

Most Truss RC's are also supported this way.

Again, sorry about the layman speak.

Cheers
Alistair

[ZeroID]

To put it bluntly a Serrurier Truss design, pseudo or not, in this size is over engineered to blazes. As I've posted on another thread when I completed my basic OTA frame I stood it on end then I stood on it on the upper OTA end. It supported my weight easily, didn't even budge.

My centre cage is a 17mm ply diaphragm of 330mm square.(IIRC) It braces the alum square frame which is bolted through at the corners.
Cage depth is about 80mm and the dovetail is secured across this metal\wood\metal sandwich along with a fair number of screws and bolts in appropriate positions and some angle bracing.
It don't move nowhere :P
Stephens cage build is welded alum box, doubled ( and it looks to be thickwall alum). All up weight for the OTA will be less than 15 kg I'd guess. (mine was 11kg) It could probably double as a 'Roo' bar on the ute without too much of a problem.
Yup, over engineered to blazes... :thumbsup:

[alistairsam]

I agree, it is over engineered and Stephen's welded mid section looks pretty solid. But at F4 (in my case), all you need is a few micron flex. I'm sure its under control though. Look forward to first light Stephen.
What mirror is it btw?

[coldlegs]

I'ts a 10" conical compliments of Mr Royce.

Cheers
Stephen

[Rac]

Mine is a square alloy welded section with the dovetail just between but just off to the side for better balance. It does have some flex only if you pull on the front or back of the scope but as I see it this should have little effect and it has no effect that i can see.
If we put the truss terminations where the dovetail is wouldn't that cause a force of pulling the back mirror support and forward ring together if there was any imbalance as the scope tilts? There is also little to no angle on the trusses to stop the forward back twisting?

The most important thing i have found is to have a very strong mirror mounting plate for the 10" Royce Conical mirror and back support ring. Mines 18mm ply with 2 layers of carbon on either side.

Here's a shot of mine (https://www.flickr.com/photos/46302893@N02/14727526938/)