Description:

'Stasis', a full-height tripod built for stability.

With my GEM nearly completed I needed to construct some sort of tripod on which to mount it. I had thought this mount would be ideal for my Vixen 90mm refractor guidescope (a little OTT perhaps for such a small scope, but I bought the Vixen as an OTA only and later decided it would be nice as a quick setup portable scope). Anyway, I thought I would see what could be done to provide a really stable platform for my home made mount.

The design encompasses no compromises in the quest for ultra stability, all mating parts were made close-tolerance, it was built entirely from alloy, stainless steel and timber. I decided it had to be a folding design to fit easily into my car for transport.

Tripod leg mounting lugs.

The top plate was turned from a 2" thick disk of solid alloy cut from a 6" diameter bar (finished to 5.975"), the central pivot for the mount (to adjust AZ) is 1" diameter and the base of the mount itself was machined with a 5.976" dia recess to exactly fit the tripod top plate. Three flats were machined on the sides of the top plate at 120 degree spacing to accept the leg mounting lugs. The pivots for the legs are 1/2" stainless steel running in reamed holes in the lugs. The legs were made from 2" x 4" timber, tapering down to 2" square at the end.

The legs have sliding extentions for levelling the tripod, the feet have flat plates on ball joints.

The legs were tricky to make, not so much because of the profiling to get the taper which was easily done with the milling machine and a long series 1/2" endmill, but they were awkward items to drill axially for fitting the feet. I eventually mounted them on the lathe cross-slide (facing the headstock with tailstock removed) in order to bore a central hole in the end 1-1/4" diameter and 6" deep. Into this hole was pressed (and glued) a 6" length of thin alloy tubing (1-1/4" x 16 gauge) to act as a liner. Into this is inserted another piece of tube machined from 1-1/4" bar which extends 2" beyond the bottom of the leg and holds the feet locking knobs. This second tube is a fairly close fit but it's by no means tight in the liner. It is held in place by cementing with silicone rubber sealant. Before it was fixed in place though, 2 disks of vibration suppessant material (1/8" thick) were placed at the bottom of the hole so they acted as shock absorbers connecting the top of the foot locking tube and the wooden leg. In theory, the rubber sealant allows a small amount of axial movement, like a piston, and the anti-vibration material should go some way to absorbing ascending vibration. Finally, the extendable feet (turned from 1" bar) are inserted into this second tube. These have a groove machined along their 5-1/2" length (not quite all the way to the top, so they can't drop out), and a knob locks them firmly in place. The feet terminate in a 1/2" ball turned from stainless steel onto which is pressed the 2-1/2" diameter pads shown in the photo. These feet allow the tripod to be levelled on uneven ground and to give a modest ajustment in height.

The leg bracing system.

The leg brace is constructed from three 1" x 3/8" alloy bars, connected to the legs by hinges at one end, and to a central sliding locking plate at the other. The pivot pins for the bars are made from 3/16" stainless steel running in reamed holes with ample bearing surface - with a dose of molybdenum grease they move sweetly without play, and there should be no wear over many years. The locking plate has a reamed 3/4" bore (0.751") and the central support rod is solid 0.750" dia precision ground stainless steel. The top end of the support rod screws into a large diameter alloy knob which itself threads into the bottom of the 1" dia lug in the base of the EQ mount. Tightening this knob locks the mount head in place onto the tripod top plate. At the lower end of the support rod is a bearing which engages a groove in the rod and is thus able to rotate independently of it. Right at the lower end of the rod is another knob (this one is black in the photos) which, when screwed onto to the bottom of the central sliding locking plate, traps the bearing between the knob and sliding plate. In this fully open (and locked) position the 4" diameter plate is clamped onto the top of the 3 bars preventing any movement. However, the mount head can still be locked or unlocked by virtue of the rod rotating in the bearing (it takes just 1/2 a turn of the upper knob) to adjust in azimuth whilst the tripod remains firmly locked up.

I'm very happy with the finished product, the tripod is amazingly solid, you can push it, twist it, whatever - and there is no flexing at all. Using wood for the legs instead of metal I hope to get improved vibration damping, but whether the extra effort to provide for the vibration damping inside the legs was worthwhile I don't know yet (not had much chance to try it out - as usual the weather in the UK sucks :().

addendum: After testing the tripod with my 90mm Vixen refractor mounted on it, I can report that the stability is all that I hoped it would be. The damping time is <1 sec, and one has to virtually kick a tripod leg to evoke vibration in the first place. Observations were made using a 7mm Lanthanum EP (143x).

'Stubby':

	Meet 'Stubby', a cousin to Stasis but a bit shorter in the leg.
	Designed as a portable tripod for my Orion Optics (UK) 12" Newt, Stubby is shown with legs set at minimum height.

Stubby was designed along very similar lines to the successful Stasis, the same leg bracing system used albeit of slightly larger section material. At it's lowest height setting Stubby keeps the end of the 12" Newt just a few inches off the ground when pointed at the zenith, although the extending legs provided an extra 8" height adjustment just in case I have to setup in long grass. You might reasonably ask why Stasis was not adapted using shorter legs which extendend further to accommodate a larger range of heights - thus the one tripod would serve both purposes. The answer is partly because it's the big, solid wooden legs that provide the stability and strong damping action, 18" extentions would have compromised this. Also, the angles of the open and locked legs needed to be different at low heights and the design is complicated enough as it is.

The same head design as Stasis was used (of course), 6" diameter and incorporating a worm gear section to mesh with the ALT adjustment worm on the mount. The leg geometry was a bit awkward, I definitely wanted to maintain the large footprint of Stasis to retain stability - but this was required in the shorter mount. Therefore the legs needed to be splayed out at a wider angle which in turn would put more mechanical stress on the leg bracing. Therefore, the leg braces were made of a larger section material (1-1/2" x 1/2"). The leg mounting lugs on the tripod top were also extended an extra inch or so outwards from the center of the tripod top to help obtain the wide footprint. The pins for all the bracing are 5/16" diameter stainless steel running in reamed bores, the leg pivot pins are 1/2" diameter. The structure is very stable when locked in the open position.

	The top-plate and leg mounting lugs.
	Azimuth worm drive for polar alignment.
	Stubby's leg bracing, 1/2" x 1-1/2" section, locked in position by the 7-1/2" diameter x 1/2" thick round plate.
	More pics showing the underside of the leg braces and locking plate (drilled to carry eyepieces).

The ends of the tapered wood legs were drilled 1-1/4" to a depth of 12", and a full-length liner (1/8" wall) glued in place. The three liners were turned from 1-3/4" alloy bar bored 1" diameter right through to accept the leg extensions, the end 2-1/2" was left full diameter to take the threaded hole for the clamp knob. The leg extentions are 1" solid alloy bar with a channel milled along most of their length, the lock knob has a 5/16" lug turned on the end to fit this channel. The extensions are positively locked in this manner (I've stood on the tripod top, stamped on it and it doesn't slip - and I'm no lightweight!).

	Stubby's extending feet, note the slot arrangement and the pin formed on the end of the 3/8" x 16 TPI lock knob.
	Close-up of the locking knob and channel milled in the leg extention.

Back to Telescope Accessories page