14"/360mm truss dob

[Moonshane]

really lovely work Alan. I do like the use of thin (bendable) ply for the UTA - might steal this idea if OK with you.

do you have any tips on drilling thin stainless steel? you seem to have done a good job of this.

[8kids]

Very neat. I get my 12" mirror in a few days for my truss dob build. I'm reading The Dobsonian Telescope by Kriege and Berry at the moment in preparation. I'll be asking various builders on this site for material sources etc. I need to get it ready for my holiday in scotland this year.

Gary.

[Rustysplit]

Thank you chaps for your kind comments.

Shane, as for drilling thin stainless steel with small drills... a steady hand, even pressure, and spare drills!! Just take your time. I put the stud connectors in a vice to drill them. My pillar drill has a dodgy chuck and it wont close up enough to allow me to use thin drills in it

The aero ply is really nice. Lovely and stiff when bent to shape. So much nicer than kydex or whatever plastic. I have laminated a panel on the focuser side so I can fix direct to the tube, without needing a focuser board. All weight saved is good in my book. The mirror is a thin blank with a ply cell board, so all weight saved in the UTA is a bonus.

I am using a revelation hybrid rack/crayford focuser as it has a nice low profile with almost zero projection into the uta when fully racked in. Allows me to use a 69mm flat too.

[Moonshane]

cheers Alan

I have a pillar drill so may buy a couple of offcuts and try them. I fancy making a curved vane spider sometime for my 6" f11 newt (and maybe my 12" f4 too if it works out)..

your scope is gonna be a beauty.

[astronymonkey]

Nice on Alan,

Very nice work, its going to look fantastic when done. I always appreciate the simplicity if wood when making a scope as its natural , easy to work with, cheap, and yet can make a fantastic structure for a killer scope.

I love that Sumerian look http://www.sumerianoptics.com/products/canopus/ very simple and understated..... form following function with low tech , tried and tested engineering

Ive only ever made one scope (but bought the mirror) and that was using an offcut of air conditioning ducting for the tube and some old polypipe for the focuser tube .... even today it serves as an excellent grab and go scope ,even though I have a couple of other scopes to use.

Looking very very good......Im dead jealous

PS what did you use for the connectors at the end of the vanes , and where did you get them from ?

cheers

[swamp thing]

Very nice Alan. Did think of using ply for the light baffle myself, only ruled it out because of weight. Yours looks really good.

Look forward to seeing it in the flesh, and maybe a little peek through the eyepiece too.........pretty please

[Rustysplit]

Thank you Astromonkey.

The connectors are simply stainless steel m5 stud connectors. I have cut a slot in them and drilled through for a fixing point to the vanes.

They came from http://stores.ebay.co.uk/kaysfasteners , as did all my stainless fittings. Highly recommended.

[Rustysplit]

Hi Steve,

I could hardly not let you have a peek, now could I? It was your dumbbell that got me into this mess

I think the trade off for weight for me, was being able to fix focuser straight to it, albeit on a double thickness section.

[swamp thing]

Yeah I see your UTA is also much shallower than mine, this would help keep weight down too.

[Tinker1947]

I like doing woodwork, i like your woodwork very much, lovely job......

[trev27]

I love that UTA and spider. Wish I'd seen this when building mine. Very jealous of your workmanship. Beautiful

[Rustysplit]

Little bit of work of the mirror cell and tailboard today.

By having nice wide ends to the tailboard, I have been able to use 3 springs per collimation adjuster. It is nice and solid. Having the cell attached to a 2nd board, which will in due course be bonded to the back board, also really keeps the cell stiff and heavy. I am wary of ending up with too little weight and the mirror end, and being top heavy. I can shave weight off the mirror box, but not the UTA.

[kirkster501]

Great stuff. You are inspiring me ! What about the mirror itself? Man, a 20" is going to take some work..... Is it doable for a newbie to grind, polish and figure such a beast?

[Rustysplit]

Glad you are enjoying it so far and getting some ideas. As for the mirror, see here http://stargazerslou...e-mirror-grind

Going ok at the moment. Just beginning to get somewhere with the polishing out. 14" though, not 20"

[Astrobits]

looking very nice indeed. I really like the secondary arrangement. you might want to screw an ali plate to the face of that to allow better adhesion of the silicone? I'd be nervous about silicone and wood but this might be unfounded.

Quite right to be nervous. Silicone does not stick to MDF very well so probably wood is just as poor. HOWEVER... just smear some epoxy over the wood where the silicone will go, rough it up a bit and the silicone sticks like crazy. I do it this way and when there was an unfortunate accident ( the shippers thought the label FRAGILE really meant MISSILE ) the MDF failed not the adhesives.

Drilling thin metal sheets, just clamp them between a couple of blocks of wood.

Nigel

[Astrobits]

Always put a spacer between the two bits you are going to silicone together if you want to separate them someday. 3mm spacers will do, enough to get a saw or knife blade in.

Nigel

[swamp thing]

Quite right to be nervous. Silicone does not stick to MDF very well so probably wood is just as poor.

Silicone is the normal method of sticking ceramic tiles onto plywood. I've seen tilers do this many times, and TBH it sticks them extremely well. Can't see why a mirror would be any different.

[Astrobits]

Sticking ceramic tiles onto plywood you can cover a large area with the silicone and the tile is relatively light so the bond is adequate. You cannot use large areas of silicone when sticking a telescope mirror or you get distortion, it has happened in my experience. Many years ago and despite my instructions to use three, and only three, blobs of silicone, one of my customers used 4 blobs and squashed them so they covered a large proportion of the mirror and then complained that the mirror was distorted. After I had remounted the mirror with three blobs, 1" dia and 1/8" thick the customer was satisfied that the mirror was perfect!

Before I started using silicone I tried to stick an 8" dia 19mm thick piece of glass ( mirror blank ) to MDF as an experiment using three blobs of silicone about 1" dia. After it was fully cured I could prise the mirror off fairly easily and the bond between the silicone and MDF was the weakest link. When I epoxied the MDF first ( just where the silicone would be) the mirror could not be removed from the MDF by prising but had to be cut off. This is why you need to put a spacer between the mirror and the backing so that you can get a saw or knife blade in to cut the silicone. My current 16" mirror is held in place with just three blobs of silicone about 1 1/2" dia. and it has travelled a lot without moving at all.

For the sake of security of your mirror a thin smeer of epoxy is not much to ask.

Nigel

[swamp thing]

Sorry Nigel my mistake I thought the secondary holder was plywood, I've just re looked and you are right where the mirror is attached its MDF. Gonna say on ply, it actually rips the plys out when you eventually prize it off. Extremely strong bond. Dunno about MDF don't use that anywhere near wet areas for obvious reasons.

[Rustysplit]

the holder is birch ply.I have epoxied it now,so hopefully fine..

[Moonshane]

this is slightly off topic but not really. as there seem to be some experts on solicone here, I wonder if someone could confirm any issues using e.g. black silicone as opposed to clear silicone? I recently used some high modulus black silicone (suitable for fish tanks per the blurb) to fix my secondary mirror and someone recently told me that OOUK had said that you should never use coloured silicone only clear as the colours added reduce life. anyone know if this is accurate?

[Peter Drew]

I've used black silicone only for eliminating stray light from joints and baffles so can't confirm suitability for securing secondaries. As far as I know, Dow Corning is the only manfacturer of silicone that advertises a 25 year guarantee.

[Astrobits]

I have always used the clear silicone particularly looking for those that claim to be suitable for greenhouse applications ( i.e. good stick to glass ) so don't know about the coloured variety. I would assume that the environment of the silicone was the determining factor as to longevity and in a telescope it will not get very hot or see much UV which are the main causes of degradation of materials.

Regarding MDF and water I have used MDF for many years without problems for mirror cells as they shouldn't get wet or stand in wet areas. The main problem as I see it is the base boards of the Dob mount which could be placed onto wet grass. If you want to make MDF almost waterproof then after cutting your profile soak the board in PU varnish diluted 1:1 with suitable thinners ( Turps ) in a tray for 24 hours. Cover the tray so the varnish doesn't dry out and make sure that the varnish mixture covers the board. Remove from tray and allow a few days to fully dry and cure before using. This is a bit expensive as it uses a lot of PU varnish.

If water at the mirror cell is going to be a problem then ordinary ply will suffer as well unless it is specifically made with waterproof adhesive ( interior ply is normally made with an adhesive that is water sensitive ) which is recommended for exterior use.

Nigel

[Rustysplit]

Here is the progress so far..

All the main woodwork is done. The formica is on, along with the teflon pads. I have also had the ali trusses trial fitted, but no pic of that unfortunately.

I have paired all the diameters down to their minimum so that I can get as small a footprint as possible. It should sit in the boot nicely, with a bag along the bag for the truss tubes.

I am going to make a set of setting circles for the azimuth rings, and probably fit a wixy for the altitude setting. I know it is sort of cheating, but having seen Si's, simonfromsussex, set up in action, the beautiful simplicity and accuracy is hard to resist.

[8kids]

That's a handsome looking pile of timber. I'm in the process of purchasing all the necessary bits for my own truss dob (i've bought a nice 12" mirror) where did you get your ali poles from and what gauge have you used.

Gary.