Home made horseshoe mount and 12" Newt

[skybadger]

Hi all

I have been building a horseshoe mount for the last 18 months or so and finally getting to a place where its worth exposing to the forum.

The design is a basic A-frame of steel on which sits a polar axle composed of a wheel hub at one end and a pair of skateboard wheels running on a 36" diameter horseshoe at the other.

The dec pivots are pillow block bearings running 1.25" shafts into bearing hubs mounted on the mirror carrier box

The 'tube' is 4 pairs of trusses with ball ends either end and the secondary ring is a sheet ring of ply with focuser and spider mounted off it. I made the secondary spider from packaging steel strap and sealing wax.

The mirror is a 12" f/6 purchased from the Worthing Astro soc after re-grinding to 1/10th wave and the secondary is an Antares from the US.

After initial problems with balance (there is no friction) reqruiring counterweighting, I am now at final assembly prior to shakedown and first light.

I'll put some pictures up here as I take them but there's also a fuller write-up at http://www.skybadger...inchscope.shtml with some existing pictures.

I have some questions that need input form the forum though...

while not motorised at the outset, I have a set of Skysensor motors I plan to use.

Would it be better to directly drive the RA wheel, drive the support wheels or install a worm wheel at the polar lower bearing instead ?

What is the best way of aligning the hubs between the sides of the mirror carrier - straight bar between them ?

I'm also looking for a simple way of aligning the pillow blocks on the horseshoe. When the thing comes apart it's by lifting the entire tube off in one piece having removed the pillow block bolts. So I was thinking of using some sort of registration and alignment block to adjust and hold the pillow block bases to the correct alignment when replacing them. Sounds like a door or levelling support or something to me but I'm not finding much more than the Axminister levellling screw which is a bit higher than the 1/2" of pillow block I have to work with ...

Getting close now, so I hope first light is soon ....!

Cheers

Mike

[JamesF]

Can't really help with the horseshoe I'm afraid, Mike -- it could easily be the first home-built one I've seen. Looks good so far though. Is the alt setting pretty much fixed for your latitude?

Interesting details in the construction of the dob, too, such as putting the spider mountings on top of the upper ring of the secondary cage. Have you posted any details on the construction anywhere? I'm always interested in plagiarising other people's good ideas for my own build

James

[Peter Drew]

Nice job Mike, the horseshoe mount is a good design and should be attempted more often. I would drive one of the support rollers as the weight of the scope would keep good contact, this assumes your disc is smooth and reasonably accurate. It would be a good plan to pass a shaft right through the horseshoe and the telescope to initially line up the DEC bearings. Once lined up you could fix a piece of wood or metal in the shape of a set square to locate the bearings when refitted. Do you have room for a largish disc between the scope and one of the inside faces of the horseshoe, this could act as a friction disc to help stabilise the DEC axis movement, avoiding the effect common on Dobs with too small a trunnion disc.

[skybadger]

Hi Peter, James,

The details will be forthcoming here but the spider is above the ring because the size of the focuser pushes the secondary up the 'tube'. It could have gone inside the secondary cage but that would make entire cage deeper to compensate.

I cut the ring with the router on a circular arm so I think that the surface is pretty flat and true. The edge was then laid with contact adhesive and worktop edging pulled hard round using bungee straps while they set.

The front board carrying the wheels is set on a pair of pillars, one at end of the wide end of the the 'A'. The pillars have a pair of studs in each. The board is carried on angle brackets with a 50 degree angled post with slotted internal edge for raising up and down. I can vary it by about 2" , which on a 24" radius is 1/12 rad ~ 5 degrees. The A-frame is also fitted with adjustable levelling feet to allow levelling.

I do need a fine adjuster on that somehow, and to be able to fasten it tighter because it slowly slips otherwise - there's a lot of weight on it..

finally, Thanks for the idea Pete. I have put it into practice with the dec dial from teh donor scope. ITs 4mm acrylic so i think its a bit flimsy for a side friction pad but maybe a strap might do. I thought of puttng it on a friction clutch and locking the edge but there wasn;t enough depth between box and horseshoe to fit that...

I'll get some pictures of these things to show what I mean.

Right now its sitting in the garage, secondary in place, collimated and waitiing for clear skies, if I get the friction issue fixed.

Cheers

Mike

[skybadger]

Some more pictures

The basic A-frame in mild steel 3x2" section with 1" angle used for the wheel seat and roller board frame.

The polar bearing is the Sierra wheel hub, sitting on shaft mounts from ebay used for 3-D routers, allows the polar bearing to be adjusted to any angle - even alt-az!

Side view is here.

.

The adjustable foot is from Beacon Hill - Barry was quite helpful and still has a stock left. The wheels are skateboard wheels and don;t run smooth unles the board is flat against them. The board rises up and down about 2" on a slot.

[Peter Drew]

Can I suggest that you screw a piece of your 1" angle to the underside of the board supporting the skateboard wheels, the board seems to be chipboard which has a tendency to warp over time.

[swamp thing]

Great project Mike. I shall watch this with interest.

[skybadger]

Can I suggest that you screw a piece of your 1" angle to the underside of the board supporting the skateboard wheels, the board seems to be chipboard which has a tendency to warp over time.

I'll take that advce Peter. The chipboard is IKEA deskop about 1.5" thick which I thought was strong enough but the weight on it is substantially greater than initially expected so I'll add some angle stiffeners.

The wood itself is sealed using varnish thinned down by about 15% with white spirit to get the penetration.

Mike

[ponytale]

thought you might like to see my horshoe mount prototype, http://youtu.be/KCp-uk46v_Q

[skybadger]

That seems to work quite nicely.

A few questions please..

Do you need the big DEC disks for friction to hold it in place or is there a big lump of counterweight behind the mirror ?

How did you determine how close to the edge of the large horseshoe disk you could go without compromise to disk strength through splay or fracture ? I have been cautious as to the width as well. Its a 36" disk with a 16" slot. The intention is to mount a 16" in this so the 12" is to find out where the flexibility and balance issues arise.

I could go wider for larger and heavier mirrors but have a back distance of 11" only.

Is there something which holds the sides of the horseshoe a constant distance apart - if you turned the horseshoe on its side would the slot slump ?

Was the bolt in the band under the horseshoe a safety feature ?- I thought about using an endless band as a high friction drive method using the entire edge area between the rollers but couldn't think of a way to prevent a bonding bump at the band ends which would ruin pointing as it went over the rollers.

I also started off with a large deep cage like yours with the intenion of the spider being inside but ended up with the spider being too deep, a spider that is not carefully made to be shallow will need a deep cage. In the end I made a single ring and mounted the spider off the top. What did you find ? the pictuire below shows the end result. The blocks are a bit ugly I know.

[ponytale]

Hi Sky badger: in answersto your queries : The aluminium Band is fixed with countersunk screws which are filled with epoxy putty and filed smooth, they are placed so as they miss the rollers in the most used zones such as perpendicular and about 30 degrees apart as the scope barely moves over 5 minutes whilst manually tracking i have not found it a problem .

the blue belt is the best way i could convert the rotation into a linear movement as i was intending to use threaded rod linked to the belt to the drive motor , i may just drive one of the rollers instead ? .

the front horseshoe is 2 x12mm ply sheets screwed and glued together giving a very strong 24 mm thick piece which with the Aluminium band added is very rigid,

i also have the same size and thickness ply on the rear horseshoe that runs against a roller on the face of the rear panel which keeps the cradle very square on the front rollers ( Three points of contact, the rear bearing pin is only keeping it central to the axis and is slightly adjustable left and right to get a true centre point so the front rollers run true and square .

regarding the side panels slumping i haven't had that problem so far i do have some play between the trunnion bearing face and the side panels i have stuck some carpet on the bearing plates which has taken up the play and adds a bit of drag /friction which helps with the steadyness /balance . in the video you can see holes in the cradle side panels this is where i put bolts through to hold the scope square while i marked out the bearing support points these to my mind must always be on the axis line to keep the eyepiece in as true an arc as possible when rotating the cradle and keep tracking errors to a minimum . when i remake in alloy i would have very small tolerances inbetween the trunnions and the cradle side panels with bolts throught into the ota so i could adjust friction and prevent sideways shift when the scope is leaning over at steeper angles.

i do have a large weight on the back of the Mirror box which helps a lot to keep it steady ,

the secondary mirror cage is a bit too heavy really but it is sturdy and in my case holding my 4 strut tubes square, as this is fundamental to colimation i am loathe to lighten it at this stage ,

i have messed around with springs and counterbalance systems but they add other problems, i am still trying to perfect counter balance issues by moving the trunnions in small increments on the mirror box as well as the bearing blocks on the inside of the cradle, it is a juggle to get balance and clearance correct.

My main intention was once i have the measurements right and the balace sorted to remake the cradle and frame in aluminium plate and tube, then cut out a lot of unnecessary material in an aesthetically pleasing style .

my base frame has pivot points where the front panel joins the lower frame so i can fold it flat for transport, and in aluminium i think i can make the cradle part foldable / dismantleable also using simple threaded holes and knurled bolts . hope this is helpful. regards Tony

[skybadger]

Hi tony

its all helpful - there aren't too many of these around, but there is a beautiful one made from ply and turned metal. I'll try to dig out the url.

The carpet idea for DEC friction is a good idea.

The question about the band was the drive band around the the rollers - they seemed to have a large lump in them where they join. For a full goto you would have to remove this lump ?

I have a plan to remount he skateboard wheels with driven axels - one will be diorectly motor driven and one will be an idler, connected to the driver by a flat band for greater torque and drive friction through the larger contact surface.

So far I have some square section tube, the bearings and and end plate. I need to machine the tube to accept the bearings as a push fit, make a motor mount to fit the end and gear a SS2K motor to the final gearing before driving the end of the axle.

In the meantime I am still faffing with this friction device. I should just knock somehting up to be able to use the scope in angr for a while.

Cheers

Mike

[ponytale]

Hi Mike , The Blue Belt was just some tie down i had to hand to test if the idea would convert the motion to linear over enough distance /time to use a threaded rod to push pull the belt in a straight /linear motion and still allow a reset by slipping the cradle on the belt , i have given up on this idea as there is likely to be too much backlash . as i intend to do short lunar and solar photography eventually, a few minutes of tracking is enough,

regards driving the roller , i have read quite a few reports of slippage at the roller / contact point , so i am probably not going down that route ?.

one idea i am going to try is a backlash free cogged belt with a stepper motor and matching pulley possibly off the back bearing /axis there is plenty of room for a large drive pully wheel

and a small motor to get a very large ratio, i will have to work out the gearing to achieve the right speeds, but around 4rpm i think for smoothness ?

I have seen a few variations on horse she/split ring mounts on you tube and mostly American astro sites , Here in Australia we have no pole star for easy alignment .

so i have a brick paved compass in my back yard, it is aligned to true south so i can basically plonk it down, level, and use . i will update as i go along

cheers Tony

[skybadger]

Right - first light report

This telescope being optically the Worthing 12" newtonian should have nothing to prove from a point of view of optics quality - I have a mirror report at lambda/10 from the polishers in the draw. So first light was about getting the mount outside, levelled and collimated and really about how it all behaves under the stars.

The positive points were..

It came out of the garage like a wheel barrow on its front wheel really tidily, onto the drive and levelled in seconds.

While i waited for the sky to darken and the optics to cool I fussed with collimation and getting the new friction clamp setup. The friction clamp is described in a later post.

Once dark, around 11pm, i roughly aligned the polar axis with polaris and set about trying to observe.

Firstly the focus point is about 2.5" outside of the focuser, so I got it about spot on for imaging but it makes life a bit unbalanced for visual use. I plonked a Baader 2" adapter in to bring out the eyepiece mount of the focuser and with a 40mm eyepiece got a lovely x45 view with a sparkly milky way field

The first object was M13, it was there, it was easily hoppable but it still took a fair length of time to get used to moving the scope. When finally found it was big and bright, a spangly octopus that made the effort of building the scope a worthwhile exercise. Clearly a brighter, larger and better resolved than my 8" or 5" refractor. Loved it.

On then to M57 and an exercise star-hopping frustration. Back in to get Nortons and work out the star hopping route, taking in Epsilon as a quick double splitting exercise and on to the ring. Nice, clear, green and still no sign of the central star.

After that on to M56 as a condensed globular with some stars resolved, M71 with clear stars and dispersed structure, M27 as a big splodge of green extending farther than ever before seen, off to M15, M2 (not found) and then M32 as a closer before realising it was now 1.30 and the sky was getting lighter again.

The telescope behaved pretty well all things told. It wasn't collimated and it showed but still split narrow doubles and gave very nice, very narrow diffraction spikes from the secondary vanes. it did 'cog' on the RA circle though. Partly this is because of the large weight on the wheel bearings, other influences may be a flat spot on the wheel or a flat spot on the horseshoe ring. I think its the first due to repeating on full circles of the roller wheel but there may be a bit of the others too. The other roller seems less subject to this problem.

The cogging meant that slewing wasn't as smooth as it should be and the telescope settled back if I didn't keep hold.

However the friction clutch on the dec axis controlled the slight imbalance and otherwise lack of friction on the dec axis which was causing it to tip all the time.

The last problem was the lack of a finder scope. I'm out of practice with a newt so pointing by eye was quite hard without kneeling at the base and sighting along the tube. The floor was gravel so that was quite painful...

I tried a reflex finder but fitting it in the dark with a clamp was a faff so I dropped that idea when it dropped off.

After a bright perseid and the surprise of a bright satellite crossing M13 while observing,it was a great night and one to be repeated.

The only downside of this scope in its new mounting I have found so far is that observing between the zenith and the co-latitude near the meridian is hard since the horseshoe ring is directly between the observer and the tube. It was possible to observer but was much more comfortable in use away from the meridian and away from that zone of declination for me between 50-30 degrees.

I packed up at 1.30 by lifting up the handles and trundling it back in to the garage. All done.

Great stuff.

Mike

[JamesF]

Excellent to hear that it's now in use Mike. Hopefully you can get the niggles sorted.

James

[skybadger]

Thought to add some pictures.

The telescope fully assembled in the drive

the telescope friction clutch - the plate is on the dec axis and presses a friction pad against a DEC disk mounted on the axle. Two pins prevent rotation of the friction pad. Two other hand screws control the friction. In this picture you also see one of the two blocks I put on, one each under the lower edge of the dec pillow blocks to allow fine bearing levelling under heavy load using a hex head screw.

finally a view down the throat

Anyone recommend a (very) lightweight finder ?

Cheers

Mike

[JamesF]

Anyone recommend a (very) lightweight finder ?

An optical finder? I made/bodged a right-angle finder using a 50mm binocular objective. It has a plastic diagonal and accepts standard 1.25" eyepieces. Without the eyepiece but including the mount it comes in at a bicep-busting 261 grammes.

James

[JamesF]

Just checked a straight through 30mm finder I had lying about and that's 214 grammes including the mount. Not the nicest to use though.

James

[skybadger]

Tony, last question - do you use skateboard wheels as rollers, where they new and do they have any hint of cogging ?

One of mine at least is bad like that, even with new bearings inside so I think it might be a consequence of the small bearings, few balls or a flat spot on the wheel.

cheers

Mike

[ponytale]

Hi mike , my skate wheels are the black rubber type which are quite hard , they are nice and round and don't cog or flatspot ( i did read a comment when researching

my project that the weight should be raised off the wheels when not in use to prevent flatspots) , i do have a 2.5 mm thick aluminium edge on my scope which is counter sunk screwed in place .

for a smooth movement . I am working on the idea of using 2 halves of a 26" alloy wheel rim off a bycycle as a running face/edge , they are very strong and true and have machined faces and would support a lot of weight with only a minimum of added structure/frame work between them to support the scope , i could even incorporate a friction brake on the rim face as per caliper brake with adjusting screw. i envisage the rim edges running in a grooved skate wheel ( machined to suit) or teflon glide arrangement to create drag . it is in the drawing stage at the moment , i will post updates as i go along , great report from you by the way i am sure trial and error will fix your minor issues . tony

[skybadger]

Hi Tony, all

a few months further on and I still haven't got to the bottom of my cogging issue on the rollers. I fixed the dec friction thing with a friction plate on the axle - to a degree. But the cogging of the rollers is still there - I am 90% satisfied its the rollers and not the horseshoe since I can rotate the roller by hand and the cogging moves to a different point that repeats with roller circumference. I guess I need new rollers or larger bearings with more balls inside them ..

I have been reviewing Highe's Portable dobsonian telescope book and there is some good information and thought provoking stuff in there. Like different approaches to the upper ring which result in a lighter assembly, mostly due to shortening the trusses. Unfortunately the telescope hasn't seen much use due to a rubbish summer, working the Olympics and then family holidays but it should come out more with the longer nights.

cheers

Mike

[LaH]

Hello Skybadger,

I see that we both did Split Ring in this same time, but when I searched inspiration, I miss you.

But, I designed and created only Split Ring without scope (I use LightBridge 12'')

Is your project finished and  you use it in standard operation?

How did you solve eyepiece position?

And there is my Split ring (from one of first missions, not all was done yet):

Cheers

LaH

[Ajohn]

This one is a bit "big" but the site may have some useful ideas. In French so needs a  browser that can translate

http://www.astrosurf.com/durey/telescope.html

There may be others about via links from this site

http://serge.bertorello.free.fr/telescopes.html

Some of the mounts shown there make me wonder about thin mirrors and maybe putting a counterweight on the end of the scope so it can be fork mounted instead of having the usual wobbly thing on the end of a projecting shaft and the scope hanging on the opposite side. Just the right thing to help it all shake about.

There is also a construction articles on a REALLY big dob.

John

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