Equatorial platform build - notes & pics

[Jim-a]

I thought I'd start this thread to document the equatorial platform I've started building. Hopefully it will help anyone thinking about building one in the future - I'll try and note down any "wish I hadn't done that" moments / mistakes / successes etc as I go along. Feel free to chip in comments / questions or whatever.

After a lot of reading around I decided to build a platform with a vertical north sector and a circular southern sector. I seriously thought about going with a single south bearing, but I think that would have meant the platform would stick out from under the scope and I wasn't so keen on that. I also decided I wouldn't try to replace the azimuth part of the existing base with the top of the platform I'm making - I'm just going to put the whole scope on the platform - I reckon that will give me a bit of wiggle room, and will mean that the measurements don't need to be as precise.

I'm planning to drive it with a bearing on the north sector directly attached via gears to a motor. Other than that I've decided not to think too much about the drive until the main wooden bits are all constructed.

I played around with a lot of the spreadsheets out there (the yahoo eqplatform group is a good source) - but found that nothing really beat getting a big bit of paper out and drawing it out myself. One of the key measurements you need is the centre of gravity of your scope setup - theres a really good explanation of how to do that here http://www.equatorial-platforms-uk.co.uk/home/cofg-or-centre-of-mass-calc . I was really surprised that the c.o.g. for my lightbridge was only 37cm - but the base is pretty hefty.

The first thing I did was to cut a custom 'set square' type thing out of MDF with my 51.5 degree latitude - really glad I did, lots of the angles you end up cutting can be checked against it.

My various bits of paper and scribbled maths cam out with a south segment radius of 12cm - so I glued a couple of bits of 12mm exterior ply together and cut a rough version with a jigsaw then finished it off with a disk sander attachment

I cut supports for the segment using the set-square and screwed them on (eventually I will glue and screw everything solidly, but for now I'm just using a few screws so that everything can be undone and re-done when I get things wrong)

With the south segment attached to the top board of the platform I attached two pieces of glued together ply which will become the north segments. I'll cut the board to shape later - but for now I'm leaving it as is so that I still have a chance to move things around if need be

To shape the north segments I made a jig to rotate the platform around its axis - again the set-square came in handy for making sure the angle was right. In the picture you can see a piece of wood attached to the south segment with a hinge.

With a second piece of wood and hinge connected to the other end of the platform I screwed it to the shed door making sure that it was properly vertical

To get a rough idea of the outline of the segments I used a piece of wood clamped to a workbench at the correct angle, with a hole drilled through it to take a pen. Then by rotating the platform and jig about the door frame I could trace a line where the cut would eventually be.

After unscrewing the segments, cutting them roughly with a jigsaw and screwing them on again I set up the drill with the sanding disk at the right angle for finishing the segments. Lots of spare bits of wood and all the clamps I could lay my hands on seemed to do the trick.

Then it was a case of rotating the jig against the sander. It took quite a long time to take off all the excess wood - but the finish looks really smooth.

Thats as far as I got this weekend.

Any questions / comments / thoughts / tips / advice - all gratefully receieved.

[Moonshane]

Great start Jim

Very novel method for getting the VNS segments right - I never really tried to work mine out like that. I went with a Poncet based design in the end.

Good luck with the build and I'll watch with interest!

[Jim-a]

Thanks Moonshane - The thread on your platform build has been a real help in getting my head around the options. Hopefully I can pick your brain at some point if need be!

The jig method for making the VNS segments was lifted from this French site (diagrams helpful though) and this guy who's set-up puts my lashed together sanding jig to shame.

[Moonshane]

feel free Jim - anytime.

I am luck to have a router, although you do need to create a jig for that too (or a fixed arm).

[Jim-a]

The bearings arrived today from these guys. I went with 10x30x9's - no science to the choice really, but with a pack of 10 for around £6 I have enough for spares (I may double then up on the north bearing - havent decided yet).

This evening I started the base board fixings for the south segment...

my plan is to get that done then fit the north bearings and then get the scope on it (first real moment of truth!!!) and check it out for smoothness / balance etc before diving into the motor stuff - that way if it doesn't work then I still have the option to move things around, tweak the design etc.

My original plan was to direct drive the north segment, but I'm struggling to find a motor + gear arrangement that would move slowly enough - if anyone has any suggestions then that would be great... otherwise I may just have to revert to a threaded rod type arrangement, but then will have to think more carefully about how to reset it at the end of its run.

[Moonshane]

I think some people use the small EQM motors that can be found sometimes on astroboot. there's one there now for about £15 - maybe just over half way down. not sure if they have the ooomph though.

[Jim-a]

Good idea Shane - I have an eq1 drive that came with a scope I borrowed from a friend before I bought the lightbridge. Hadn't thought to experiment with that - at least I can try before I buy, though like you say, I suspect that the scope would have to be really well balanced for it to have enough oomph - but watch this space

[Moonshane]

actually I am sure others have used that exact one. see how you go.

[Jim-a]

Made reasonable progress yesterday.

I cut some stainless steel strips from an old cooker hood we had lying around and epoxied them to the north and south segments. Have to say my cutting is pretty rubbish, but once the epoxy is fully cured then I'll take a file to the edges and make sure they are smoothed out a bit so they don't catch on anything.

To drive the platform i'm going to use an eq1 motor drive. I figured that a 10mm drive shaft would be about right and seeing as the bearings I bought have a 10mm hole in them I can use them to support the shaft. The bearings sit in two notched pieces of wood and are held securely by some brackets I made from a spare stainless steel strip (they don't look great - see my comment early about my lack of metalworking ability - but they seem to do the job). The shaft is a 10mm bolt. The connection for the motor (which has a 6mm connector) was made by filing down the head of a 6mm bolt so that it would fit snugly inside a 10mm threaded connector and then filling the gap with epoxy and hoping that it would set straight (which it did near enough). Thats then just screwed onto the 10mm shaft.

I'll need to hacksaw the thread off and then file a flat section for the connector screw to engage with, but hopefully that will do me for a drive.

Mounting everything on the base board was pretty tricky. I did the south section first then balanced the top board on the south rollers with the north end propped up on some bits of wood so I could mark out where the north bearing and drive shaft should go. I think if I was doing it again I would first screw the top and bottom boards together with some pieces of wood to separate them so that they were exactly parallel. That way I'd have been able to fit all of the rollers so that contacted the segments much more easily.

So... the first moment of truth... I can move the platform around and it stays in contact with all of the bearings and the movement feels smooth and 'equatorial' (ie it moves roughly how I imagined it would). I don't think I have all the bearings exactly square on though - some of them are contacting on the edge, but have a fraction of a mm or so clearance on the other edge. I'm not sure how much this will matter - I guess it may cause there to be more wear over time? I'm going to have a fiddle to see if I can improve things.

Using the spirit level app on my phone I measured the amount of 'safe' movement as 11.5 degrees... so this should give me 40 mins or so of continuous viewing between resets - which I'd be really happy with.

So next steps will be to see if I can improve the bearing contact and then put the scope on it to check it takes the weight / doesn't fall over etc - after that I'll put some end stops on to limit the movement to the 'safe' zone and fit the motor.

[Mike73]

Enjoying seing the build progress Jim, looks very similar to my EQ platform.

[Jim-a]

I had a tweak of the bearings today and got a much better fit, then while I was on a roll I fitted the adjustable feet and the eq1 motor. Eventually the motor will need hacking about a bit to put a remote on/off switch and speed control on the end of a long cable - but for testing purposes I'm just going to have to get on my hands and knees and fiddle with the controls under the platform.

With the motor mounted, I really noticed how much the platform would just slide over the drive shaft with the slightest pressure. I think the solution will be to put some heat-shrink rubber tubing over the shaft.... however, because I'm impatient I racked my brains for a quick hack and decided to wrap a rubber band around the shaft and attach it with a cable tie at each end. Seems to work pretty well - the platform doesn't slip at all.

Seeing that the moon is up I thought it would be a good idea to give it a spin while it was still reasonably light, so I levelled it - pointed it in a vaguely north direction and popped the scope on top.

It adds quite a bit of height to the scope - at zenith the eyepiece is exactly at my eye level - any higher and it would be a tip-toes job!! I think if I were building one again I'd put far more effort into making it low profile.

So the moment of truth...lined up on a moon crater with a 9mm eyepiece (so 168x mag)... I switched the motor on.... peered through the eyepiece... and.... the crater gently drifted out of view... but ... in the opposite direction from normal !!!!!!! - so I tweaked the motor speed and after a few visits to the bottom of the platform had things pretty steady. Went into the house for 10 minutes and the crater was still in the field of view.

Result - I'm very chuffed!!

There are still a few things to do - I noticed that the scope is much more wobbly on the platform than it normally is - even a breath of breeze was causing seasick-inducing views and the settle down time after moving the scope is really long (to the point where it would be pretty annoying when trying to find things). I suspect that the ply is moving around too much. Having only used 12mm ply, I'd always planned to add some bracing once the basic build was done, so hopefully that will help. I also need to give it a proper long test, put a bubble level and maybe a compass on it, build the remote cable, and put in a motor cut out for when it reaches the end of its travel... but for now I'm pretty pleased.

[earth titan]

That's a mighty fine looking scope on top of the platform ;-)

Good job!

Typed by me, using fumms...

[Jim-a]

Tom.. surely you mean thats a mighty fine looking platform under your old scope ;-) Hows the 16" doing?

[Moonshane]

looks good Jim

you could make an equatorial platform out of very thin ply as long as your scopes' feet sit on the three bearing points that is. if they do then there's no flex at all. I found the 'wobble' annoying too and this is one thing I like more about the threaded rod design as it seems to provide a little more stability. ideally if you make the bearing spacings low and wide stability must improve I expect. as with most projects, this will be a trial and error and you have done well to get the thing working so well and so quickly! I made mine initially quite square but you can make it triangular in shape with no detriment

[Jim-a]

So I've used it a bit more this evening. The motor stopped working half way through, but I'm hoping thats just the battery wearing down (I have no idea how long it had been in there).

The wobble is the biggest thing that needs sorting out. It's so bad that you start nudging the dob, you think its actually moved on its alt/az bearings, but when you let it go it just springs back - so it must be easier to deform the platform than overcome the stiction on the dob itself. I'd maybe expect a bit of play in the arc that the platform is meant to move in, but this seems to be in every direction.

Let operation 'eliminate wobble' commence

[Jim-a]

Unfortunately the motor stopping last night wasn't because of a low battery. It looks like its fried (the electronics seem ok - but the motor itself is kaput).

I'm wondering if it was ok at first as the initial part of the motion would be where the weight of the scope is helping - perhaps it died when it started having to push the scope uphill and it couldn't provide enough oomph. (

Not really sure what to do next - I could buy another similar eq drive motor, but it may end up going the same way, so I'd have to get to the bottom of 'why' it didn't work. I've looked and I cant find a replacement part for just the motor bit anywhere.

So right now I think my options are....

- Buy another EQ style motor and reduce the size of the drive shaft to try and reduce the amount of torque required - I know that these drive units are used on other platforms, so they must work ok. If someone has one I'd be grateful if you could measure the drive shaft diameter for me.

- Buy a low rpm motor + a further gear reducer (a worm drive or similar) to get something beefier that would be slow enough to direct drive

- Abandon the direct drive and go for a threaded rod type arrangement - although with this I'm not sure what the best way to reset it at the end of the run is?

I may load the scope up on the platform again and attempt to measure how much force is required to move the scope (I'm thinking of some heath-robinson style contraption involving ropes and a pair of kitchen scales), so that I can get an idea of how much of a problem this is.

I knew it had been going too well so far!!

[Moonshane]

I have the threaded rod type and like it, when running with a 4rpm DC motor and a PWM controller it works really well. at the end I just manually turn the threaded rod but would ideally recommend a small rotating handle to allow this. it takes me less than 30 seconds to do it my way.

[Jim-a]

Shane - do you have some kind of mechanism to decouple the shaft from the motor when you manually rewind?

[Moonshane]

hi Jim - see pics from thread

I used an M10 bolt with a central hole and an M4 grub screw to tighten and loosen on the flat of the motor shaft. luckily I had a mate who did this for me. this allows me to decouple the (stationery) motor and then manually wind back. also pics showing the overall set-up so you can see how things fit together. hope it helps a bit.

[Jim-a]

I've weighed up the options - direct drive vs threaded rod drive as in Shane's pics above and I've decided to go with the threaded rod.

Whilst I started wanting to build a direct drive, it just seems really difficult to find a motor that goes slowly enough (other than the EQ1 type drive, which I'm worried doesn't have enough power). I could buy a motor and then buy gears to reduce it down further, but decent worm gear type arrangements are quite expensive and would need pretty accurate metalwork to mount them on - so I've concluded its too much of a gamble when the rod drive is a relatively cheap, tried and tested mechanism.

I've ordered this motor and this PWM controller which should do the trick. It will be a while before the snail mail from hong kong arrives, so in the meantime I'm going to brace the platform to reduce the wobble, and I'm going to rework it to lower the overall height as I think that will make observing a bit more comfortable.

[Moonshane]

I think that's a good decision Jim. if you need more detailed pics of my set up then just ask.