Equatorial Platform for Dobsonian Telescopes

[SniffTheGlove]

Photo's look good and the ball joint looks really simple.

Question Shane, from the last set of photo's what does the north sector rest on. In the photo's on the first page you have bearings and the sector rides on these but in the last lot of photo's these have disappeared and the sector just seems to be resting on the sofa cups from behind. Have I missed something here?

[Moonshane]

hi Sean, no you are quite right. the sofa cups are temporary but allow me to establish the range of movement (it's effectively an arc with a radius of about 16").

The force is directed at 90 degrees to the face of the wedges on the ground board so there's no need for 'vertical' support. the steel I have bought will be on the back face of the sector to roll against the ball transfer units when they replace the sofa cups.

Assuming this works, it's a really simple solution - Nils Olson Carlin uses this method and assures me it does (although it's only suited for latitudes of 45 degrees or more - for less northerly / southerly latitudes you need some 'vertical' support due to the very vertical faces of the ground board wedges).

[Moonshane]

I think I have finally sussed my electronics with the help of my SGL colleagues!

I have ordered one of the se 12V 15A DC Motor Speed Control PWM HHO RC Controller on eBay (end time 26-Feb-11 23:58:41 GMT)

In theory this should 'pulse' at a higher rate than my dew controller which created heavy duty pules and vibrations when the motor speed / voltage was lowered.

Once this arrives (and my ball transfer units) and my machining work is done by a mate, I should then be able to complete the job. I am hoping this will all be next weekend or the one after. my plan is to be ready for SGL6 so people can see it in action.

[SniffTheGlove]

I think I have finally sussed my electronics with the help of my SGL colleagues!

I have ordered one of the se 12V 15A DC Motor Speed Control PWM HHO RC Controller on eBay (end time 26-Feb-11 23:58:41 GMT)

In theory this should 'pulse' at a higher rate than my dew controller which created heavy duty pules and vibrations when the motor speed / voltage was lowered.

Once this arrives (and my ball transfer units) and my machining work is done by a mate, I should then be able to complete the job. I am hoping this will all be next weekend or the one after. my plan is to be ready for SGL6 so people can see it in action.

Hope the new PWM works OK, not bad for $21.

Hope to see it at SGL6 as I only live 20miles away.

[SniffTheGlove]

Doing some calculations late last night and by coincidence the 50mm rubber caster I had planed as the segment roller for the motor has it's circumference at 157mm with is nearly spot on to the 159mm tracking distance (@15deg = 1hr).

So my will may castor be OK for the job only doing 1 revolution if I track for the complete hour. Will the shorter times create a flat dampened spot on the caster wheel?

I am now going to rethink to see it my stepper motor will be viable doing 0.016rpm (1 rev per hour), will that really slow speed create some vibration?

The other route to take is to put a rubber sleeve over the stepper motor's output shaft which will then allow the motor to rotate faster and maybe not produce vibration.

I am getting close with finalising my circuit design and my stepper Arduino code and I shall try both methods before settling one one way.

[Moonshane]

cheers Sean - ain't having a play great!?

[Moonshane]

today's update. trying to sort myself a small but achievable target each night as time is a bit limited in the week.

made a small bracket for the two electrical controls, the PWM controller dial and the forward / off/ reverse switch. this was just made from angle ali and then shaped a little on the grinder. I'll paint this black probably along with the rest of the EQP. my colour scheme will probably be all matt black to match my OOUK dob bases, but with a white edge to highlight where the platform is when used with my smaller scope.

BTW on reason I cut off the back corners is that I am slightly paranoid about treading on a corner in an observing session and flipping the scope off balance. I may even put some small LEDs on the bottom of the top board to outline the shape (for home use - will probably have to disconnect at star parties).

also permanently glued and screwed the two outer sections of the north base board wedges and added some threaded rod to one of my ball joints along with some two pack glue to provide a longer fitting to allow me to raise the platform a little if needed when I fit my front ball transfer bearings.

[Moonshane]

hi all. Last night I made some feet for the platform. As it needs to be level (and although my main observing site - garden - is pretty level and flat) I have built in some adjustment.

I had previously made some round approx 60mm pads (they could be square etc of course) and these were drilled out to accept an m8 stud (cut from a length of rod). These were then glued in overnight with a two pack epoxy resin. At the same time I used some rubber sheet to provide some damping and glued this with the left overs.

The adjustment is about an inch although I'd normally expect the ground to be fairly level without any adjustment.

Moving on to tonight I received my ball transfer units and these were a little bigger than expected (the plate anyway). Luckily it just fits onto the bearing wedge. I used a forstner bit to remove enough material to allow it to sit in the recess and then screwed them into place. I'll remove them when it comes to paint. They work well and are quite smooth. When I add the steel plates on the back of the north sector they should run very well indeed and with little resistance, thus straining the motor less.

Final job for tonight was the addition of some braces under the top board. These are possibly OTT but I though that as I will be using this platform for a 6" and a 12" scope it will have the main weight in different places. For the sake of 10 minutes work I would sooner have the extra strength. It will also help keep dew off some of the electrics.

I have realised that the quality of this plywood is not great and will be using far better when I build my next platform and my dob. It's been a great experience so far though and I calculated today that if I do another, I should be able to sort it completely in about 16-20 hours - so a long weekend.

Here's some pics of the latest work.

[SniffTheGlove]

, but with a white edge to highlight where the platform is when used with my smaller scope.

More like to highlight where it is before tripping over it.

The trouble with Plywood quality is trying to acquire a good local source. I looked high and low locally for some decent birch ply but no one does it. I could get it off the internet but the carrier charges were high (as you can expect for a few sheets of 4x2ft).

I have found that the plywood from Wickes I am using is OK, the only problem I have had was splintering when using my circler saw but this was OK after sanding and the middle ply of the 5ply is incredible tough. I tried using my circular cutters to cut some round feet and they just get stuck on the middle ply and I am having to use a hell of a lot of force pulling the pillar drill lever down, then I had to come at the ply from both sides.

Glad you ball units have come and are OK.

So nearly there then Shane. I am a bit behind with mine, getting caught up with my Arduino playing and other matters but feel I must get the stepper motor all working fine so I can measure where to place it on the bottom platform, then measure everything else from there to join the top and bottom together.

[Moonshane]

cheers Sean

I fitted some steel plates to the back of the north sector tonight. more on this tomorrow when I'll add pics but suffice to say that cutting 3mm stainless bar is easier said than done. I ended up gluing with epoxy to avoid having to drill the thing!

one mystery was solve when I fitted the ball units. if you look at the arc created on the back of the north sector by the grease on the ball bearings in the last pic posted, you see the effect of the rear pin/ball joint method rather than the two sector method. it effectively shunts the whole platform back and therefore the radius is much reduced. I now fully understand this thankfully.

[Moonshane]

I managed to take some quick pics so here's today's update. doing about an hour each evening after work.

using some 50mm x 3mm stainless steel flat bar I cut the required length and then worked out the best shape with a cardboard template of the cut piece. I then cut two pieces the same shape and it has worked out well, covering the whole travel of my 'balls'.

I used some two pack epoxy to glue them to the back of the sectors rather than using screws, partly as I didn't want to risk the screws getting in the way if I adjust the height at all and also as stainless is so tough.

my recommendation for anyone using steel in future for this sort of thing is don't use stainless. it's ridiculously hard to cut, especially in the dark. ended up grinding the corner cuts to shape them and it took a lot more effort than normal steel which I'd wager would be fine too.

also, here's a pic I'd forgotten about with the 12" dob temporarily perched on the platform. it just fits nicely.

[SniffTheGlove]

Looking good.

I see what you mean about the radius when using the ball transfer and ball joint, the grease shows the arc quite nicely and it is quite different to the original radius arc.

The Dob fits OK on the platform, your a lot closer than me finishing. I thought it would take a couple of day (12 hours) but this is way off espcially when you have to sit back and do the maths.

[Moonshane]

yeah, I agree mate. taking longer than I thought too. I reckon though having done it once and learned the theory / geometry / mechanics we could do the next one a lot faster. for me a job is always worth doing at a slower pace and get it right than a faster pace and get it wrong - that said, you learn both ways

the 12" is the biggest scope I have currently and really I designed this with the 6" in mind but thinking they'd both fit on and thankfully they do!

[Moonshane]

a bit more progress today (after a big shed tidy).

• added a few blocks to hold the battery.
  
• ground down the sharp corners on the bracket holding the ball joint to the frame (this would have been guaranteed to result in cut ankles otherwise)
  
• added a small piece as a 'trim' for the above
  
• drilled and set out the drive pin on the north sector.
  
• also cut some 'soft ground pads' out of scrap for use away from home. these will also fit in the main body.
  

Tomorrow will be mainly prepping and painting the thing and then rebuilding ready for the final bits which will hopefully arrive next week.

[JAO]

Shane

Your platform is looking really good and not far off completion. You must be chuffed with it

Adrian

[Moonshane]

cheers Adrian - I'll be chuffed when I test it and it works!

[SniffTheGlove]

Have you tried the motor yet with the platform and weight of the dob?

[Moonshane]

hi Sean

not yet but it seems quite a torquey motor and with the steel plates the platform glides with almost no effort.

[Moonshane]

Made some good progress today.

Anchor system

• I was a little concerned about the high centre of gravity (36") of my 6" f11 dob so I felt I had to create an anchor system for this scope. I thought about several ways to do this and in the end went for M6 cap head bolts and Tee nuts.
  
• I drilled some 10mm holes in the weights I have added at the feet (previously) for stability and this gives me a little play to help in location of the holes.
  
• Then I drilled 7mm holes for the tee nuts and added these to the underside.
  
• I had to offset the one at the south side due to the bracket I have used.
  
• This works well and it seems really secure, avoiding the need to remove the feet and use the EQP as a ground board.
  

Just in case blocks

• Being paranoid about my big clumsy feet treading on the front portion of the EQP in the dark, I thought it best to add something to prevent this 'tragedy'
  
• I cut some small blocks, just less than the height of the feet and this should prevent any mishaps in most cases
  

Paint finish

• Everything was rubbed down and prepped for paint and a flush of primer added as a guide coat.
  
• I'll fill it tonight before bed and then undercoat tomorrow, final coat Tuesday.
  
• As mentioned it will be clack with white edges and I may add a single red LED under the ground board to discretely highlight the shape.
  

Motor brackets

• I made some motor brackets from 25mm x 25mm x 2mm ali angle. They seem to work well and I'll fix them once I have the whole drive train running.
  

I am now awaiting some machined parts and my PWM controller and will have a crack at the electrics when they come. If all goes well I'll be having my test run next weekend sometime.

[SniffTheGlove]

Looking good Shane, I had to re-read you last sentance as I looked at the photo's and saw a diffent coloured platform then reliased you had put a primer over it.

I an undecided about my finish. I have a big opened pot of Sandolin Exterior Black that I have coated my outdoor table with and have loads left. It takes ages to dry like 4 days but then becomes quite hard finish.

[Moonshane]

did a little more tonight but not too much as I am waiting on some bits:

• I generally observe at home on hard paving but occasionally go to other sites (like SGL6!!) so need to spread the weight a bit. Therefore made some 'soft ground pads'. I wanted to have these handy but not in the way so I made them just big/small enough to fit under the top board and secured them with a bolt (finger tight) and a tee nut on the underside of the ground board. I'll prime them before bed and then it's a rub down and undercoat everything tomorrow.
  
• I fitted the motors to the brackets I made and they are spot on height wise. Just laid them on the board where they will be.
  
• Finally I did some thinking about the electrics today and created this diagram.The logic is that there are two circuits operating via the toggle switch (forward / off / reverse) to the right of the battery although with only one circuit operating at once. The one concern I have is the splitting of the wires from the positive on the battery and the joining of the wires at the negative side of the battery but I am sure it will be OK with the toggle controlling the flow (i.e. I don't think splitting the wire (or in fact doubling it) will affect the voltage?)). With the toggle switch on forward (fwd) the tracking motor will run at about 2.5 rpm (approx 7 volts) and on reverse (rev) the reset motor will run at 60 rpm at 12v - returning from end to start in about 90 seconds. For the forward circuit the power goes battery > toggle switch > PWM unit >cut off end switch > motor > PWM unit > negative on battery. For the reverse circuit you switch the toggle to off and then reverse and the power goes battery > toggle switch > cut off end switch > motor > negative on battery (the PWM unit is not required on the reverse circuit as this will run at full power).
  

Hope this all makes sense!

circuit for EQP.pdf

[spaceboy]

Looking forward to seeing the finished product at SGL6 Shane. Will the base have to be leveled or are adjustments made to the slew etc? I must admit I would have concerns to the durability and stability of that M8 ball joint given the weight of the dob and regular use.

[Moonshane]

yep, I have built in adjustable feet to level it - will do so with the Wixey I think or maybe small bubble levels.

the M8 ball joint is from trike suspension joints and can stand 175kg apparently so 30kg max moving very slowly should be fine I think. I have bought 5 for £10 so I'll always carry a spare just in case and inspect every now and again (certainly initially).

that said, for future versions, I'll be putting it the other way around with the ball pressing down into the horizontal housing. this one is mainly for my 6" planetary dob which weighs hardly owt.

[spaceboy]

hi all. Last night I made some feet for the platform. As it needs to be level (and although my main observing site - garden - is pretty level and flat) I have built in some adjustment.

I had previously made some round approx 60mm pads (they could be square etc of course) and these were drilled out to accept an m8 stud (cut from a length of rod). These were then glued in overnight with a two pack epoxy resin. At the same time I used some rubber sheet to provide some damping and glued this with the left overs.

The adjustment is about an inch although I'd normally expect the ground to be fairly level without any adjustment.

yep, I have built in adjustable feet to level it - will do so with the Wixey I think or maybe small bubble levels.

.

Doh.... Yes I see you have addressed levelling.

I have some thing similar to this on my dob base Circular Bullseye Spirit Bubble Surface Level on eBay (end time 20-Mar-11 14:15:52 GMT) as it can get you level in all directions quick and easy. I see the one for sale on UKB&S has one also so should work for your EQP.

[Moonshane]

cheers mate - just the job - have ordered one.

put the second undercoat on last night so can get the final matt black coat on it tonight and tomorrow am.

then (still) waiting for the PWM controller and some machined bits (my next purchase will be a pillar drill - sick of relying on people!) being done this weekend. so hopefully can trial before SGL6 and get working properly.