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Equatorial Platform for Dobsonian Telescopes


Moonshane

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hi Nick

the radius of the north (big curve) sector is technically slightly bigger on the outside edge than the inside edge but the bearing run approximately in the middle so this does not affect things. My mistake here (I am pretty sure) is that the centre point of the rear (south) pin is correctly pointing at 53 degrees but critically is pointing at 53 degrees at a higher elevation (in the EQP) than the face of the north bearing. This effectively means that the mount 'thinks' it has a bigger radius than I have created.

What I need to do is pull apart the (of course glued and screwed) south bearing and then make the pin mid point the same height as the face of the front bearing. This should then make everything line up (in theory).

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Your drawn lines are confusing, the drawn lines should be indetical as they follow the rotation of the north radius.

I am only guessing (could tell you tomorrow as I made a pigs ear of mine today with the router so got my father coming round who knows alot more about woodworking than me) but perhaps the south bearing is off centre thus inducing a slight twist to the rotation?

As long as the reference plane from the bottom of the north bearing goes through the polar axis centre line of the south pivot teflon washer then it should be fine.

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Your drawn lines are confusing, the drawn lines should be indetical as they follow the rotation of the north radius.

I am only guessing (could tell you tomorrow as I made a pigs ear of mine today with the router so got my father coming round who knows alot more about woodworking than me) but perhaps the south bearing is off centre thus inducing a slight twist to the rotation?

As long as the reference plane from the bottom of the north bearing goes through the polar axis centre line of the south pivot teflon washer then it should be fine.

cheers Sean

You might be right and I think I have concluded that it's the height of the teflon washer (being the centre of the pivot axis) being too high (about 30mm or so higher than the face of the north radius (i.e. that which touches the skate bearings).

I'll carve off the south pivot support blocks and get them closer and see what happens.

I'll update things as I go.

Hope you get your woodwork sorted too.

the good thing is the curved sections don't use too much timber so I have a few more goes yet! might also try a curved south radius too.

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Your drawn lines are confusing, the drawn lines should be indetical as they follow the rotation of the north radius.

I am only guessing (could tell you tomorrow as I made a pigs ear of mine today with the router so got my father coming round who knows alot more about woodworking than me) but perhaps the south bearing is off centre thus inducing a slight twist to the rotation?

As long as the reference plane from the bottom of the north bearing goes through the polar axis centre line of the south pivot teflon washer then it should be fine.

Just had an epiphany this morning based along the lines of your comments Sean - thanks. You are right that the top and bottom axes of the 'pin holes' (i.e. those in the top and bottom segments of the south bearing) were drilled separately and therefore not 100% accurate. I have concluded that unless you are very skilled you should avoid the south pin method. One way to do it is to use a single (unsplit) block for the south segment and then sandwich this in position between the top and bottom boards. You'd then need to create a jig for your latitude and drill through the whole lot.

Personally, after some discussion on the EQP yahoo group with Nils Olof Carlin (who has been really helpful) I have decided on a south sector M8 'quick release' ball joint M8 Quick Release Balljoints x 5 Trike Chop Race Rally on eBay (end time 05-Mar-11 14:15:25 GMT). As long as this is centrally positioned (much easier than the pin!) then I think my original method will work fine.

I'll report back once the bits arrive and I've had a go. I am off work today (dropping my daughter off at airport for a school trip) so my challenge is removing the glued and screwed blocks currently in position on the south section. :(

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despite my enthusiasm earlier, I fear I am banging my head again a brick wall.

I have removed the wedges I needed to remove and have temporarily screwed them on the end of the two which support the N segment. I then fitted the skate bearings wider than suggested in the spreadsheet I found online which seems to be wrong as it suggested 2 inches from centre line.

This leaves the south end free to move and as a result the north bearing runs really well and smoothly runs on the skate bearings.

Unfortunately, as shown in the pics below, the south end moves left and right by about 50mm at the far end of travel of the N bearing.

This surely cannot be right and despite measuring all of the dimensions along and across corners, splitting the N sector into segments (measuring and marking with a pencil) and measuring the sagitta at each point, and checking that everything is centred, I cannot understand why this is happening.

Even a ball joint as the south bearing would not cope with this degree of movement. I have never actually seen an EQP working but surely it must rotate evenly about the centre point and not 'swivel' like this?

I'd appreciate anyone with a clue as to what's going on advising me if this is possible. everything seems square and accurate (or as accurate as you can be with wood anyway).

cheers

Shane

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I then fitted the skate bearings wider than suggested in the spreadsheet I found online which seems to be wrong as it suggested 2 inches from centre line.

I found one spreadsheet that did that and ignored it as it works the spacing difference from the amount of arc travel the segment uses and places the "skate" bearings at the end of travel, so in effect then are quote close to the centre line as there is only approx 15cm of travel. In my case I have 15.9cm of arc travel.

Another spread sheet suggest putting the "skate" bearings just out side the horizontal radius of your scopes base thus in my case there would a distance of 44.44cm and this is what I hope to do

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Thanks to Nils I think I have another option now. That it to put the skate bearings into the 'face' of the radius and create an old fashioned Poncet Design. This seems illogical but he assures me it works!

Pics will follow when my ball joints arrive (I hope by Saturday am.)

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Aha!, possibly a good moment for me to join this very interesting thread. I've always been a big fan of equatorial platforms as they seem ideal for DIY enthusiasts and potentially offer a stable and accurate driving platform at affordable prices. When the original Poncet design was described I have to admit that I actually had to make one to get my head round how they worked. Since then, as usual, many advanced hybrids have evolved, the majority look somewhat complex to build. This has brought me back to the original, after all, what's a platform required to do?, basically, just turn the telescope round at the correct angle and at the correct rate. The simple one that I made had a shaft running in a self aligning S bearing set in a block at the correct latitude, the N component consisted of a flat, inclined stainless steel plate on which two suitably spaced ballraces fitted perpendicular to the plate carried the top board. Drive was by the usual threaded rod. As threaded rod drive isn't inherently accurate, I now think that driving one of the N bearings would be a better bet, it would also make it simpler for resetting as you could just slide the top board back to "square one".

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cheers Peter

well, my ball joints have arrived today thankfully so I should be able to use one as my south 'pivot' and when I put my skate bearings into the face of the north bearing (and get the height right) then in theory, it should work, just then requiring a drive which is in prep.

assuming this all works, I will then be able to duplicate it and make more as required but of a more traditional design (i.e. like yours). I am planning a dew controller speed control and hopefully this will get the accuracy a little more like it (when I eventually GET to the drive system!).

I take your point about the ease of reset.

I am having two motors (one 4rpm controlled to 2.5 approx and the other 60rpm). the idea being that with a grub screw/thumb screw I can have the flat held (for turning) or not held (to allow the opposite motor to run). it will be clearer when I post pics!

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Hi Moonshane, you may have checked this, but if the platform cannot turn around it`s pivot, which should be inclined to your latitude then I suspect that the radius of the north bearing could be wrong. Did you get the radius from the triangle which the angle of the latitude creates?

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Hi Moonshane, you may have checked this, but if the platform cannot turn around it`s pivot, which should be inclined to your latitude then I suspect that the radius of the north bearing could be wrong. Did you get the radius from the triangle which the angle of the latitude creates?

cheers Phil

after some discussion, thinking and trying out various things, I have come up with a way of creating a more traditional Poncet based EQP.

that said, I think you are right, I suspect I have tried to combine a two sector dimension with a one sector EQP. I think that the platform would need to be a lot longer to use the method I was trying.

I suppose this is what prototypes are all about - it's gonna have a lot of holes in it but when it's done and working, I can then repeat it more neatly and possibly larger for my 12".

it's all good fun (apart from the frustration, anger, pain, heartache and wasted time). ;)

I have some time anyway as my connecting components for the drive rod are being drilled and tapped next week. might be ready for a first light by next weekend with any luck (he says.......:)).

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wow - Phil - that's incredibly simple and looks really solid too! coincidentally, I came up with a similar idea this morning while dozing in bed. I'll be using a ball joint at the south end though and (as suggested by Leolion) some ball in cone bearings for the north end with my existing sector. I don't think you even need to cut a sector with your/my arrangement. we'll see!

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In theory, the south bearing would not need to be a swivel joint, because when the radius is correct the base should swivel around that joint.

With my one the south bearing is a stud which fits into a small bearing set such as the ones used for skateboards.

You might have already gone into this but to check the radius, a little bit of trigonometry or a full size drawing would give you the value of the north bearing radius. As long as the height of the north bearing rollers is adjusted accordingly they can be moved towards or out of the centre of the curve to give the required amount of movement.

TBH I havn`t actually tried my board yet because I went on to aquire an HEQ5 mount.

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cheers Phil

I managed to fit my ball joint south bearing today (temporarily at least) and it works great. I also bought some 'glide easy' stickers which I have stuck on the face of the blocks on the ground board and it all works remarkably well.

I'll be making more permanent versions of these with something like two of these Unit, Ball Transfer 25mm Dia on eBay (end time 16-Feb-11 11:50:02 GMT) on the north bearing base wedges. I get about 5-6" of travel (equates to about 30 mins of tracking between resets) which I am quite happy with.

I even put both scopes on today and although they are edging toward toppling at the ends of the travel I reckon if I add some feet fixing it should all be fine.

during this week I'll be working on the drive system and will hopefully get a test run by next weekend.

I'll upload pics when I get the chance.

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Well done Shane on getting this far. My wife has banned me from the garage for a few days as my back is bad thus I am grumpy.

I can not wait to get my bits fully assembled.

Shane, on using the ball transfer on the north bearing support (If that is what you mean) then you might need to cover the rear side of the segment of Aluminium as the balls may end up grooving the segment rear and imped rotation if the balls are kept in one place with a load on the platform.

I have seen a few photos of what has happened and how they fixed it.

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hi Sean - I keep adding to your thread and forgetting mine!

I'll be doing some work at the weekend I think now as I have to work outside and it's rubbish in the dark :)

Yes, the dark is rubbish when working with with jigsaw etc ;)

Good luck for the first prototype, how it works fine straight away to save the urgent need to make a mark II

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hi Sean

If you could possibly send me a circuit drawing for this I'd be delighted and it would really help me. I may need to go to a circuit board basis I thin to reduce the voltage but we'll see.

I made some progress today cutting the back corners off for a more shapely shape.

Also recently added a ball joint as discussed for the south sector, this allows much more flexibility all round and negates the need for anything but straight cuts at the front end. As I have already cut the sector I am using this but for future EQPs I will just use straight sectors for the north end.

I have on order some ball transfer bearings (running against a steel plate) for the north sector but for now have used some 'floor gliders' (like the ones used on the base of a sofa, chair leg etc) to allow me to see how the whole thing 'swings'.

I can comfortably get 5 degrees of motion either side and this will give me 40 minutes of tracking potential.

photos of these updates added below.

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hi Sean

If you could possibly send me a circuit drawing for this I'd be delighted and it would really help me. I may need to go to a circuit board basis I thin to reduce the voltage but we'll see.

The circuit details for the simple spin motor is

CIRC-03 (Transistor & Motor) .:oomlout:.

This is the link for the starter set example page and Arduino code.

For the stepper motor build I am doing I am following the code here

Dan Thompson: EasyDriver 4.2 Tutorial

I am using a different stepper motor that here but my motor is 4.5v and not the 12-15v one he has, just means I must have a lower voltage on the power rail. Apart from the arduino I am using the EasyDriver as it handles all stepping code and functions better, beside it's only £11.58 from EasyDriver Stepper Motor Driver

I got my Arduino Starter set from oomlut

Starter Kit for Arduino (ARDX) from .:oomlout:.

for £56.17 though the actual Arduino UNO only is £24.17

I got the starter set to play with (and I am having fun)

If you go down the Easy Driver route, the board (red in the blog photo's) needs additional header pins soldered into the connection holes to mount it to a breadboard (which is how I will do it) and then mount the breadboard within a plastic box enclosure to be weather tight on the platform. The header pins cost £1.57 from

Male Header (36 pin) (x4) .:oomlout:.

Hole that helps you Shane.

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