Build Project: Dob Equatorial Platform

[Moonshane]

Hi Sean

Started my thread today http://stargazerslounge.com/diy-astronomer/127311-equatorial-platform-dobsonian-telescopes.html#post1666070

[SniffTheGlove]

I have just read your first post. Good luck as it's not as easy as I first thought it would be.

I have identified one error I have been making and this is why my maths was not tying up with some of the spreadsheets out there. The error is based on the Centre of Gravity of the scope.

When looking at spreadsheets and plans I just thought that especially the graphics in plans were badly drawn and even reading the EQPlatform Yahoo group posts I saw not post mention this. What the graphics show is that the CofG must sit on the Polar Axis, but I also thought that the northern bearing was also based on the CofG point. This was making for very long platforms and ridiculously high and off centred segments.

I even draw my plans like this thinking that is the way it should be but after redoing my main drawing for the 4th time I have seen the light and realise that it's only the CofG that must be on the polar axis and not both axis's and that way the math trig function I am working with now produce a much shorted platform with short segments.

I am now redoing my main drawing for the 5th time in pencil to a scale of 1mm = 10mm and things are now looking better and I can then put the paper written equations into my spreadsheet.

[Moonshane]

hi Sean - sorry I should have mentioned this but thought it was me being thick. I have a COG of 35" on my 6" f11 dob and working like you, I got to a platform that was 5 feet long!! I now realise, as you have, that the it is the radius of the north sector in partic that is affected by the COG - i.e. that the higher the COG of the scope, the larger the radius of the conic section for the north sector (and thus the shallower the curve). I think this is reflected by your earlier comment that a six foot diameter sounded a bit large for my platform?

the only thing that has to affect the size of the platform boards is the physical size of the dob base/feet - i.e. it should fit on the thing! I also think that you can use a platform based on a higher COG than the scope you are using but not lower. In my case I have two dobs which have 35" and 25" COGs so as long as I base my dob on the former and they both fit on I can use it for both. I hope you agree with this?

[SniffTheGlove]

hi Sean - sorry I should have mentioned this but thought it was me being thick. I have a COG of 35" on my 6" f11 dob and working like you, I got to a platform that was 5 feet long!!

And then you start thinking this is not right as I can't fit it into my Obsy let alone the car for transportation.

In my case I have two dobs which have 35" and 25" COGs so as long as I base my dob on the former and they both fit on I can use it for both. I hope you agree with this?

Yes, you are correct. A scope with a lower CofG can use any platform that was designed for a higher CofG. No problems.

The other little I had and you have just touched on above is the segment size. Plodding through the maths using simple trig there is a point you need to define and that is your reference plane, the plane at which you latitude angle is derived from and where the segments are measured from.

The big thing here is what dimension to use from the bottom of the rockerbox platform to the reference plane.

A few of the resource sites and spreadsheets all used a user input number to calculate this measurement but there are no explanations as to why they chose that number. Some were 10cm, some were 6 and one was 14cm.

It was only last night that I finally got it sorted in my head and became clear as one of the things I was doing with the maths was trying to work out this dimension from the available equations, which is possible to do but produces a very large sagitta for the segments which makes the platform rather high and also gangly in appearance as the north segment would always appear to attach to the platform near to the CofG Vertical centre line. What these other users had done was to maybe guess (as no one stated otherwise) a smaller number which allows the north segment attachment point to move nearer to the north end which makes the platform lower in profile but crucuially more stable from the weight of the scope above. Of course the actual movement of the platform as it tracks is not comprised as the angles are correct wherever the segments attach.

So now I am reworking the northern segment to produce a lower profile but I still want it to be done mathematically as when I am finished I want to build my own spreadsheet that others can use without any of the guess work the current spreadsheets seem to want (Don't get me wrong, these spreadsheets do work but the simple question of platform height is not discussed or indicated whey the default value was chosen)

My design is now going to be reworked today and tomorrow using my new knowledge from last night to produce a design that will work and also look good. Well I hope it will as I should of built the damn thing by now!!

[noakesy]

good luck bud research it well as it can become costly

otherwise

[SniffTheGlove]

Thanks noakesy.

Another issue has had me thinking. That is stability.

There needs to be stops in place to stop the segments rolling off it's bearings at the end of the tracking (Also at the start during resetting). This is easily accomplished by putting a simple block at the end of the segments.

But for stability is the above good enough. If an accident happens and you fall against the scope pushing the rockerboard off it's bearings you will have everything come crashing down.

If you have a 2 segment design, that actually means that the groundboard and the rockerboard are 2 distinct items that are not joined and therefore could be more susceptible to inadvertent movements, whereas a single segment with pivot design then the platforms are joined therefore only the north segments needs protecting from inadvertent movements.

As a consequence the pivot design means that the platform is heavier to move as it can not be broken into to 2 pieces for moving.

So I am thinking about what would be better (for me) as I have a spinal problem and can not lift/twist/walk very well.

The 2 segment design looks better for moving the platform around (into car etc) but could have a problem if I fall/trip against it.

[Moonshane]

hi Sean

yes, I agree about the height issue. I have personally decided to keep the platform as low as I can get away with and my north section is 18" wide and about 1.5 inches high but is right at the front of the platform.

I am also using the pin/pivot design for the south segment. I cannot see why this cannot be removed, it would just mean a very slightly tricker alignment when putting the top board on? I could try and scan my side view drawing if you like? it's only rough but will show what I intend - and it's subject to change!

I think this will be a more stable platform though as a result.

Furthermore, I will be using limit/micro switches to sop my motors at both ends of the drive rod and also a cap head bolt into each distal end of the north segment to act as a double safe.

I might also restrict the angular movement to 10 degrees rather than 15 as this will reduce the tilt. This will still give 40 mins of tracking between resets which in my case will be a 2 min job.

I suppose if you fall into a dob on the ground the chances are it's going over?

[SniffTheGlove]

I suppose if you fall into a dob on the ground the chances are it's going over?

I fall over quite often due to my spine when my legs give way or I trip over things easily as I can not lift my my foot high enough sometimes and then catch it on objects.

Today, I brought a 5mm thk (100 x 70) piece of Teflon sheet for £1.49 plus £1.49 for postage off ebay. So this means I have gone back to a south pivot design and not a 2 segment design. This saves a little money and makes the design stronger and sturdier I believe.

The teflon sheet will be used to make two 5mm thick 60mm dia washers that will be used to make a nice smooth south pivot bearing.

[Moonshane]

I fall over quite often due to my spine when my legs give way or I trip over things easily as I can not lift my my foot high enough sometimes and then catch it on objects.

Today, I brought a 5mm thk (100 x 70) piece of Teflon sheet for £1.49 plus £1.49 for postage off ebay. So this means I have gone back to a south pivot design and not a 2 segment design. This saves a little money and makes the design stronger and sturdier I believe.

The teflon sheet will be used to make two 5mm thick 60mm dia washers that will be used to make a nice smooth south pivot bearing.

ahh I see, sorry.

I am doing a similar design but was told a while ago that teflon does not work well against Teflon. I am therefore using two 50mm M10 stainless washers which I will glue (two pack epoxy) to the top and bottom of the pivot surfaces and then with a teflon washer as 'the meat' in the washer sandwich. This will do two things 1) retain the 10mm pin in position strongly 2) prevent the wood wearing where the pin turns. I will probably also glue the pin into the groundboard (bottom board) bearing so it sticks upwards and this will also strengthen everything and make putting the top board in position a bit easier too.

been to get some bits today (threaded rod, washers, bolts and grub screws) and will be starting (and hopefully finishing) the woodwork next weekend. will take pics for my thread as I go.

good luck - sounds like we are both about to get serious!

[SniffTheGlove]

ahh I see, sorry.

I am doing a similar design but was told a while ago that teflon does not work well against Teflon. I am therefore using two 50mm M10 stainless washers which I will glue (two pack epoxy) to the top and bottom of the pivot surfaces and then with a teflon washer as 'the meat' in the washer sandwich. This will do two things 1) retain the 10mm pin in position strongly 2) prevent the wood wearing where the pin turns. I will probably also glue the pin into the groundboard (bottom board) bearing so it sticks upwards and this will also strengthen everything and make putting the top board in position a bit easier too.

been to get some bits today (threaded rod, washers, bolts and grub screws) and will be starting (and hopefully finishing) the woodwork next weekend. will take pics for my thread as I go.

good luck - sounds like we are both about to get serious!

Moonshane, thanks about teflon V teflon, I make the teflon sandwich instead.

I was thinking of using M8/M10 threaded rod as well but I was thinking of sleeving it in some Nylon tubing to prevent the area around the rod being enlarged through the twisting action of the rod and bearing and if the nylon tubing does wear and the rod starts to have lateral motion it would be easy to replace with a new section of tubing.

Ends of the thd rod will be having large dia metal washer to spread the load and then terminated with a nyloc nut (groundboard) and a the upper platform will have a the washer and wing nut so the platforms can be disassembled for transportation.

I am wanting to make the first cut in the ply but I keep going back to the plans and the equations to check everything is going to be OK.

So getting serious? yes. I just want it to work because it will last me a lifetime if it is built well.

[Moonshane]

nice one Sean - I could have written all of that! sounds like we are on the same track. measure three times and cut once - always the best way! the good thing though is that the top and bottom boards are straight forward and only the curved sector affects things greatly (assuming the angles are correct on the north sector and the bearing supports) and this could be redone a few times if needs be. I get the impression that as long as the angles are correct, the radius only matters approximately. I am considering adding a few weights in the base and some clips to hold the scope on the top board to reduce the chances of mishaps. adds to the weight but worth it I reckon.

[SniffTheGlove]

nice one Sean - I could have written all of that! sounds like we are on the same track. measure three times and cut once - always the best way! the good thing though is that the top and bottom boards are straight forward and only the curved sector affects things greatly (assuming the angles are correct on the north sector and the bearing supports) and this could be redone a few times if needs be. I get the impression that as long as the angles are correct, the radius only matters approximately. I am considering adding a few weights in the base and some clips to hold the scope on the top board to reduce the chances of mishaps. adds to the weight but worth it I reckon.

Clips to hold the scope are great idea. I was going a little further myself as I hope to remove my Dob's swivel base (Square in shape) and cut a new circular one that will be able to be placed on the EP CofG Centre Line and then attach a 12v motor to use as a Azimuth motor to swing the scope under powered control using a GemRad to measure the angular rotation to help provide a simple setting system, then of course put another motor with GemRad on the Alt bearings to provide Alt movement so together I can (when EQ Platform is in it 0 deg position ie horizontal) I can then use Az/Alt coords and as a simple goto and then when on target the EQ tracks.

Instead of adding weight you could look at bolting on a lateral extendible legs to the groundboard to provide better ground loading and spread the scope weight over a great amount of area. ie they slide out sidewards from the groundboard

[SniffTheGlove]

I am going around and around in circles (Well triangles really)

I really thought I had it all sorted out this morning with the final load of hand drawn and ported to excel. I had done all the calculations apart from the final cell equation which should give me the segment height as specified in the user input cell. If these 2 results where inentical then all my other loads of loads of calculations were correct and cutting could begin but I am 3cm out so now I have to back track and go through all the calculations again. It's probably a decimal point is the wrong place or the wrong angle used in the trig.

I am trying to achive a spreadsheet that others can use after me and hoping that it will just need 2 or 3 pieces of information to generate all the dimensions.

Currently I just need the Scope CofG (The spreadsheet works this out for you from scope dimensions and weight), Latitude and platform plywood thickness.

The the spreadsheet should calculate all the measuring points. Not just radius and sagitta. By doing all the possible dimensions it will give you the entry and exit points of the pivot points on the South Bearing plus all the dimensions of the segment (Chord, Sagitta) the gap need if you do not chamfer the Sagitta etc.

Back to the drawing board!

[SniffTheGlove]

I have completed the spreadsheet calculations and I am 99.99% happy with it,just a little bit of work with bearing placement calculations but will fine tune these when I have make my platform as I can alter and recheck how well everything went.

I have included my spreadsheet here if anybody wants a look.

It is a North Segment with south Pivot, though the spread sheet will give the entry and exit points for the south pivot there is not actual picture to show how everyhing goes at the moment, but all you need to do is put your details into the green boxes and then check out the "Final" sections at the bottom of the spreadsheet for the North radius and Sagitta

eq_platformNsSp.xls

[Moonshane]

this is top work Sean. I am taking a much simpler approach. making sure the N&S points are level and that the sagitta is high enough for my battery and the board large enough for my scope.

other than this the main thing I need to get right is the radius of the north segment.

will be interesting to see if my less techie approach works too!

[SniffTheGlove]

That's what it all boils down to really. North Radius and correct sagitta.

Anyway, not bad for 2 1/2 weeks of maths and drawing boards. Now I have tested my spreadsheet with one of the original poncet ones and my details now work out very very close. I think in the end my Sagitta is 8.61cm and the other poncet one is 8.39cm so that is .22cm (2.2mm). Not bad I think seeing how the way every was done triangle by triangle moving slowly.

Anyway hopefully first cut tomorrow

[Moonshane]

ooooh bravery!

good luck mate - I'll be doing mine either this weekend or next I think as things stand. will update my thread as things go.

[SniffTheGlove]

Damn, as usual you get things all out and ready to start then something else goes wrong. My internal IP network went down early this morning and I lost 2 bits of equipment which resulted in no external internet traffic. Thought it was the Sky Sagem that had gone faulty but as it can only handle 2 wired connections I have the Sagem plumbed into a Netgear 8 port 10/100/1000 Switch and it was this switch that had died meaning that all my internal connections went down as they all route through this main switch. Also one of my Wireless Access Points had died at the same time taking out most of my wireless traffic.

So with the main switch down plus one WAP the only thing do do was bung a temporarily solution together (Moved my Astro WAP to replace the died WAP which means that my Obsy is without net access and shoehorned an old router to replicate the main switch)

I have just brought 2 new network switches and also 40m of Cat5 pre-terminated patch cable. The patch cable is to run down to my Obsy so I don't have to worry about the replacing the WAP as I'll now have a wired and faster connection now to the Obsy.

Now that it has been sorted and awaiting delivery for tomorrow I'll try and do my first EQPlatform cuts tonight.

[Moonshane]

well, Sunday's the day for me for cutting at least!

[SniffTheGlove]

Now that it has been sorted and awaiting delivery for tomorrow I'll try and do my first EQPlatform cuts tonight.

Well the IT stuff was delivered at 8am, really good. All plumbed in including a hard line to the Obsy now.

Did no cutting last night though I did measure out the segment ready for the router cut but will need to practice first as never used a router before and no cutting today as I was installing the It gear and also had to go shopping. Oh life!!

At least I'm alone tomorrow.

[Moonshane]

hi Sean - did the bulk of the woodwork today. Hope this will help you - it's not too terrifying once you plunge in.

http://stargazerslounge.com/diy-astronomer/127311-equatorial-platform-dobsonian-telescopes.html

[JamesF]

I have just brought 2 new network switches and also 40m of Cat5 pre-terminated patch cable. The patch cable is to run down to my Obsy so I don't have to worry about the replacing the WAP as I'll now have a wired and faster connection now to the Obsy.

In case you weren't aware, the outer sheath on normal cat5 isn't usually considered UV stable, so if you have it out in the sun it's likely to degrade. You can get cat5 with a UV-stable sheath, or the other obvious alternative is to run it through a length of hose pipe or something similar.

James

[SniffTheGlove]

In case you weren't aware, the outer sheath on normal cat5 isn't usually considered UV stable, so if you have it out in the sun it's likely to degrade. You can get cat5 with a UV-stable sheath, or the other obvious alternative is to run it through a length of hose pipe or something similar.

James

Yes James, the cable is housed inside a seperate electrical conduit down the length of my garden fence at least 18inches away from the main electrical supply to the Obsy to avoid EMF and possible crosstalk

[JamesF]

That's probably better than mine will be if I can ever persuade SWMBO to let me build one.

(Actually, I have a plan, but, you know, walls have ears...)

James

[SniffTheGlove]

It took several attempts to router the segment radius just right. The first couple I made a right pig's ear with the router, then tried a jigsaw and that was as bad, then I used the jigsaw to cut a 5mm larger arc then used the router to take off the final 5mm, this 2 attempts as the first one I was out by 1mm so redid the arc 2mm back and just took a smidgen off. Perfect.

Tomorrow I am cutting my main test wedge at 52deg and once that is done and correct I can then use that test wedge to check the angles on the circular saw cuts. One the segment is cut and chamfered I shall do another segments while all the gear is still set up incase I mess up the segment later on, and if all goes well I have another segment ready to do another platform (just in case)

I'll take photos tomorrow as apart from routering the segments today I had the slab men finishing off my observing patio which look nice and is level.