Build Project: Dob Equatorial Platform

[SniffTheGlove]

This is my first post of this build project.

I want to achieve the ability to have tracking to my Dob for visual observation plus SPC880(900) webcam astrophotography.

Hopeful for at least 60mins tracking which will be the 15deg of total motion.

I am designing the north circle segment with a south pivot bearing on 18mm ply. The electronics I have a good idea with but these will be at the end of the project. I want to build the platform first then add the electronics when I have more time after the wood making build bit.

I am trying to source some decent wheel to use and looking at inline skates or some heavy duty but small fixed wheels that can handle 100kg load though my current 10in Dob is 40Kg and I don't want to skimp of getting 50Kg wheels.

My latitude is 52deg

Currently plodding away with the Maths but very near to getting the final size calculated but the wheel are need to finalize the platform height.

I'll post photos and descriptions aong the way and anticipate cutting the first wood in approx 7-10 days time. Depends on my spinal condition and the weather being drier than it currently is (raining)

[Doc]

Best of luck with the project Sean and I look forward to seeing it progress.

[SniffTheGlove]

Thanks Mick.

I must also say thanks to Fred the Astrodwarf, he lives in Belgium and it is his design (which is a take on the original Poncet with Chuck's south bearing mod). I have emailed Fred to clarify a few things as the Google translation was poor from his German website language but we got it all sorted out between us now. I do like his design though I will do a few thinks different like the support legs. I will use 12mm threaded rod instead of his 6mm so the platform will be a bit more supportable.

Additionally I shall build a lowrider truss dob after the platform, so the upper part of the platform will act as the groundboard for the scope's lowrider rocker box. I plan to motorise the this scope in Az and Alt so the Platform will have the mountings to the motors and Gem encoders. The idea is when the pltform is at 0 deg horizontal, I can then use the Gem encoders to display Az and Alt, all this will be controlled by the Platform handset.

Anyway, back to the platform first. The upper platform will have simple bubble level indicators to help horizontal alignment via the adjstable legs (could I be fancy and motorise the leg adjusters as well Hummm!)

To provide the central (rocketbox) pivot I will use M12 so the side forces when at an angle of 7.5degs will be able to handle the load of the Dob. I will have to centre tap for the Gem encoders I belive.

To provide the Az rotational I shall use Ball transfer bearings as I have see this work very well though hopefully the friction of the Az motor will stop any rotation of the dob when it moves through different angles of tracking.

I have brought the wood on Thursday. Looked online for Birch Plywood (18mm) and could not find any local supplies and the supplies who could were a long way a way and the cost of shipping an 8ft by 4ft panel would of be very costly.

So I went with normal External 18mm Plywood. My local Focus DIY store would only do 4ft by 2ft and then only in 12mm and the assistants look dumb when I asked if they could get 18mm, so I went to Homebase and even though they did 8ft by 4ft panels it was only 12mm again though I was shocked at the cost of £57. I then tried Wickes and they did Birch plywood but only in 6mm however they did External plywood in 8ft x 4ft in 18mm priced at only £30 so I brought the sheet and then to get it home cut it up in the car park with a battery circular saw into 4ft x 2ft panels. This is roughly the size of the platform I shall design as well.

So if you 18mm external plywood then look at Wickes. I also brought 2 x 1m lengths of 15mm x 2mm flat aluminium strips to put on the circle segments for the runners to run agains, these were £1.99 each.

[Moonshane]

I'll be watching this with a great deal of interest as I am about to start one soon myself. I am currently amassing parts (just got some 12v DC 4rpm geared motors from ebay) and will be starting work properly in a few weeks. just trying to work out a reversing mechanism - at 4 rpm it will take about 10 minutes to reverse the thing. might have to reconsider and get a faster motor. they were £6 each so not the end of the world and will use them for something eventually if not for this. whatever we do it will be cheaper than the £500 the ready made ones go for.

good luck!

[Doc]

Shane build a mechanism that allows you to dissengage the drive and swing the platform back to the start by hand then engage the motor and start tracking.

[Moonshane]

cheers Mick

I was starting to conclude this looking at some designs online. I think a gear on the motor drive shaft, matched with the threaded rod might be best and then a plate to move the motor back and forth to disengage the gear might be the best way.

always other options though and hence I am going to take my time. once it's designed properly though I reckon I can make them for every dob I own in future and for less than 10% of the price of a ready made one!

[SniffTheGlove]

Moonshane, I too will be checking on yours as I have probably forgot something or you have thought of a better implementation than me.

To reverse back I am going to put a physical runner stop coupled with a microswitch cutoff and then use one of the following options:-

1) Main motor/gearbox on a spring loaded arm so it can be disengaged manually and swing the platform back.

2) Automate the response from the microswitch that will then use a servo to retract the tracking motor and then engage a faster motor to swing the platform back and stop.

3) Run the original motor/gearbox in reverse but at a higher voltage load to increase the rpm speed.

No the fastest method from the above would be No.1, followed by No.2 and then No.3

I have currently choose the motor I specified before being from Maplin that will run between 2 and 6 rpm based on the voltage from the speed controller. If I go with a stepper motor design then I can just reprogram the reverse at a much higher stepped rate. Still working on these ideas though!

[Moonshane]

cheers Sean

some excellent ideas there.

my motors being 4rpm (controlled by a dew controller to the appropriate speed) will be a bit slow for reversing so I'll rig up a release mechanism for the drive shaft and then a hand crank to return to base. easier said than done though! I need to find a supplier for a pair of small cogs for the end of the motor shaft and the drive shaft.

PS thanks for the heads up re Wickes. I hate B&Q and have been after another source of materials. I think there's a Wickes quite near to me.

[SniffTheGlove]

cheers Sean

some excellent ideas there.

my motors being 4rpm (controlled by a dew controller to the appropriate speed) will be a bit slow for reversing so I'll rig up a release mechanism for the drive shaft and then a hand crank to return to base. easier said than done though! I need to find a supplier for a pair of small cogs for the end of the motor shaft and the drive shaft.

PS thanks for the heads up re Wickes. I hate B&Q and have been after another source of materials. I think there's a Wickes quite near to me.

For gears Maplins do some gear : Maplin Electronics

and they come from mfacomodrills

MFACOMODRILLS.COM

[Doc]

For the cogs and gears, try a model shop the ones who specialise in airplanes, trains etc.

[Beulah]

I'll be keeping an eye on this thread. I want to build one myself eventually.

Can an EQ mount be hand powered only?

[Moonshane]

great stuff re the gears guys!

an EQ platform could be powered manually but it would be tricky and probably more costly than battery/motor power.

funny how ideas come to you as you drive along - 'ahAAA' I said out loud in the car on the way back home just now as I realised that one of those flexible drill/screwdriver bits like this flexible drive drill shaft 36" on eBay (end time 24-Jan-11 21:24:16 GMT) might be useful for disengaging the motor and manually winding the threaded rod back to the start. might need to find a shorter one though.

[SniffTheGlove]

'ahAAA' I said out loud in the car on the way back home just now as I realised that one of those flexible drill/screwdriver bits like this flexible drive drill shaft 36" on eBay (end time 24-Jan-11 21:24:16 GMT) might be useful for disengaging the motor and manually winding the threaded rod back to the start. might need to find a shorter one though.

Moonshane, I have one of these and had modified it myself to make it shorter 5 years ago. It is a square shaft that runs all the way, so just cut to size and then reattach the end coupling. All I used was heat shrink tubing to keep it in position.

[Moonshane]

nice one, cheers Sean. I think that's another bit of my plan coming together

don't worry by the way I'll upload pics etc when I start. I'll start a new thread but put and link to it here and to yours on mine and then anyone interested can follow both. will be interesting to see how they work.

[themos]

I've thought about making one too but I realize I'd have to build a toy-model first just to grasp everything that is involved.

[Astrodread]

Im thinking of making one for me 10" homemade dob. I saw some images online of some that some people had made. they used an motor drive for an EQ1 tripod mount. i was wondering if i could do that as i own one. any one got thoughts on this whether it would work or not? just wondered if it could cope with the weight.

[Hypernova]

I am considering making one of these fairly soon, yes another project for this list

My plan is to make a direct drive example, with a motor with a drive wheel of the correct size to move the platform by 15 degrees. The wheel would contact a smooth wooden or sheet metal bearing surface to make the required movement. I have seen someone else make something along these lines and I have no reason to think that it wouldn't work just as well as the kind with the nut and threaded bar.

Resetting would be done entirely manually as I don't think I have the required electronics skills to knock up a circuit board to control it all.

I also can't seem to find anywhere that tells you what size each of the bearings need to be.

[SniffTheGlove]

The maths is proving quite difficult. For ease I went with an already created spreadsheet, then I went to check the results with a different spreadsheet and I have got different measurements of the North Segment Radius. I found another different spreadsheet and again this one produced a different North Segment Radius.

To calculate the North Segment Radius you define your reference line and then work out the radius vis Trig Equations. They all should produce the same radius when working from a reference point and CofG (I have taken the additional measurements out like platform thickness, segment wheel heights etc so the calculations should agree)

So I am now doing the Trig equations myself and see what I get when I am finished working things out.

[SniffTheGlove]

they used an motor drive for an EQ1 tripod mount. i was wondering if i could do that as i own one. any one got thoughts on this whether it would work or not? just wondered if it could cope with the weight.

Technically any motor could work, it's all about balancing the scope on the excat CofG then a small motor would work as everything is balanced but move the scope around a bit then the motor needs to be beefier as more torque is needed to push the platform uphill for the last 7.5deg.

[Moonshane]

the size of the circular sections will depend upon the centre of gravity of the scope and base as well as the latitude of the observer. My north circle will have a radius of about 30 inches but of course you only use a small section of that. getting my plywood tomorrow so work will start at the weekend. will hopefully have it done by the following weekend.

cheers!

Shane

[SniffTheGlove]

So I am now doing the Trig equations myself and see what I get when I am finished working things out.

A week has gone by and I am still working on the equations and to be honest I only wanted simple design and the main dimensions, however as I am working my way through the design I am calculating every single dimension for the segments and also the supports.

I have got my old "Technical Drawing" board out of the attic that I have not used since school 30 years ago and have drawn out in scale the plans, then modified them and redrawn.

[Moonshane]

cool. wish I had a tech drawing board still. been doing some rough drawings myself at work in my lunch hour.

decided on a conic section at the front and a pin at the rear. also got some roller skate bearings for the front end. just need to get down to my father in laws for some 'router time' and buy the hardware etc.

[SniffTheGlove]

cool. wish I had a tech drawing board still. been doing some rough drawings myself at work in my lunch hour.

decided on a conic section at the front and a pin at the rear. also got some roller skate bearings for the front end. just need to get down to my father in laws for some 'router time' and buy the hardware etc.

I have plenty of rough drawings everywhere on scraps.

I went for some fixed 50mm dia rubber castor wheels from Screwfix as they were £9.15 for a pack of 4. They look good and there is an M6 bolt which goes through the central bearing to the mount so they should come apart easy from the mount and then be fixable to the direct drive shaft of the motor.

Depending on eventual platform style as if I go for 2 bearing design then I shall need another pack for the south bearing.

Oh what design to go with, each has a different style and look. I like a south pivot but I all so like a south bearing. There is only one way to find out! As Harry Hill says "FIGHT"

[Moonshane]

maybe we should not say any more and have a '10 years younger' style 'reveal' at the end

only joking - will be great to see them as they develop (and if they work!).

[SniffTheGlove]

(and if they work!).

I do hope so else I will be