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My Equatorial Mount Build


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Hi im Mike from Taunton

After a lot of reading i decided to go out and build a platform. I followed the directions on this webpage Building an equatorial platform for a Dobsonian telescope

At first i was stumped, too much info but after reading it about 100 times it started to sink in how things worked. I made an educated guess of the centre of gravity of my scope by measuring to the handles I have a skywatcher 10" dobsonian. I then used the computer program on that site to calculate all the other areas I needed.

The details I defined into the program were

Center of gravity = 23inches

Depth of top platform I was going to make = 1.1 inches

Distance from bottom of top platform to bottom of sectors i used recommended = 2 inches

Geographical latitude was 51.02 degrees

length of drive arm = 0 as I wasn't using one

Measurment from front to back of platform (north sector to south sector) = 24 inches

Distance platform potrudes beyong sectors = 0 as I had my sectors flat to the surface.

These details told me the diameter of my north sector arc would be

51.49 inches but I wanted radius so divided by 2 and got 25.745inches

South sector was diameter 18.02inches or radius 9.01inches.

sector parts depth from further most point of the arc to where i had to cut straight edge would be 3.18 inches. This would mean my straight edge on the north sector is 24.79 inches long and south sector 13.74 inches long refered to in the computer program as width of northern or southern sector at platform.

I actually changed all these measurements to millimeters before starting to make it easier to calculate and because I was going to put a wooden trim on the edge of my arcs which was 3mm thick I had to take 3mm off the radius of the arcs before I cut them.

Armed with all this info I got my dad to help me cut the arcs out using a jigsaw. We measured a thin piece of wood in a straight line to the same distance as our radius and put a nail in one end to act as pivot and to use the wood as a compass to make a smooth arc. i measured back 3.18 inches and angled the jigsaw at 39 degrees as this is the angle i needed (90 degrees minus my latitude 52 degrees gave me an angle of 39 degrees) once cut out I checked the long edge was acurate to what the computer told me it should be and I was amazed how accurate it was :).

This is how far I have gotton at the moment in regards to the platform it self. I have identifed the wheels I want to use to move it just waiting to get some parts. In the mean time my attention changed to the motorisation. I bought a stepper motor (1.8 degree 200 step) and a stepper bee control board from pc-control (google them) the stepper bee meant i could connect the board to my usb port on my computer and control the rate of the motor. (im no electrician so building the board wasnt for me).

From the site I linked above it told me that I would need to push 6.93 inches of tyre away in one hour to match the rotation of the earth and suggested I would need to run my motor at 25 revolutions a minute. I calculated this to be 5000 steps at 12milisecond intervals

(60 seconds / 25 rotations = 1 rotation every 2.4 seconds or 2400miliseconds.

2400 miliseconds / 200 steps = 1 step every 12 miliseconds.

25 rotations x 200 steps = 5000 steps a minute).

But this is way to fast for tracking the stars so I need a way to gear down the motor even slower. This was my next teach ya self area I looked into. I had to work out what ratio i needed. so the site said the stepper does 25 revolutions x 60 minutes = 1500 rotations an hour but I need a rotation of

6.93inches amount of tyre pushed an hour / 3.71 size of my drive wheel I will use to push the platform = 1.868 rotations an hour.

To work out how much reduction I need to do

1500 current motor rotations an hour / 1.868 rotations an hour needed = 803 reduction. I was going to go to maplins to buy cogs but then thought i have loads of technical lego wander if it would work. In lego the cogs I had were 40 teeth 24 teeth 16 teeth and 8 teeth wheels and a plastic worm drive. i also found my stepper motors 3mm shaft was perfect for use with lego how wierd B)

I decided to work backwards i new that 20 x 40 would give me 800, and a wormdrive would be equal to the number of teeth on the gear wheel it was driving. so I started off with the 40 tooth gear wheel attached to the worm drive this gave me the 40 in the equation so now all I had to find was 20 in actual gearing. This was achieved by attaching a wormdrive thingy to my stepper motor and the other end to a lego axle which had a 8 tooth gear fitted on the end. this drove a 40 tooth gear 40/8 = 5 giving me a gear ratio of 5:1 I then attached a 8 tooth gear to the axle of the 40 tooth and drove a 16 tooth 16/8 = 2 giving me a gear ratio 2:1 we now do 5 x 2 = 10 giving gear ratio 10:1. I now attach another 8 tooth to the axle of the 16 tooth and drive a 16 we do 16/8 = 2 this gives me another ratio of 2:1. now i do 5 x 2 x 2 = 20 now we do 20 x 40 from the worm drive = 800 close enough for what i need.

I have some pictures of my work so far and hopefully the lego ones will help explain what I just said above :grin:. I plan on glueing chipboard or plywood to the outside of the lego to stop it breaking and strengthen it and see how it goes it might work it might not but at least from doing this I now know how to work out gears. Oh the actual drive wheel we be connected to the axle on the wormgear. Youtube has some good videos on gears as well. I will keep this thread updated as things develop and hopefully be tracking some stars soon :p. Please leave any comments or corrections.

Equatorial platform photos | Facebook

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Welcome to SGL.

There are a number of people here building EQ Platforms and I'm sure as soon as they sign in they will comment on this. You have done alot of working out here and I wish you every success in building this.

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