Hi all, am playing with the idea and trying to confirm the overall budget. Need motors with moderate torque. Any thoughts would be appreciated, starting with the best Arduino board to use, shields, motors and up to heat sinks.
Many thanks.
am playing with the idea and trying to confirm the overall budget
What idea?
Need motors with moderate torque
OK get some having first defined what you mean by moderate.
the best Arduino board to use
Uno
Use these as drivers Pololu - A4983 Stepper Motor Driver Carrier
Note we don't charge by the word you are allow to use full sentences.
Torque can be increased almost to the point of infinity if you're willing to gear down far enough - how much torque do you need, and at what RPM? "Moderate" torque isn't specific enough.
Stray
Grumpy_Mike:
am playing with the idea and trying to confirm the overall budget
The idea is to motorise the alt-az telescope a bit. It's a balanced, but a pretty heavy thing that - very simply needs two motors in the horizontal and the vertical plane (assuming the base has been made perfectly horizontal 
)
So, once aligned, the device can take a reading of the onboard 3D compass that will give it a rough bearing and inclination. I need to send this data to the attached laptop, compute the speed for each motor required to track the object. Continue doing this until a realignment is required.
Any thoughts on the type of *duino board that I need that would take two stepper controllers and an interface to a compass chip?
iPhone or an android phone is a nice to have remote controller but forget about it for now to simplify things...
Thanks.
I would have thought you needed a geared system for this.
(assuming the base has been made perfectly horizontal
The great thing about using a computer is that it does not have to be perfectly made. Basically you have a coordinate frame for the telescope and another for the sky. So all you need is a transformation matrix to convert one to the other.
the onboard 3D compass
Not sure what you mean by this. You can have a compass which is essentially giving you an angle, you can have accelerometers which basically give you an orientation. But these are not accurate enough to guide a telescope, just a naked eye sighter. However with stepping motors you can align on three known stars and then work out your transformation matrix and so point anywhere in the sky.
Have you seen this book?
Grumpy_Mike:
I would have thought you needed a geared system for this.(assuming the base has been made perfectly horizontal
The great thing about using a computer is that it does not have to be perfectly made. Basically you have a coordinate frame for the telescope and another for the sky. So all you need is a transformation matrix to convert one to the other.
the onboard 3D compass
Not sure what you mean by this. You can have a compass which is essentially giving you an angle, you can have accelerometers which basically give you an orientation. But these are not accurate enough to guide a telescope, just a naked eye sighter. However with stepping motors you can align on three known stars and then work out your transformation matrix and so point anywhere in the sky.
Have you seen this book?
http://www.amazon.co.uk/Telescope-Control-Mark-Trueblood/dp/0943396530
Gears are ideal, but a single speed would do. I'd say clutches are far more important as you will want to operate the system by hand for sure. With regards to the 3D compass I was referring to a chip e.g. HMC6343, and have seen others.
So, my question remains open, i.e. what would be the best *uino board and controllers for the steppers?
Thanks.
The HMC6343 is not nearly accurate enough for a telescope, a look at the data sheet shows the accuracy is over two degrees. That is four times the width of the full moon.
what would be the best *uino board
Still an Arduino Uno - why do you need anything else as it is just an SPI interface?
controllers for the steppers
As you haven't actually chosen any steppers it is impossible to say but I stand by my earlier recommendation in the absence of any actual facts.
Thanks. As I mentioned before, I don't need the thing to point directly to the object - this will be done visually. The task will be to track the object, i.e. calculate the speeds for each stepper motor. The calculation should be sufficiently robust given the position is alt-az position is known to about a degree or two. In the worst case the coordinates could be entered manually from stellarium. The laptop then does the job of re-calculating the speed as often as it can.
Now, when you mentioned gears did you mean the clutch or multiple speeds, i.e. the slew and the tracking speeds? A relatively slow slew (e.g. 2-5 degrees per second) shouldn't be an issue - we have the clutches and manual overrides 
Sorry to be a pain, but appreciate if someone could give me a more or less complete list of components and shields (starting with an Uno? Mega?) that people use to drive a stepper (Unipolar? Bipolar?) (e.g. Pololu A4983 has been discontinued).
Much appreciated!
Pololu A4983 has been discontinued
Yes but:-
Discontinuation Notice: This board has been replaced by the newer A4988 stepper motor driver carrier, which it is a drop-in replacement additional integrated protection.
So what's the problem?
some what related to this project was a sun tracking heliostat that this guy made
one of the problems he ran into with his project was the arduino handles doubles the same as floats as far a decimal places and he couldn't directly get the arduino to be accurate enough to on the fly calculate, in his case the sun's position at a set time. you might need to have the iphone handle this task.
the motors he used where the astrosyn 28bb-h151-11
I found some on jmayes.com on the deal sheet for 4.50$ea, I just received a order of 8 and the torque seems rather good, I can get around 24 in oz of torque in unipolor mode with one phase energized, the only issue is its 7.5 degree steps so you might need some gearing or maybe some microstepping.
during my testing I started running out of known weights like bottles of water, canned soup with a poptop, and a new 1lb spool of solder all hanging from some yarn.
it took some work on the conversion of fl oz to oz and then to convert the torque, from yarn on the shaft, into in oz. Goodluck.
I forgot to mention this guy used some relays to drive heliostat, which isnt a good design for the driver board.. you could make you own driver board with some mosfets, resistors, blocking diodes, and a heatsink for each phase. to drive them as unipolar