The blog of 'The Arduino Guy' aka Mike McRoberts, author of Beginning Arduino.

04 October 2013

Telescope Focus Controller - Part 1 - The Idea.

One of my hobbies is astronomy and more specifically, astro-photogaphy. I have a pretty sophisticated setup with a computerised GoTo mount with a cooled CCD camera, motorised filter wheel, guide camera, etc. Most of the gear is automated and can be controlled remotely (i.e. it is out in the cold while I sit in the 'control room' indoors and operate it. however, the one thing that still has to be operated manually is the focuser. I have no choice but to focus the kit out in the cold then come indoors. Not only will automating the entire kit make it more comfortable to image when i'm at home, but it will also save a lot of time and time is precious when doing astro-photography, especially in windy and rainy old England.

Me & my astro gear - Brecon Beacons Astrocamp, Sept 2013
Now a commercial motorised focuser isn't cheap. They can cost a lot of money and most of the cost is unnecessary. After all, it is simply a stepper motor attached somehow to the focus knob and then controlled with a 'box of tricks' connected to a PC. So in the hacker spirit I decide to make my own. I had an old 3.15v Sanyo stepper motor somewhere that I scavenged from a printer or scanner or something so decide to use that. I also had an Easy Driver 3.1 board (i've also got Adafruit motor shields and the official Arduino Motor Shield).

As the completed product needs to be small and light I went for the Easy Driver motor control board as it is tiny and easy to use. 

So far I have hooked it up to the motor and have it stepping in tiny 1/8th microsteps. The next stage is to create a control box with knobs and buttons to select the motor speed and to focus in and out (Stage 1). This will also have a USB interface so I can connect it to a PC and control it from software (Stage 2). This will have a temperature sensor so it can automatically compensate for the changing temperature throughout the night as the telescope cools down and heats up. 

Once I can control it from a PC I can then operate it remotely over VPN and sit in the warm whilst imaging. 

Stage 3 of the project will be to write some ASCOM complaint drivers for the device so I can hook it up to either FocusMax or my imaging software (APT & MaximDL) so the focussing procedure can then be fully automated.

As you can see in the picture I am providing the motor with 6.2 volts. The motor is a 3.15v motor. However, that's 3.15v per coil and so double that is needed to get it going. Powering it with just 3.15v volts and it goes nowhere. When running continuously it was using about 500ma. 

Next I am going to make a hand control unit. The plan is there will be a knob to select the step speed and number of steps. Then buttons for focus in/out, another for selecting continuous operation or one iteration of number_of_steps and another for selecting either speed or steps. This will be displayed on either a 7-segment display or an LCD. 

Below is an example video showing the motor in micro-stepping mode. it is doing 1/8th micro-steps controlled by an EasyDriver 3.1 motor control board. The motor is a 3.15v Sanyo stepper motor salvaged from a printer or scanner. I am running it at 6.2v (3.1v per coil) and have dialled down the ED3.1 so it is being given no more than 100-150mA. Any less and it stalls and any more and it gets too hot. This current is enough to give me the torque I need. Next I am prototyping the circuit and testing it out. I will post an update soon.




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