Over the past couple of months I have been designing a tracking mount that will allow me to attach my DSLR to my standard camera tripod allowing my camera to track objects at a precise sidereal rate.
The design of the mount is based on a Barn Door mount using a Stepper Motor driven lead screw, The motor drive system is based around 2 Microcontrollers 1 which is dedicated to controlling the display and control buttons and the other for controlling the motor drive technique. My Prototype was based on the very popular “Arduino Mega” platform.
Initially my original design was only going to drive the mount at sidereal rate and occupy 1 microcontroller. Unfortunately I soon discovered a couple of problems, one of which when I started to use interrupts to allow me to start and stop the mount at any point during its travel caused the motor to have an annoying tick every rotation which transmitted itself into the mount causing vibration, the other problem was that I was having to program lots of delays into my code to allow the display to reflect accurately what was happening and what the operator could do. I spend a fair amount of time working on how the Stepper Motor was going to be driven; the motor I am using has a 1.9degree per step resolution giving the motor 200 steps per revolution, although this sounds a lot because the motor rotates relatively slow this can cause vibration between steps, I spent some time figuring out a simple method that would allow me to micro step the motor and ended up writing my own library file. My Library file allows me to micro step the motor to a resolution of 16 PWM micro steps to 1 motor step, I also added some code to tell the motor to rotate at a specific RPM.
Maths: Okay so there are 24 hours per day, 1440 minutes per day and 86400 seconds per day. But these numbers are of no use as that is a Calendar day. The rotation of the Earth is very slightly quicker. Our sidereal day is actually 23.93446972 hours long; this gives us 1436.068183 minutes per day and 86164.09098 seconds per day. You may be thinking why does this affect us? Well if you are performing exposures that may run for several hours the last thing that you want to see on an exposure that has been running for a couple of hours is star trails. When you build your mount you measure everything very precisely and with the numbers above you can work out exactly how far you need your motor to drive the mount per minute or second.
I am adding a Lunar tracking rate to the mount and in time I will also have a Guiding facility.
I will be providing pictures of my Prototype mount and some of the results I have achieved in the next couple of weeks. Providing I am able to get a nice clear night 🙂
