Meade Autostar DS motors on a homemade DIY equatorial telescope mount
Category: Homemade Telescope Mount
Posted by: Tom How
Meade Telescopes did a great service to us crazy astronomers who like to build their own mounts when they produced the Autostar DS motors.
The system consists of two black plastic motor boxes containing the electric motor, various electronics, and a reduction gearbox. There is a central plug board into which you connect the two motors, the power and the handset. The power requirement is 12v (I use a PC power supply) and the handset is the Autostar 497 controller - the same one I use on my LXD55 mount.
There are two major advantages to the Meade DS motors. First, it is physically quite straighfoward to engineer something to fix the motors onto any type of DIY mount. Secondly, it is possible to adjust the speed settings in the Autostar 497 controller.
All telescope mounts have large reduction gears external to the motor gearbox - usually a worm-wheel and gear arrangement. This converts the fast electric motor to a the very slow speed / high torque drive requirements of a telescope mount. Depending on the various tooth counts and ratios of the worm gears, you have adjust the motor speed specific to your setup. A large worm gear means the motor must go faster to keep up with the sky than with a small worm gear.
It is up to the motor controller (the handset) to calculate the correct motor speed, and the joy of the Autostar 497 controller is that you can effectively tell it the size of your worm wheel reduction - meaning the motors can be used to run more or less any mount.
The added benefit: Once you've told Autostar about your mount, all the GOTO and autoguiding features will also work.
The real trick is mounting the motors properly on your homemade telescope mount. Most mounts are driven using a worm gear that turns a large worm wheel. The worm gear, which is usually 10-20mm in diameter needs to be driven by the motor output shaft. One option is to directly couple the motor output shaft to the worm gear - however, this is rather unstable and impractical.
It is much neater and more effective to use gears or timing belts/pulleys to transfer the drive from the motor output shaft to the worm gear. I use timing pulleys and belts as they are easier to tension and get correctly adjusted than toothed gears.
The key to a successful implementation of DS motors on a telescope mount comes down to ensuring both the worm gear and motor shaft are rigidly fixed with respect to each other. Unless both the motor output shaft and the worm gear are properly mounted with bearings and bushes, there is a chance of things bending and flexing - resulting in serious tracking errors at the telescope end.
For example, if the distance between the worm gear and motor output shaft on my mount changes about a millimetre during the course of one rotation, that translates into several tens of arc seconds error in tracking - an unnessecary cause of periodic error.
The images below show the parts I have made to connect my Meade Autostar DS motors to my telescope mount.
The first picture here shows the motor on the RA axis attached to the mount.
The second picture here shows a breakdown of the parts. I have made an anodised red round aluminium insert. This insert can be bolted to the worm gear block on the mount, and it has a thread onto which I can screw the Meade DS motor. The motor output shaft runs through the middle of this block supported by a bearing at one and and a bronze bush at the other end.
This picture shows the worm gear itself - mounted between two bronze bushes and spring loaded against the worm wheel. The motor is bolted to the sprung section.
The system consists of two black plastic motor boxes containing the electric motor, various electronics, and a reduction gearbox. There is a central plug board into which you connect the two motors, the power and the handset. The power requirement is 12v (I use a PC power supply) and the handset is the Autostar 497 controller - the same one I use on my LXD55 mount.
There are two major advantages to the Meade DS motors. First, it is physically quite straighfoward to engineer something to fix the motors onto any type of DIY mount. Secondly, it is possible to adjust the speed settings in the Autostar 497 controller.
All telescope mounts have large reduction gears external to the motor gearbox - usually a worm-wheel and gear arrangement. This converts the fast electric motor to a the very slow speed / high torque drive requirements of a telescope mount. Depending on the various tooth counts and ratios of the worm gears, you have adjust the motor speed specific to your setup. A large worm gear means the motor must go faster to keep up with the sky than with a small worm gear.
It is up to the motor controller (the handset) to calculate the correct motor speed, and the joy of the Autostar 497 controller is that you can effectively tell it the size of your worm wheel reduction - meaning the motors can be used to run more or less any mount.
The added benefit: Once you've told Autostar about your mount, all the GOTO and autoguiding features will also work.
The real trick is mounting the motors properly on your homemade telescope mount. Most mounts are driven using a worm gear that turns a large worm wheel. The worm gear, which is usually 10-20mm in diameter needs to be driven by the motor output shaft. One option is to directly couple the motor output shaft to the worm gear - however, this is rather unstable and impractical.
It is much neater and more effective to use gears or timing belts/pulleys to transfer the drive from the motor output shaft to the worm gear. I use timing pulleys and belts as they are easier to tension and get correctly adjusted than toothed gears.
The key to a successful implementation of DS motors on a telescope mount comes down to ensuring both the worm gear and motor shaft are rigidly fixed with respect to each other. Unless both the motor output shaft and the worm gear are properly mounted with bearings and bushes, there is a chance of things bending and flexing - resulting in serious tracking errors at the telescope end.
For example, if the distance between the worm gear and motor output shaft on my mount changes about a millimetre during the course of one rotation, that translates into several tens of arc seconds error in tracking - an unnessecary cause of periodic error.
The images below show the parts I have made to connect my Meade Autostar DS motors to my telescope mount.
The first picture here shows the motor on the RA axis attached to the mount.
The second picture here shows a breakdown of the parts. I have made an anodised red round aluminium insert. This insert can be bolted to the worm gear block on the mount, and it has a thread onto which I can screw the Meade DS motor. The motor output shaft runs through the middle of this block supported by a bearing at one and and a bronze bush at the other end.
This picture shows the worm gear itself - mounted between two bronze bushes and spring loaded against the worm wheel. The motor is bolted to the sprung section.
Andriy wrote: