Dobson Telescope Drive

MatKub's picture

Introduction

        What was inspiring for this project were countless hours spend with the telescope in the open air. It is natural for amateur astronomers to aspire for more beatiful view in an eyepiece. Unfortunately appropriate equipment is very expensive. The best quality to prize ratio have the simpliest constructions, for example newtonian telescopes which are big and heavy. They are usually mounted on Dobson mountage, so they are not as handy as the ones equipped with equatorial mountage.

        The project's aim is to upgrade a Dobson mountage with an automatic drive. It would be steered from smartphone or PC (Stellarium). An custom remote analog controller based on Playstation or XBox pad is predicted. Project discuss the problem of tube positioning and calibration.

          Project files are stored in SVN repository. It consists of:

  • eagle PCB project containing schematics and PCB layout;
  • device's firmware as Eclipse microhal project;
  • mechanical solution photographs.

Mechanical Solution

        The object of our interest is Synta SkyWatcher 8'. It's mirror diameter is 200mm with a focal length of 1200mm. The original Dobson moutage had a few disadvantages. The most annoying one was static friction caused by plastic plain shaft bearings. On contrary the mountage is very solid, so it was a perfect candidate for multiple mechanical upgrades:

  • The round bottom slab was equipped with three bearings hence the upper is able to spin freely with no friction. Additionaly a roller bearing in the middle centers the upper slab. This bearing was mounted on amortising rubber so the elements fit to each other.
  • 20mm width and 10mm deep round channel was milled in the lower slab. There the cogged belt is glued. It forms a planetary gear for horizontal movement stepper motor.
  • For vertical driving a stepper motor with clutch and additional bearing was prepared. It drives an axle of 8mm diameter. It is mounted to the side slab and holds the tube through a worm gear. The worm gear was tapped in a textolite block which is pegged down to the tube.

        All mechanical work was done without any specialized tool. The most helpful tool was a standard mag drill with a cutter set. However good imagination was required to make it possible in house workshop. And a lot of patience :)

Electronical Solution

         As the above diagram shows the main part of the electronics is STM32F4-discovery board. It is widely known board which does not incorporates many peripherals. Hence it has many free pins awailable for custom use. Firmware runs on FreeRTOS operational system. Such decision was made, because of the project complexity.

Power supply

The device could be supplied by an optional accumulator which voltage does not exceds 20 volts and could sustain a load of few amperes. It could be for example an acid 12 volts accumulator or 3-4 li-ion 18650 cells. If the drive will be used in the open air it could be at last supplied by a car battery. To sum up - more than 10 volts should be provided of viability of few aperes.

Stepper motor driver

DRV8825 is responsible of handling stepper motors. It is fully automated driver with current regulation. The current could be set by DAC0 and DAC1 for each motors. The connection with MCU bases on GPIO ports steered by appropriate software.

Connectivity

Two communication channels are provided. Both bases on UART. The first one incorporates BT SPP link. A simple protocoll which is implemented on it allows most Android BT SPP aplications to handle the communication. The second UART could be connected via FTDI converter to a PC. In the future a Celestron Next star protocoll will be implemented on this link, but now for the purpose of interaction with the telescope an Command Line Interface was implemented.

 

 

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