When I built my telescope in 1995, I had already planned to implement Mel Bartels' System to computerize a dobsonian telescope . I finished building the electronics 2 years ago, but could not find the time afterwards to make the necessary modifications for the mount. Only now (April '99), after building my observatory , I managed to implement the drive. I want to use this space also to thank Mel for the tremendous work he has done for us fellow ATM's by providing this magnificent drive system. I also want to thank him for his neverending patience in helping me and others to setup the system. I hope I can pass on the lessons I learned to others in the future.
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The groundboard is a 20mm thick plywood circle with a diameter
of 50 cm. It is attached with 4 screws to the concrete pillar of
the observatory. (Click on the small icon to see larger image) |
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The RS-components 1:250 gear-reducer and the 4.1V
/1.1A 6-lead Nema 23 stepper motor are mounted directly to the
ground board. (Click on the small icon to see larger image) |
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The two 32mm ball-bearings, attached to the groundboard via 10mm
shafting, and the drive wheel form an equilateral triangle. (Click on the small icon to see larger image) |
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I initially used a rubber friction drive wheel for both azimuth
and altitude drives, but replaced those with steel ones as the
rubber wheels introduced unpredictable PE. (Click on the small icon to see larger image) |
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For the altitude drive I replaced one original 20 cm trunion
with a 40cm plywood disk rimmed with aluminum. (Click on the small icon to see larger image) |
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The motor and gearbox are attached to the rocker with a plywood
bracket. I sandwiched thin strips of foam between the bracket
and the mount to avoid the transmission of vibrations. (Click on the small icon to see larger image) |
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Since the tube assembly is relatively light, I need to
artificially tense the altitude trunion and the altitude drive
wheel. For that purpose I installed a chain and spring
tensioner. That simple device avoids slippage between the two
drive wheels. (Click on the small icon to see larger image) |
  
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Following an advice found on Chuck Shaw's excellent page , I installed
a device to always keep the tube centered between the sides of
the rocker. It consists of two small ball bearingsmounted at
right angle to the altitude axis. To support the bearings on the
rocker I drilled vertical holes in the rocker, mounted the
bearings on screws and wrapped teflon tape around the screw to
achieve tight fit to the hole. (Click on the small icon to see larger image) |
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I use my old 386SX Laptop equipped with a 387SX (donated by Jim
Burrows) mathematical coprocessor. The display is covered with
red filter. The computer has no CD-Rom drive, but I installed a
partial version of GUIDE on the 40 MB harddisk, as described in
the GUIDE FAQ available on the Project Pluto
home page . (Click on the small icon to see larger image) |
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 To
adjust the microsteps I glued a dial to the back of the motors.
That made it much easier to identify which steps are to wide or
to short. The same dial also provides a PEC mark to synchronize
the motors precisely. Due to the high power consumption of my
motors and low computing power of my Laptop, I need to use PWM
values of 38 downwards. Therefore I do not get a microstep
resolution as fine as I would like to, but thanks to my very
small microstep size on both axis (~1.3 arcsecond/microstep)
tracking is still very smooth. Since I run the system without
batteries, I can afford a higher power consumption than a field
user. For slewing and tracking my motors consume about 0.2 Amp
each.(Click on the small icon to see larger image) |
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