Saturday, May 09, 2009

Friction Telescope Drive, a new design

I will detail the development of a new design that started out as simple way to achieve accurate telescope tracking for astrophotography, free of periodic error and backlash. By simple, I mean that the basic embodiment can be constructed by hand tools.
Initial tests exceed my expectations, even with known fabrication deficiencies.


Arcturus photo using 411 sec exposures. New drive design by Ed Johnson
High Wind Warning in effect and windy. Seeing = bad (see insert, short exposure crop)
Field 1.05ยบ high, North=down, Mag limit ~~ 14-15, ISO=100
11 day old moon not far away from target. Right image is contrast enhanced.

8 " f/4 scope, see
Closeup of drive lashup
Friction drive at 76.5 Hz and 0.500" drive roller = best (right photo)
Left exposure used 75.5 Hz drive rate.

Pointing and guiding is not easy at this time (keep unguided exposures <2min)
Wind showed the friction drive is very sturdy-rigid-stable.
Pole finder tube used - only.... recheck axis elevation
Next time, start off by taking Polaris with drive off, then on.
Next drive roller to be about 0.6375" for 60 Hz drive (adjustable accurate source (this part is a game, to see how close the mechanics can be made))
Next is to make it transportable and easy setup.
Motor=4RPM, gears 20:66 (15 / 20) * 66 = 49.5 sec @ input of 40:1 worm = 1980 sec (33m) rotation on drive roller. Roller ratio s/b 86164 / 1980 = 43.5171717
Small roller r s/b 13.88 / 43.5171717 = 0.318954552, Dia = 0.6379 for 60Hz
Measured r of drive arc sector = 13.88"

Friction drive efficiency is high, very smooth, many times stiffer than any other gearing system, has zero backlash and periodic error. Machining is very straight forward as only cylindrical surfaces are needed (
Large scope using a friction drive:
The Earth's rotation relative to the stars is called a sidereal day, it has a length of 86164 seconds (a Solar day is 86400 seconds)

Coincidence of Dimensions: 0.6379 - 0.6375 = 0.0004" = Rob's calculation from my by trial speed setting vs my calculation from moter speed\gears\roller sizes


Test photo link: