After purchasing a Canon EOS 1100D recently, I decided it was time to upgrade my 8” observatory scope to get longer guided exposures. Although I’ve been using it with my Sac 7 CCD, I was only taking short unguided exposures for confirmation images of comets .
Here’s what I‘ve upgraded ( See images )
1. Turned up a new friction drive, with knurling to get more positive grip to the R.A wheel. The only down side is that when I changed speed with the observatory control switch, a small wobble was noticed.
2. Upgrade observatory controls, with the two speed switch introducing a wobble when switching, I decided to wire in momentary bypass button, this is pressed when changing speeds to keep constant current to the drive motor. Then on one side I wired in a rheostat, for variable control. This works well, don’t know why I did’nt do it years ago.
3. Upgrade focuser, this is actually a helical focusing segment from an old practica zoom lens, it had terrible optics, but the housing was well made, I just had to turn up a housing for the eye piece and and put a locking ring on. The whole unit is only 50mm high and gives me 30mm of focus.
4. Guide scope, The guide scope is a 300mm mirror lens, with the Sac 7 CCD attached, this gives me approximately the same field as the Canon 1100D at prime focus. While the optics in the mirror lens are no great shakes, it’s only for guiding.
Now all I need is some clear skies to test it all out.
Justin. Is that a home made friction drive EQM. If it is, I love it. At a guess, I'd say that tracking is very accurate. Would love to build something similar.
Yes the friction drive is home made. The R.A. wheel has a stiff dimpled rubber glues to it and the drive is spring loaded to place pressure on the R.A. wheel. There's a cable release for disengaging the drive.
The motor itself is a crouzet one revolution per hour 12v DC synchronise timer. I picked this up from a second hand shop years ago.
The main thing in this design is to have good balance, so as to not put too much strain on the gear box.
I recently made the same drive for the 16" scope, originally I had a chain drive with an extra worm wheel drive inbetween the motor and the friction clutch. Too much back lash and torsional forces!
So I simplified it to the straight friction drive, less things to go wrong in the drive chain. The drive is reasonably accurate, but does need some controlling, hence the rheostat on the observatory scope.
I designed a small EQM a while back. Driven by a stepper motor timing belts and a planetary gear box. Eventually, I decided that a larger disk version with friction drive would be more accurate.
