ATM tune-up 17.5inch Dob

[Adam Stephens <adamstephens@altavista.net>]

I've just done a tune-up on my 17.5inch Dob, with somewhat mixed results.

Visual results are excellent so far, with the best view of Jupiter/Saturn
I've had with this scope.

The good bits

1. Added a mirror cooling fan. Looks as though it works well and the image
while running is vibration free at x200 
2. Added below the focuser and across from the focuser baffles.
3. Wired the scope for 12v, for fan and secondary heater.
4. Replaced centre spot -ring binder- hole with custom made hole. The hole
centre is about the same diameter as the whole old hole.  I was sure using
a Cheshire, that collimation accuracy could be improved if I could see the
whole of the centre hole of the Cheshire within the centre spot.  I can now
align with a Cheshire much more accurately.  This is highly recommended if
you can follow me.
5. Simple visual indication of which way to turn collimation adjusters
incorporated in centre spot.
6. Focuser made adjustable for squaring on.

The bits that need more work

7. Reduced included angle of the alt bearing Teflon from 145 deg to 70 deg.
Scope is now easy to move in alt, but very sensitive to balance.  Adding
weights to base of top face of mirror box makes things unpredictable as the
moment arm changes with alt angle.
8. I suppose by fan cooling the primary mirror quickly, it has become
susceptible to dew.  It was dewed up sufficiently to give up w/i 4 hours.

So, having cooled the primary, how do you heat it to stop dew?

Adam Stephens
adamstephens@altavista.net