Another quick write up of a GSO 10:1 focuser tweak I did
this afternoon.
This is a 2nd hand unit, bought from here on IIS, (thanks Peter!)
This unit had a few minor problems that I solved by having a look
inside the 10:1 mechanism.
It was very 'grindy' and had a wobble on the 10:1 knob, possibly
from a knock or drop
It also had a tension screw missing. I made a replacement on the lathe.
How the 10:1 mech works:
The 10x knob rotates between a 3 way pressure point of 3
ball bearings, which in turn, roll on the inside face of a larger aluminium
wheel (the RHS 1x knob)
Obviously the 10:1 ratio is from the ratio of the diameter of the ball bearings and the
10x shaft size.
You can see that the pressure/ contact point of the balls has left an indent
on the 10x shaft. I'm not sure if this is original or if it has worn in this way.
I'd guess it is original, and the idea being the curve enacts more surface contact
on the ball bearings. If it was worn this way eventually the 10x knob would just
slip. It also would prevent the 10x knob from slipping out sideways if tension
was lost.
Anyway, the 10x shaft is approx 2.47mm in diameter and a 3/16" pop rivet
shank worked as a replacement. (2.45mm)
After heat treating it (to harden the metal) I polished it with some Brasso and
fitted it to the focuser, It now has no pressure point curve. I'll soon see
how that goes with time.....
To fit it all back together I followed the reverse order of the disassembly
making sure I pre-loaded the bearings until it was driving smoothly through the
10:1 mechanism. Not too tight...not too loose...a wavy spring washer holds the
pre-loading nut where you leave it....it won't loosen by itself.
This unit has better orthagonality than my other 10: 1 (
see here how to fix that)
Back together and it was fitted to my 8" F7 Homemade Newt (now my son's scope)
Works lovely.
Steve
