ETX TECHNICAL TIPS Last updated: 12 May 2004

Improved focus control for ETX 125

Sent:	Tuesday, May 11, 2004 11:38:43
From:	"Mike Barrett" (mm.barrett@btinternet.com)





After being slightly disappointed with the performance of a flexible
focus extension I tried, I made a refinement of the old "clothes peg"
idea, which uses a simple clothes peg clipped to the existing focus knob
to provide smoother control and finer focussing due to the extra
leverage and much larger apparent diameter of the focus knob.

I turned a new knob from 1" dia. aluminium rod, reduced to 5/8" at the
end closest to the OTA so that a " Terry clip will fit over it to form a
friction clutch. The operating lever was made from the plastic handle of
one of my wife's discarded make-up brushes - it needs to be light to
prevent the focus assembly rotating due to the weight of the lever when
it is in the horizontal position. A more rugged version for field use
could be made by pushing a length of stiff plastic tubing over a 1" long
screw passed through the hole in the clip and fixed with a nut. A white
lever is easier to locate in the dark.







Once the new assembly has been fitted in place of the original focus
knob, coarse focus can be achieved by turning the knob as usual, while
supporting the tip of the lever in a fixed position with the other hand,
or just letting it rotate until it touches the rear extension of the OTA
and starts slipping. The grip of the Terry clip should be adjusted by
careful bending until it allows the lever to slip during coarse
focussing, but grips firmly enough to rotate the knob without slipping
when the lever is used.

In use, the lever is controlled by gentle pressure with the side of the
finger (there's no need to grip it), so very little vibration is
transmitted to the scope, and the effective 6" diameter turning circle
of the lever permits extremely fine adjustment (ideal for CCD!). If the
OTA is swung between the fork arms to the zenith position, the lever is
automatically turned as it strikes the fork and although focus may be
lost, after resetting by turning the knob, the lever still gives about
40 degrees of adjustment.

Mike Barrett

Hertfordshire, U.K.

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