Last updated: 12 May 2004
Sent: Tuesday, May 11, 2004 11:38:43 From: "Mike Barrett" (firstname.lastname@example.org)
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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