FOCUSSING UNITS.

Rack and Pinion Type.

These were the first focussing units I made, using a crome plated brass tube with rack teeth cut in it and a spindle with teeth milled in it. The tube moved on teflon strips. I will not go into detail on these as it was of the conventional type. The drawing below illustrates my design. I only mace a few of these. The rack and pinion was not as smooth as I would nave liked. I belive this was caused by the hard chrome irreguianties in the rack that would not wear in. Also I had difficulty keeping the teflon strips in place.

Helical Focussers.

I was told when I attended an Astronomers convention that Helical Focussers were the "in Thing". They were low in profile and allowed the eyepiece to be positioned closer into the tube so that a smaller diagonal mirror with less obstruction could be used. Now if it allows the diameter of a 32mm. minor axis diagonal to be reduced to 30mm. here .s a reduction of 28.2 sq. mm This repesents 0.0015% of a six inch mirror area. Can the human eye detect this difference in image brigntness ? I very much doubt it. However I made up some 1-1/4" and 2" helical focussers which are illustrated below. The body was cast aluminium and painted while The rest was machined from aluminium tube and black anodised.

On my small lathe, 48 different threads can be selected just by moving levers. However I use 26 TPI brass thread in the faceting machines I make as I need a fine thread from ¼" up to 1 ¼" To get this number of threads requires different gearing and the lathe is set permanently to cut 52. 26 and 13 threads per inch. As I do not like changing the gearing, I used 13 TPI. For the focussing units. This is far too tine for this application in my opinion. A draw tube was fitted and I reconed to slide this in and out until the focus was found and then fine tune it with the heilcal adjustment. However I found that I could focus well enough just by sliding the draw tune. The screw adjustment was not necessry. If the mirror is correct the image snaps in and out of focus There is no need to screw the eyepiece in and out trying to decide the best focus.

I was told that a unit that did not rotate the eyepiece was best. Such a unit is more difficult to make. I can make a unit with the eyepiece accurately centered and which moves in and out very smoothly with no wobble. The thread was free turning and when greased had Just the right drag. As the thimble covers the thread there was no risk of the grease picking up dust. Below are illustrations of a 2" and 1 ¼" units.