by The Bolton Group

Version 1.2

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Contents:-
A Centring Device - guide star finding problem solved.
Crayford Focusers - backlash free focusing and electric too! Updated with drawing.
A Filter Wheel - manually operated filter wheel.

This device was invented by Gerald in response to my request for a moving eyepiece so that a guide star anywhere in the focal plane can be centred. The focal plane might typically be 2 inch in diameter and a guiding eyepiece has a field lens of less than 1/2 inch so there is plenty of scope for moving around.

At first glance you might think you need to move in two directions but by making the head rotate together with a simple slide, all parts of the focal plane can be reached. With this device finding a guide start is a doddle - you just rotate and twiddle the knob and a star moves to the centre in seconds. Two inch diameter ones are available ex-stock at £100/$170.

3.5 inch Crayford

The Bolton Crayford

Internal Crayford

Introduction

We use these focusers exclusively now being far superior to rack and pinion types. They have absolutely no backlash and can hold heavy loads such as CCD cameras with ease. Our design has evolved from the original concept which held the drive rod against the draw tube by springs to a more robust solid hinge design. Pressure to the rod is applied by means of twin vertical thumbscrews. When these are tightened into the baseplate, there is enough movement in the solid hinge to force the drive rod against the drawtube.

Construction

The Crayford design requires 4 small bearings on which the tube runs. These bearings are expensive to buy new but can be obtained for next to nothing from scrap 3.5 inch floppy disc drives. These are often thrown away by computer maintenance companies. You will need to get several drives before you find 4 identical bearings but at this price (next to nothing!) that's no problem. Check out your local PC maintenance company - they will be glad to get rid of them!

The two focusers here have been fitted with electric drives for CCD work. They are connected by means of worm gears which provide a huge reduction - one stab on the drive button moves the drawtube 1/1000th of an inch. This is too low a gearing for visual work. They are however both fitted with a single knob and as the wormwheel has a small friction clutch, they can be turned easily by hand. The 3.5 inch one has a giant knob which makes manual focusing a doddle.

The last Crayford shown was made by Brian for his 5 inch refractor. It fits internally to the main tube so is largely invisible with just the two focusing knobs protruding plus a tension (compression really) adjusting knob underneath.

Inside view

This filter wheel was built by Gerald several years ago before they were widely available commercially. Judging by what prices they now go for, making your own is still good sense. With CCD work it is impractical to change filters by unscrewing the camera when taking tri-colour exposures. Disturbing focus and pointing is not a good idea and an in-line filter wheel overcomes that problem very simply.

The filter holder is nothing more than a light tight case in aluminium with an internal wheel holding the filters. It is manually operated by means of an external arm. This is simply rotated to align with the appropriate colour dot. A spring loaded ball-bearing engages is a hole to align the filter precisely. It adds about 1 inch to the light path to the camera.