Doubling the precision (or more) of the focusing of an SCT.

 Doubling the precision (or more) of the focusing of an SCT in a non invasive way

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Since I made this post, a friend 3D printed a knob to fit on the focusing knob of our Skymax 127 Maksutov. This has increased the precision of the focusing by a factor of 2.63, which will make focusing a lot easier.

3D printed knob for the Skymax 127

Another friend 3D printed a knob adapter for our 8" SCT

This will give the same increase in precision as the one on the Maksutov. Focusing is going to be so much easier from now on.

One of the problems with SCT and Maksutov focusers is that it is so easy to go through focus as you turn the focusing knob at the back of the scope. One solution is to add an external focuser to the visual back. This can be of a rack and pinion type or a Crayford type. The external focuser can be of a motorised type if required. Either way, such focusers can be problematic if you are using a focal reducer/field flattener on the visual back. This is because for optimal performance, the focal reducer should be a constant, optimal distance from the camera. This is not possible with an external focuser.

We have found that there is a simple way to double (or slightly more) the precision of the stock, mirror-shift focuser. The focusing knob at the back of an SCT is normally quite narrow. On our 8” Celestron Nextstar SCT it has a diameter of about 21mm giving it a circumference of about 66mm. This means that a point on the focuser knob moves through 6.6cm to make a complete revolution. If the knob had a larger diameter, the point on the knob would have to travel commensurately further to complete a revolution. This means that the greater the diameter of the focus knob, the finer the focusing precision. The circumference is directly proportional to the diameter by the relation:

C = π D

Where C is the circumference and D is the diameter.

We found a piece of PVC tubing that was a snug fit over the focusing knob. The tube was cut to length and a plastic gear wheel of 41mm diameter was attached with araldite to one end of the tube. The tube can be pushed onto the focusing knob and the gear wheel used to turn the focuser.

We find that this increase in the diameter of the focuser knob gives us significantly better control of focusing and makes it less likely to go through focus too quickly.

It can be seen from the images that there is the possibility of using an even greater diameter gear wheel, commensurately increasing focusing precision. Using a gear wheel allows for easily gripping the focuser, and also adds the intriguing possibility of motorising the focuser.