FOCUSER
The purpose of the focuser is to provide a quick mounting for the eyepiece, and to be able to adjust sharpness for different eyepieces. This would not be a difficult task, if not for the fact that the focuser and primary mirror must be precisely collimated, and stay collimated whether the focuser is operated or not.
There are five types of focusers, helical (rotating and non-rotating), crayford, rack-and-pinion, drawtube and the sled focuser.
Helical : The helical focuser is a rotational focuser. The image is brought in-focus by turning the eyepiece holder. The best way to use this kind of focuser, is to make all your eyepieces parafocal, i.e. to put a stop on your eyepieces so that when they are inserted in the holder they go in up to the stop. After focusing once, you can then change the eyepiece without having to do a lot of adjustment. The major advantage of this type of focuser is that it can be very short.A variation of the helical focuser is constructed so that a rotating ring moves the eyepiece holder in and out. Much like the manual distance setting on a photo objective.
Crayford : Here the eyepiece holder is pressed by a wheel to four bearings. When the wheel is turned, the holder moves in- or outward. The wheel and the holder are coupled by friction only. This type of focuser can be very stable and operated without backlash.
Rack and pinion : This focuser looks very much like the Crayford, but the coupling between drive wheel and holder uses a pinion gear and a teethed rack. This type can be very stable, but often experiences backlash.
Drawtube : Here the eyepiece holder is pulled or pushed by hand. It moves inside a slightly larger tube and is hold by friction between the holder and it's enclosing tube.
Sled : Here the eyepiece holder and secondary mirror are mounted together on a 'sled'. Focussing is done by moving the sled to and from the primary mirror. This is probably the most demanding type of focuser, and should only be build for specific purposes by knowledgeable amateurs.
There are also two sizes of focuser, 1.25" and 2" diameter. The smaller focuser can cause vignetting especially when used for low f/# value's. As an approximation, the focuser should have a lower length/D ratio as the mirror in order not to cause vignetting.
The main requirements for a good focuser are: a linear path of travel along the optical axis of the eyepiece, no sagging when loaded and fully extended. This is especially important when making photo's because of the weight of the camera.
When the focuser is fully racked in, it should not cause an obstruction. It is advisable to make the focuser as short as possible, since a long focuser would need the focal plane to be further from the optical axis of the primary. Then you would need a bigger secondary, and thus have more obstruction. On the other hand, a short focuser needs a bigger area to be 'baffled' opposite it. This is especially true for scopes that use the truss tube design.