Projects In Metal

Starship Part 2

One of the critical parts of a telescope is the eyepiece focusing arrangement. In my telescope, I am using a Crawford style focuser, that I constructed from 6061-T6 aluminum alloy. If you look close, you can see the 4 tiny ball bearing assemblies that are supporting the back of the focusing tube. The eyepiece tube is being tensioned against these bearings by the focusing shaft visible on the front side of the tube. This is a very simple arrangement that beats a rack & pinnion style focusing system, as the eyepiece tube has zero lash other than back and forth as is desired. The focusing shaft is turned by the focusing knob that is mounted at the end of the focusing shaft, at the very top of the telescope.

Initially, I built the focuser with a focusing knob that had a 1 to 1 ratio. Once the telescope was completed, and I was using the telescope while observing the night skies, I realized that a ratio of a 1 to 1 did not cut it. Back at my drawing board, I came up with a new design that I felt would fix the focusing problem. The focusing knob that you see in the photos of my telescope is quite unique. This focusing knob uses a planetary gear drive that has a final drive ratio of 6 to 1. In other words, one complete revolution of the focusing knob moves the focusing shaft 1/6th of a revolution.

This new focusing arrangement is everything that I had hoped for and then some. You may find it interesting that the planetary gear train that I am using in the focusing knob assembly, was sourced from a tiny cordless screwdriver with a burnt out motor.

It goes without saying that your telescope is only as good as the eye pieces being used. The selection of eyepieces that I have for my telescope are built by "TELE VUE" and are very good. I have them in various magnifications and are selected based on what you will be observing.

Another crucial factor in a telescope is Collimation. Collimating a telescope is lining up its optical components (lenses, mirrors, prisms, eyepieces) in their proper positions. This should be accurately done, or else the image quality will suffer. The main, or primary, mirror is the large mirror at the bottom, with a concave, aluminized face figured to an extremely accurate paraboloid surface. It concentrates the light from a star into a sharp image - not really a point, but a diffraction pattern with a small circle of light surrounded by small, faint rings.

My mirror is fastened to the mirror cell with silicon. The mirror cell rests on 3 set screws. By adjusting these screws, you can finely adjust the tilt of the main mirror, this is an important part of collimation. Once collimation is complete, these set screws are locked.

The smaller mirror with an elliptical face is suspended by the ring spider, and the face is at 45 degrees to the focusing tube. It is used to deflect the light from the main mirror sideways, so that you can see the image without having your head in the way of the incoming starlight. The secondary mirror holder that I described as a ball type mount is adjustable by unlocking a set screw and allowing the mirror to be tilted or rotated slightly as required. Poor collimation...... continued in Part 3....

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