The simple sight tube

This is a piece of tube with a peephole cap. To center the secondary mirror, the length is not very critical. If the length is close to the inner diameter of the tube times the focal ratio of your telescope, you could also center the main mirror within the inner opening of the tube. Here is how to make one.

You cannot, of course, see the inner opening and the mirror edges perfectly sharp at the same time, but the small peephole will improve your "depth of focus" and keep the unsharpness tolerably low.

To center the secondary mirror (step 3) you can hold a strip of white paper (or a piece of cardboard) inside the tube, below the secondary mirror (with a truss tube, you can use the lid of the mirror box!). You see the whole of the secondary face reflecting the white paper - without it, the outer rim of the secondary will reflect the dark inside of the telescope tube around the main mirror, and what you see is the edge of the main, not the secondary, mirror. Pull the tube in or out to make the secondary appear barely smaller than the tube opening, and lock it with the focuser locking screw. Move the secondary (try to avoid changing the tilt) to center it accurately - or at least to within acceptable tolerances. To center it "sideways", you may have to move the spider vanes - if this is not possible, shim the focuser sideways instead.

To center the main mirror (step 4), take away the paper strip (or take the lid off the mirror box) and push in the tube enough to see the main mirror reflected within the secondary. Tilt the secondary (try to avoid moving it) to center the main mirror inside the tube opening, just as you centered the secondary.

To aid in checking tolerances, you could put small protrusions inside the tube at its inner end (I have used strips made from the tubing, 2 mm high and slightly wider and longer). These act as tolerance gauges - if you divide the height by the tube length you get the angle in radians (multiply by 57.3 to get degrees) that the gauge is seen by. My f/6 tube is 168 mm long and the gauge is 2 mm high, the angle is 0.68 degrees (appr 2/3 degrees). If I have centered as shown above, and the black rim is almost gone at one side and as high as the gauge at the other, then the error type 1B is half of the angle, or 1/3 degree - I believe this is a very small error for visual use.

To judge the centering of the secondary, you can multiply the gauge height by the ratio (secondary size)/(inner diameter of peephole tube). If secondary size is 84 mm and inner diameter of tube is 28 mm, errors should be multiplied by 84/28=3.0. In the illustration above, error is half the gauge height times the multiplication factor - that is ½*2*3 mm=3 mm.

On some telescopes, it may not be possible to center the mirrors as described. If the focuser is high and the focus is far from the tube and/or the secondary is too small, you cannot see the edge of the main mirror inside the edge of the secondary.

On the other hand, if the drawtube is too long and/or the secondary is too big, you cannot see the edge of the secondary to center it. In both these cases the basic design is flawed (unfortunately this is rumored to happen sometimes with commercial telescopes), and if you are an Amateur Telescope Modifier you might consider modifying it. Otherwise, you may use a crosshairs tube as described below - your telescope may give a very good performance, nonetheless.

Date: 2015-12-11; view: 796