Thats a very good article.
It covers all the important BASIC issues very well. It does neglect one step which I consider important. Particularly, if you use the scope for long exposure photography, or require "PINPOINT" pointing accuracy from your digital setting circles. This involves making the focuser axis coincident with the mechanical axis of the telescope, prior to centering the secondary mirror under the focuser.
This is what you need to do:-
Part 1:
Firstly remove the focuser completely off the tube. Lay the tube at 45 degrees or lower, so if you drop a screw, nut or spanner, it slides down the inside edge of the tube and doesn't hit the mirror. Measure around the outside of the telescope tube from the edge of the focuser hole on each side. Add the radius of the focuser hole to this number. Write the number down. Measure this distance around the tube. Mark the tube with a line, lengthwise along the tube. This line should be directly opposite the focuser and coincident with the mechanical axis lengthwise down the tube. The line needs to be about 70 to 100 millimetres long, use a steel rule and a whiteboard marker so you can remove it later.
Part 2:
Measure from the top of the tube on the focuser side, to the closest point of the focuser hole, along the "approximate" central axis of the tube. Add the radius of the focuser hole. Write this measurement down. Measure "down the line" on the opposite side of the tube this distance. Put a little Mark across the line at 90 degrees. The intersection should now be a point directly opposite the central axis of the focuser. At this point 1mm accuracy is fine. Now drill a 1mm or 1/32nd hole in the tube to "permanently" mark this point on the telescope tube. Yeah Yeah!!! I can hear you all cringeing. It won't hurt anything. I remove all the optics from the scope whenever I drill it and hold a magnet at the exit point of the drill, to catch any filings. It has to be a very small hole that you can see on close inspection, but not big enough to let any stray light in. If your tube is "flocked" all the better, as the flocking material "conceals the hole and stops stray light. My tube is flocked and I find the hole and insert a 1" x 1mm nail through it to "mark" the hole, when I need to reposition the focuser after removal.
We now have a small hole marking a point directly opposite the central axis of the focuser, which is also coincident with the mechanical axis of the tube.
Part 3:
Remove the secondary mirror and holder. Remount the focuser without overtightening the screws. Move the focuser to the "centre" of its travel position. Put a laser into the focuser and then "wiggle" the focuser to get the laser aligned on the hole. Once you get it right start to tighten up the screws a little more. Now move the focuser the full length of its travel and the laser "SHOULD" stay centered on the hole in the tube (Ha!! Ha!! bet it doesn't). You then need to work out which corners of the focuser are high and low and shim the focuser near the appropriate bolt with small thin pieces of cardboard or plastic. Thin plastic is the best because it is fairly stiff and you can force it into position without having to loosen all the focuser mounting bolts. After a lot of "trial and error" the focuser should be able to be moved over its full travel and the laser will remain fixed on the hole on the opposite side of the tube. This means the central axis of the focuser travel is perfectly aligned to the mechanical axis of the telescope. You now tighten the focuser mounting bolts right up.
Part 4:
Put the secondary back in the telescope. Using a sight tube centre it perfectly under the focuser. Two things to adjust. The secondary distance from the primary so it sits right under the focuser and also the "rotation" so it's face is at 90 degrees to the focuser central axis.
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Alternatively you can follow the instructions at the beginning of the article provided with the cats-eye collimator. It is the very 1st section titled "The focuser should be square to the tube axis" and the second section titled "the diagonal should be centered on the tube axis".
http://www.catseyecollimation.com/Co...e%20-%20R1.pdf
Whilst these initial mechanical alignments don't have an effect on the optical performance of the telescope, they can significantly affect it's pointing accuracy when using digital setting circles and a high power eyepiece. Whilst time consuming, these things normally only have to be done once, if you don't continually pull your telescope apart or remove the focuser. This is the reason many manufacturers of high quality focusers, also produce a collimateable base plate.
CS-John B