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The laser collimator

WARNING:Never look at any laser beam directly! Normally, the beam will stay within the optical path of the telescope, but if it is badly out of collimation, the returning beam may escape beside the secondary. Do not look down the tube if you don't see it return as expected - instead, try picking it up on a piece of paper in front of the tube.

This is a solid state laser, mounted in a tube to fit the eyepiece holder, and critically collimated to make the laser beam centered in (1A) and parallel to (1B) the tube. The "inner" end has a bright face where you can see the spot of the returning beam, with a small hole for it to exit. It can be used in steps 1, 3, 4 and, with reservations, step 5.

The laser collimator can be expensive and impressive, and favored by many. It makes step 4 very simple, as you can stand by the tube opening and easily see where the beam hits the main mirror while you adjust the screws on the secondary holder. Seeing where the beam returns can be harder, particularly if the returning beam has been focused by the main mirror to make the spot smaller than the exit hole.

Some laser collimators are sold with holographic attachments, that project a reticle that covers a fairly wide angle. This helps in step 3, where you can see the centering of the reticle on the secondary, and possibly also step 4 and 6.

Potential problem using a laser to collimate the main mirror:

After step 4, if the laser beam is off the main mirror center, the return beam will be parallel to the optical axis, but displaced (by the same amount) at the return. If you then adjust the main mirror to center the return beam accurately, you unwittingly miscollimate and get a 1A error (of half the displacement)! For instance, the strict tolerance for a f/4.5 mirror is only some 1/3 mmor 1/64 in., so the allowable error in step 4 should be much less (than 2/3 mm or 1/32"). With many telescopes, it is not really possible to read the position of the spot (or even adjust the secondary holder) to anywhere near the required precision. On the other hand, if you allow a type 1B error of 0.2 degree (assuming a 70" = 1.8 m focal length), the spot can be 1/4" or 6 mm off!

For this reason, if you collimate step 5 with a laser, you should check with a Cheshire or star test that the collimation is acceptable, or else tweak it - but in the section on Experimenting, I have described a workaround!

You can make your own laser collimator from a cheap keychain laser pointer.

The autocollimator

The name may suggest that this is a device that will automatically collimate your telescope - sorry, it won't. It is another development of the simple peephole tube, with a mirror inside the cap. There is a transparent spot in the center of the mirror, used as a peephole, and the mirror is set accurately perpendicular to the tube (there are other devices called "autocollimator" too - if you search the net for more info, don't be confused).



It is used (after step 5) to check the alignment of both mirrors. If the main mirror is accurately perpendicular to the axis of the autocollimator, what you will see in the peephole is your own eye pupil reflected, and infinitely magnified, via the secondary - primary - secondary - autocollimator - secondary - primary - secondary mirrors.. That is, the autocollimator mirror appears entirely black. Otherwise, you can see reflected light from the bright sky/ceiling via multiple reflections. However, the autocollimator, like a laser, does not distinguish between type 1A and 1B errors - a "black" reading means nothing unless the Cheshire shows the main mirror spot is centered within tolerances. It can help you to get the relatively unimportant 1B error vanishingly small (although a laser collimator is far more convenient for this), but it does not detect the crucial 1A error (that still must be determined with the Cheshire) - in particular, if you do a laser collimation and make the beam return, the autocollimator will go black regardless of where the beam hits the main mirror (or else the focuser is likely at fault), and in this situation it will not detect even a gross 1A error.

The finer details of using the autocollimator are given in the booklet "Perspectives on collimation" by Vic Menard and Tippy D'Auria (the authors do not distinguish between the errors called 1A and 1B here, and make no attempt at analyzing the tolerances involved). If you make one yourself, make sure the mirror is very well squared to the tube. I would not recommend its use (with one exception - you can make a simple but possibly useful autocollimator for wide-field photo).

The centering mask

Take a piece of paper or semi-transparent plastic, large enough to fit over your tube opening. Make a circle exactly as large as your main mirror, and center it over the tube opening. This is useful in step 6 above, when you want to check the centering of the optical axis within the tube. If you have decided on a maximum allowable error, multiply it by 2 and draw a circle so far outside the first circle (if you like, you can cut a hole inside the first circle). Look inside the sight tube (any kind) - if you can see the inner circle or cutout, the optical axis is not exactly centered, but as long as you cannot see the outer circle, the error is acceptable.

Is this the end of the FAQ?

Yes, unless you want to go to the separate section about some special situations and the DIY projects - you may have followed some links already.

Special situations are:

  • A workaround to get more accurate collimating with a Laser collimator - the Barlowed Laser
  • Collimating the main mirror without a center spot
  • Collimating a rotating secondary cage
  • Auto-collimating for wide-field photo

Thank you for staying with me so far - I hope you feel it has been worth your while and that you notice the difference, now that your telescope is well collimated. If not, deliberately miscollimate step 5 to make the error fall outside tolerances. Try it on a night with good seeing and see what it does to the image, particularly of planet details.

Nils Olof Carlin (mailto:nilsolof.carlin@telia.com) - comments are welcome


With special thanks to Mel Bartels for his help in making this available on the net, and to Ken and Mike for maintaining the ATM site where this is mirrored.

 


Date: 2015-12-11; view: 926


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