The Art of Collimation

Refractor owners can take a break here, though if you're thinking of joining the ranks of reflector owners, please read on. In order for a reflector of any type to produce a sharp image it is absolutely required that its mirrors always be in precise alignment. Before a telescope is shipped the manufacturer will usually perform a check using a laser to ensure that the mirrors are properly aligned with each other to produce the best image quality possible. Over the course of time, the secondary mirror may work its way out of alignment and need to be adjusted. This can happen as a result of bumping or jostling of the telescope or through mechanical flexure caused by repeated heating and cooling of the telescope. You might ask why the manufacturer does not design the scope so that it is inflexible one the mirrors are in alignment. The answer to that is because there is no substance known to our science that does not expand and contract to some degree in response to changing temperatures. The tolerances we are talking about are infinitesimally small. Remember that the spherical aberration that rendered the Hubble Space Telescope myopic is the result of an error in mirror shape of only a few angstroms; we're discussing billionths of a millimeter. In addition, changing temperature conditions may cause the mounting and telescope tube to flex, grow and shrink. The secondary mirror therefore is always equipped with an adjustable mounting to allow its alignment to be corrected periodically.

Though it is possible to precisely collimate a telescope these days with a laser, it is fairly easy to do without high technology. It just takes some patience. When you unpack the scope, one of the first things you should do is remove the covers and look through the eyepiece holder without an eyepiece. What you should see is a perfect reflection of your eye in the center of the secondary mirror. If your eye's reflection is dead center in the mirror, then you have a perfectly collimated mirror. If not, then you will need to adjust it. At the front of the telescope on the secondary support you will find three small screws with either Phillips or Allen heads. Your telescope will usually be supplied with the right screwdriver to adjust these. Turning the screws will move the secondary mirror. As you look through the eyepiece holder or visual back, turn one of the screws slightly and observe the result. The adjustment is very coarse, especially if you consider that even a few degrees of rotation of the screws will change the collimation completely. Carefully adjust the screws one at a time until the image of your eye is perfectly centered in the field of view. This technique will allow the telescope to function well enough to let you do a more precise collimation under the stars.

Once you are outside, center and focus a moderately bright star at low power. Move the focuser gradually out of the focus position. As you do so, watch what happens to the star. As its image blurs and expands, it forms the shape of a doughnut. The hole in the center is the shadow of the secondary mirror and is called the airy disk. Using the appropriate screwdriver if necessary, collimate the mirror until the airy disk is precisely centered in the blurred star image. Refocus the telescope and notice the improvement in image sharpness. Repeat the procedure then with a high power eyepiece in place. You may discover that the alignment was not so perfect as you thought with low power. With the airy disk centered using the highpower eyepiece, you have now achieved perfect collimation. Many observers like to collimate now with the aide of a laser aide. Generally the technique involves shining the laser into the eyepiece holder and watching where the dot falls on the primary mirror. It works and makes money for the laser sellers, but I must admit that I prefer doing it the old fashioned way. Owning a Schmidt-Cassegrain is helpful to me here since the optics are much more ruggedly mounted than on typical Newtonian reflectors. In sixteen years, I have only needed to collimate the telescope twice. On both occasions, it was after being transported over a long distance and in rough conditions, something I do not generally do to the telescope. Still I check my scope's collimation regularly because if it is off, the scope's performance will be too.

Telescopes Mastery

Telescopes Mastery

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