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C'mon Baby, Do The Collimation

Latest Revision: September 29, 2002

DISCLAIMER: All this information is accurate to the best of my knowledge; if there are any omissions or errors, please let me know. This document is intended to be an overview, not the end-all-and-be-all of a given topic. If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section.


Collimation -- Contents:

  1. Huh?
  2. The procedure
  3. Tools -- Who Needs 'em?
  4. Where to get them
  5. Other resources

Huh?

"Collimation" refers to the alignment of optical elements in a telescope. In portable telescopes, particularly reflectors, these elements can be moved enough out of position (through minor bumps and jolts when moving or transporting the telescope) to seriously affect the quality of the images they produce.

All scopes need to be properly collimated at some point, but routine maintenance collimating is more commonly needed on some types of scopes. In particular, conventional (Newtonian) reflectors and some catadioptrics (particularly classical Cassegrains and Schmidt-Cassegrains) frequently need recollimating. Refractors generally need it less often.

The procedure

This is a very brief overview for a Newtonian reflector and is not necessarily a substitute for a more detailed account. An excellent online illustrated outline is given on Mel Bartel's collimation page, which also outlines the procedure for a Newtonian reflector. For more details or for collimating a telescope other than a Newtonian, see the Other Resources below.

In brief, for a Newtonian reflector, you need to do the following:

  1. It may simplify the job if you make sure the optical axis of the focuser is perpendicular to the optical axis of the telescope (sometimes called "squaring"). To first order, this will be perpendicular to the telescope optical tube assembly; some further adjustment may be needed if the OTA's physical axis and optical axis don't exactly coincide.
  2. Ensure that the diagonal (secondary) mirror is directly beneath and aligned with the focuser (i.e., the two are concentric). This can be checked by looking into the focuser, with no eyepiece, and adjusting screws on the secondary and spider to move it back and forth as needed.

    A potentially important aside: Short focal length reflectors may benefit from a slight offset of the secondary mirror from this perfectly centered position, to improve the mirror's ability to catch the incoming cone of light from the primary. This has no effect on collimation, except to make the secondary mirror's correct position slightly off-center underneath the focuser. In general, the offset is small and can, for initial purposes at least, be neglected.

  3. Adjust the tilt of the secondary mirror, using the screws on the back of the cell (or whatever is appropriate) until the reflection of the primary mirror is concentric with the edge of the secondary.
  4. Adjust the tilt of the primary mirror so that the reflection of the focuser tube in the secondary is perfectly centered in the focuser. I.e, all circles (focuser edge, secondary edge, reflection of primary edge, reflection of focuser tube, and reflection of secondary if no secondary offset was used) are concentric.

At this stage, most Newtonian telescopes should be fairly well collimated. If you want or need further adjustment, this can be done with the star test, or a highly sensitive collimation tool such as the autocollimator (see the Other Resources section for further pointers).

Tools -- Who Needs 'em?

In general, if you have a reflector, especially one with a fast focal ratio (for our purposes, f/6 or shorter), it helps to have some tools to aid you in the dirty work. This is because the shorter your focal ratio is, the more sensitive your optics are to misalignment and the harder it is to get them aligned solely by eye.

There are two basic tools: the sight tube and the Cheshire eyepiece. The sight tube is designed to make the preliminary stages of collimation easier. It's nothing more than a long tube with a pinhole at one end. It fits in the focuser, like an eyepiece. What it does, in effect, is to project the focuser downward, so you can see more easily if the secondary mirror is concentric with the focuser. Many such tubes also have cross hairs at the far end, which makes it easier to align the tilt of the secondary and get the reflection of the primary properly centered.

The Cheshire eyepiece is used to adjust the primary mirror -- the last major step. It's an eyepiece-like plug with a washer-like insert tilted at 45 degrees to the focuser axis. When you illuminate the washer (there's a hole in the side of the eyepiece for this purpose) and look through the top, you see a donut shaped reflection in addition to the usual reflections. You then turn screws on the primary cell until the center of the primary lines up with the center of the washer. Many amateur astronomers (carefully!) mark the centers of their mirrors to make this adjustment easier; if you have a large commercially made Dobsonian, odds are its primary was marked in this manner during assembly. It's very difficult to use a Cheshire eyepiece without some sort of reference marker.

Several places, for example Orion Telescopes, make combined sight-tube and Cheshire eyepiece "collimating tools." These are useful since you don't have to change tools all the time.

There are a number of other tools out there too:

  1. There is a device called an autocollimator which can be used for very fine adjustments (no, it does not automate the collimation process, I am sorry to say) -- it's much less important than the others I mentioned. You'll probably only need something like this if your scope is especially picky about collimation; the sight tube and Cheshire combination is generally adequate even for fast scopes.
  2. Star testing. On nights with good seeing, the shape of star images provides a sensitive test for almost all optical problems your telescope may be prone to, including miscollimation. The March 1995 issue of Sky & Telescope reviews the basic ideas behind star testing in an article on a recent book on the subject.
  3. There is also a new breed of laser based collimation tools. They use a small laser, like the ones used in laser pointers, and shine it down the optical path; you then turn adjustment screws on the mirrors until the laser reflection behaves a certain way. The cost is greater than that of the sight tube/Cheshire pair, but requires less experience to use successfully. Sky & Telescope reviewed one such device in the February, 1996
    issue.

Where to get them

You can make a sight tube yourself out of a plastic 35mm film canister, by poking a pinhole in the lid and putting it in the focuser. This does demand some respect for mechanical tolerances (that pinhole should be accurately centered, and the canister shouldn't have any slop in the focuser). If you have a very fast telescope, say an f/4, you're probably better off getting a precisely machined version (you can do it yourself if you're skilled enough, or just be a consumer and buy the darned thing). Other tools have to be made or purchased elsewhere. The principal vendor for collimation tools in the US is Tectron Telescopes, which makes a line of large Dobsonian reflectors. The address is

Tectron Telescopes
2111 Whitfield Park Avenue
Sarasota, FL 34243

They also sell a booklet which goes into gross, disgusting, and useful detail on the procedure for collimation.

Other resources

  • Local astronomy clubs. There's usually one or more people there who have done this a zillion times and can show how it's done.
  • "Perspectives on Collimation: Principles and Procedures", Vic Menard and Tippy D'Auria. Booklet coming with the Tectron collimation tools and going into detail on the procedures for all kinds of scopes, even refractors. Highly recommended.
  • Sky & Telescope tends to have occasional articles on the topic. Two thorough articles are:
    • March, 1988 p. 259 (covers collimation of Newtonian reflectors and includes a review of the Tectron tools mentioned above)
    • April, 1988 p. 363 (covers refractors and catadioptrics).
  • Also, as mentioned above, S&T has recently has several directly related discussions (e.g., star testing, laser collimation devices). Especially relevant ones:
    • March, 1995: Richard Suiter's Star Testing Astronomical Telescopes, a book on how to use the star testing method successfully. Includes photographs showing how different scope defects, including poor collimation, appear in the star test.
    • February, 1996: AstroSystem's laser collimating tool.
  • Mel Bartel's WWW site on amateur astronomy, which has a good
    overview of collimation procedures. URL: http://www.oarval.org/collimatE.htm

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