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What is Collimation?

What is collimation?

AstroTelescopium Team |

You've purchased your telescope, acquired the necessary accessories and are ready to start observing the celestial objects in the night sky. The evening is all planned out. You set up the viewing spot, called your friends over and gave them the rundown of events. The telescope is pointed towards the first planet on your list, but as you take a look through the eyepiece, you're unable to see the image clearly, no matter what adjustment you make.

"Why isn't this working?", you ask yourself, "Are my optics faulty?". Probably not. Sometimes telescopes just need to be collimated (or, more simply said, aligned).

Collimation is the process of aligning all components in a telescope to collect more light and provide optimal image clarity. It often involves the adjustment of the telescope's mirrors to ensure that light reflecting on the main mirror forms an image in the precise center of the eyepiece.

There are 2 types of collimation:

  • Optical collimation
  • Mechanical collimation

Optical collimation aligns a telescope's optical surfaces to bring the image to the correct orientation in the focal plane.

Mechanical collimation aligns a telescope's physical components. For example, this is required if a secondary mirror is misaligned, a focuser isn't square to the tube, or a mirror isn't centered.

How do I know if my telescope needs collimation?

Newtonian Reflectors and Schmidt-Cassegrains require collimation each time you set them up. However, refractors, due to their fixed lens, hold collimation well and do not require the same frequency of adjustment. In fact, unless you drop your refractor or it is significantly jostled, it typically will not require any collimation over its lifetime. Maksutov-Cassegrains also tend to hold collimation well.

A simple method for checking if your telescope needs to be collimated, referred to as the "star test", is outlined below:

  1. On a clear night, focus your telescope on a bright star.
  2. Center the star and zoom in as much as possible (200x or more).
  3. Slowly begin to defocus on the star.
  4. If you see concentric circles (symmetrical circles with a common center) around the star, collimation is not needed.
  5. If you see non-concentric circles (circles not having a common center), collimation is needed.

How to collimate a reflector telescope

There are 3 items that require collimation in a reflector:

  • Primary Mirror
  • Secondary Mirror
  • Focuser

In order to verify that your telescope's secondary mirror is properly aligned with the focuser, you can utilize a collimation cap. A collimation cap, also referred to as a sight tube, is a plastic cap with a small hole in its center and a reflective underside. You can think of it as a plug that fits in your telescope's focuser. This tool can also be used to verify alignment between the primary and secondary mirrors. An alternative, slightly more advanced, tool you can use is a Cheshire eyepiece. This collimation tool comes equipped with a peephole, crosshairs in the barrel and a reflector to shine light on the secondary mirror.

Depending on how unaligned the optics are, you might have to start by adjusting the four spider vanes and making them equal lengths. Then you will need to rotate the diagonal mirror to center it in the focuser's aperture. However, most of the time, this part is unnecessary unless the telescope has been completely disassembled. Below, we will walk through the 3 steps for collimating reflectors.

  1. First, you need to make sure to adjust the angle of the secondary mirror so that it is lined up with the perimeter of the main mirror. The secondary mirror is located at the front of the telescope tube, typically held in place by a glass pane. There will be three collimation screws on it that you should very gently tighten or loosen, one at a time, in order to make the circles around the object you are viewing more concentric. Before adjusting the screw, make note of its original position. If your initial tightening (or loosening) does not fix the focus, you may need to restart the screw in its original position and turn it in the opposite direction. When the main mirror is centered in the diagonal mirror, you can advance to the next step.
  2. Now, you must center the spider vanes and the diagonal holder. To do so, you must adjust the three collimation screws until the diagonal mirror is centered in the reflection of the primary mirror. Again, be very careful and gentle as you adjust each screw.
  3. Finally, take your telescope outside, wait for it to properly assimilate to the temperature, and complete the "star test". Find a bright star above the horizon to aim your telescope towards, center the star and zoom in until it's out of focus, then use the three collimation screws on the primary mirror to fine-tune the star until you see concentric circles around the star. During this final step, make sure not to make any adjustments to the secondary mirror.

It is important to note that it is best practice to make these adjustments while the tube is in a horizontal position. This will help you avoid accidentally dropping your tools on the primary mirror below.

How to collimate a refractor telescope

Refractor telescopes do not require the same high frequency of collimation as reflectors. The main reason for this, is due to their fixed lens that does not move. If an unfortunate situation has occurred, such as you dropping your refractor or it getting bumped very hard, than it does not hurt to check if collimation is needed. In most cases, the manufacturer will have to make the proper adjustments to ensure your refractor is safely collimated.

Let's get started

As you have just discovered, the collimation process is not a difficult task to complete. The entire process should take you less than a few minutes. However, it is well worth it to get the optimal use out of your telescope.

If you are in the market for upgrading or purchasing your first telescope, be sure to review How To Choose The Right Telescope. Alternatively, if you already have an idea of what you want, feel free to directly browse our curated collection of telescopes from industry-leading brands at value prices.



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