If you have a reflector telescope, you’ll need to collimate it now and then. Collimating aligns the telescope’s optics (the primary mirror, secondary mirror and focuser) to ensure you are able to focus when you observe various celestial objects.
Depending on your telescope, you may need to collimate it every time you take it out into your backyard for a stargazing session or once every few weeks.
You’ll usually know when the optics of your Dobsonian or Newtonian telescope are out of whack. You’ll find it hard or even impossible to focus on a planet or star, no matter what you do and how clear the skies are.
How do I know if my telescope needs collimation
To tell for sure if you need to collimate your telescope, use the following methods.
1. Look through the Focuser
Remove the eyepiece on your telescope and look directly down the focuser. You’ll be able to see whether the primary and secondary mirrors are off-centre.
Note that this is just to check whether the telescope needs collimation. Don’t attempt to begin collimation without using a collimation cap or a laser collimator.
Also, note that this method will only reveal extreme instances of misalignment, which can happen if the telescope got knocked or fell or if someone messed with the alignment screws.
Usually, the misalignment is too subtle to notice with your naked eyes.
That’s why we recommend the second more accurate method below.
2. Look at a Star
Looking at a star will reveal even when your optics are slightly misaligned.
Setup your telescope and mount and locate a bright star in the sky. Use your telescope’s red dot finder scope to focus on that star.
Check your eyepiece to see the star. If it’s visible in your eyepiece, increase magnification as far as it can go.
Now, defocus the star such that it begin to appear blurry and you see a pattern of concentric circles.
If the circles are perfectly centred and concentric, your optics are aligned and you do not need to collimate your telescope. If the circles appear off-centre, it means your mirrors are misaligned and you need to collimate your telescope.
We recommend using this method every time you go out to stargaze. Even if your telescope holds its collimation well, it’s good to be sure that everything is okay before you start. In any case, it takes just a couple of minutes.
You don’t want to stop midway through your observations because you realize your telescope needs collimation. You then have to find what you were looking for again, focus on it…a lot of unnecessary work.
Read also: How To Polar Align A Telescope During The Day?
What About Refractors?
The above applies to reflector and Schmidt-Cassegrain telescopes.
That doesn’t mean that your refractor telescope doesn’t need collimation. All telescopes do. The difference is that refractors are collimated at the factory. As long as it’s not involved in an accident, a refractor can hold its collimation for the duration of its life.
If you think your refractor has gotten misaligned, use the star method to be sure. Unfortunately, if it is misaligned, the only option is to send it back to the manufacturer for collimation.
Most refractors are not designed to be collimated at home or in the field.