How to Achieve Accurate Collimation of Newtonian Telescopes

The collimation (alignment) of the mirrors of the telescopes is extremely important for visual observation. In the case of really fast telescopes (focal ratio < 5) and astrophotography, the collimation becomes critical.

This article provides the instructions and important considerations for collimating a Newtonian telescope to the highest accuracy level.

What do you need

• Center spot in the main mirror. Most Newtonian telescopes already have that.

• Reflective Cheshire Eyepiece. Do not confuse with traditional Cheshire Eyepiece.

• A grey or colored sheet as big as the diameter of the optical tube.

• A white sheet.

• Laser collimator. It's optional but highly recommended to accelerate the collimation process.

Laser Collimator Considerations

Single beam laser collimators are relatively affordable and easy to use. Unfortunately, they can not help to correct the off-axis position of the secondary mirror or ensure the collimation of the surfaces of both mirrors. While the laser collimator really helps to speed up the process, in order to ensure the perfect collimation between the two mirrors, a Cheshire eyepiece is always required.

If you already have a laser collimator, consider checking if that tool is actually calibrated. In order to do so, consider following these directions:

1. Place a circle sticker on a flat surface as far away as you can. A door at the end of a long hallway will work.

2. Place the laser collimator on top of a V-shaped surface and adjust the height. I use a 3D printed model on top of a tripod. It is very handy but a piece of timber will also do the job.

3. Turn the laser collimator. You will notice that the red dot draws a circle when you rotate the laser collimator. Place the sticker in the center of that circle.

4. Rotate the laser collimator and adjust the three screws in order to move the projected laser beam in the center of the sticker.

5. This is an iterative process. You may need to move the sticker and repeat steps 3 and 4 until the laser beam does not move when you turn the collimator on the rig.

Cheshire Eyepiece Considerations

Traditional Cheshire eyepieces are pretty subjective, and collimation becomes a challenge as they cannot achieve the precision required for astrophotography and relatively fast telescopes (focal ratio < 5), which is usually the case of medium and large Dobsonians.

My suggestion is to acquire a Reflective Cheshire Eyepiece and a laser collimator. While the last one is optional, it really helps to get a quick collimation. Obviously, I recommend getting a Skylabs Cheshire Eyepiece but there are other options in the market.

The reflective Cheshire eyepiece relies on a reflective spot perfectly centered in the main mirror of the Newtonian telescope and a reflective circle at the edge of the Cheshire eyepiece. The image of the circle is reflected several times by the mirrors of the telescope, providing an incredible objective and accurate telescope collimation.

Tri-Bahtinov Mask Considerations

The Improved Sensitivity Tri-Bahtinov masks are designed to obtain a very accurate focus and very precise collimation of reflector telescopes, including Schmidt-Cassegrain, Maksutov-Cassegrain, Classical Cassegrain, Ritchey-Chretien, Maksutov-Newtonian, Dall-Kirkham, and Newtonian Telescopes.

The Tri-Bahtinov mask improved by C.Y. Tan, Satoru Takagi and Jordi Blasco (Skylabs NZ) has incorporated several enhancements compared to the traditional Bahtinov mask. It allows evaluating an even more accurate focus and it also allows detecting and correcting a misscollimation of a reflector telescope. This affordable tool is the result of vibrant cooperation and open-source contributions inside the astrophotography community.

While the traditional Bahtinov masks deliver a diffraction pattern, creating a set of three fine spike lines around a bright star, the Tri-Bahtinov masks generate a set of 18 spikes, which is crucial to assess an accurate focus in astrophotography.

I recommend using the Skylabs Improved Sensitivity Tri-Bahtinov mask because it is based on eco-friendly high-density cardboard (kraftboard). The material is not only durable but also delicate with the telescope optics, especially for MCT, MNT, and SCT. The thin material and the CNC precision cut shape ensure an extraordinary precise diffraction pattern.

The initial collimation must be done with another tool, like a laser beam collimator (any telescope) or Cheshire eyepiece (Newtonian telescopes). The secondary mirror needs to be correctly adjusted because the tri-Bahtinov mask will only help you to align the primary mirror. I will cover this topic in a new article. Stay tuned! In the meantime, you can read the instructions available here.

How to collimate your Newtonian telescope with a Reflective Cheshire Eyepiece

Most Newtonian telescopes already have a donut or triangle in the center of the main mirror. If your mirror doesn't have that, consider getting a mirror spotting template and center spots. It is very important to select the right size of your spot and template. Request the small triangle for 1.25” Cheshire eyepiece or the big one for 2” Cheshire. Obviously, if you have a 2” focuser a 2” Cheshire eyepiece is highly recommended.

If you already have a center spot, you can jump to the next step (Secondary Alignment).

Setting up the center spot

If you don’t have a spot in the center of the main mirror of your Newtonian telescope, follow these steps:

1. Remove the main mirror from your Newtonian telescope.

2. If the surface of the mirror is dirty, consider following these instructions to clean the mirror. https://www.youtube.com/watch?v=0GUh2ldsVJs

3. Place the center triangle shape of the template over one of the stickers (triangle center spots) aligning both shapes.

4. Use a small piece of transparent sticky take and place it over the center of the template. Press the sticky tape over the hole located in the center of the template. This will hold the center spot perfectly aligned with the template.

5. Remove the paper from the back of the center spot.

6. Place the template over the mirror from your Newtonian telescop