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:
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.
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.
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.
Rotate the laser collimator and adjust the three screws in order to move the projected laser beam in the center of the sticker.
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:
Remove the main mirror from your Newtonian telescope.
If the surface of the mirror is dirty, consider following these instructions to clean the mirror. https://www.youtube.com/watch?v=0GUh2ldsVJs
Place the center triangle shape of the template over one of the stickers (triangle center spots) aligning both shapes.
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.
Remove the paper from the back of the center spot.
Place the template over the mirror from your Newtonian telescope aligning the circle of the size of the mirror with the edge of the mirror.
Once the template is perfectly aligned, press the center of the template in order to stick the spot in the center of the mirror. If the size of the mirror is large, you may need some help.
Finally, remove the transparent sticky tape from the template carefully.
Use a microfiber cloth to remove any potential air bubble captured between the center spot and the mirror.
Return the mirror back to the telescope and align each corner of the center spot with the collimation screws. This way, it will be easier to evaluate which screw you need to use when collimating the primary mirror.
Be careful when tightening the clips, they are used for locking the mirror but they shouldn’t pinch it.
Secondary Alignment (usually not required)
Before you start with the secondary alignment, be aware that usually, high-end Newtonian telescopes do not require this operation. The secondary alignment does not need to be absolutely perfect. The secondary alignment is only critical if your telescope has a relatively small secondary mirror, which is usually the case of telescopes tuned or designed for strict visual observation.
The objective of this exercise is to place the secondary mirror in order to intercept the light cone coming from the main mirror by aligning it with the focuser.
In order to achieve that, follow these steps:
Introduce the Skylabs Cheshire Eyepiece in your focuser without using the locking screws.
Introduce a grey or colored sheet underneath the secondary mirror. This will avoid distractions from the main mirror.
Introduce a white sheet at the opposite side of the focuser, behind the secondary mirror. This will create more contrast.
Untighten the locking screws of the secondary mirror.
The position of the secondary has three degrees of movement.
towards the walls of the tube. You can move this position by adjusting the appropriate spider vanes’ thumb knobs. This is the less common adjustment.
along the tube axis (between the main mirror and the spider). You can move this position by loosening or tightening the secondary holder center.
Tilt and rotate. You can adjust this position by loosening the secondary mirror adjustment screws, rotating the secondary mirror, then finally adjusting the set screws to re-align the focuser axis. This is the key movement to point the secondary silhouette offset towards the primary mirror.
If your secondary is perfectly aligned, you should be able to see a circle form the secondary mirror perfectly aligned and centered with the circular edge of the focuser tube. If you can not see an image like in figure 3, iterate step 5 until you achieve it.
Collimate the mirrors
With time, this will become a quick and easy routine. If you are new in the telescope collimation, we strongly suggest practising this operation at home before getting into the field. This way, you are going to spend more time observing and less time setting up the telescope.
Important note: modern Newtonians have an offset on the secondary mirror. If you are also using laser collimation, you will notice that the laser beam impacts in the middle of the short axis of the secondary mirror but not in the middle of the long axis. When using a Cheshire, you will also notice that the center spot is not located in the center of the shadow generated by the secondary.
Use the circle reflected by the Skylabs Cheshire Eyepiece as a reference. Note that with a traditional Cheshire is not possible to evaluate that. That is one of the key differences between a traditional and a reflective Cheshire eyepiece.
The illustrations on the left are courtesy of Jason Khadder.
Use a laser collimator for a quick and rough collimation
Before getting started, check if your laser collimator is actually collimated. Consider using this method to review and fix the potential misscollimation of the laser. You can use a Barlow lens to reduce potential collimation issues of the laser.
Introduce the laser collimator in your focuser without using the locking screws.
Use the collimating screws of the secondary mirror to point the laser beam to the very center of the hole in the triangle center spot. You may use one or two screws at the same time to achieve it.
Untighten the locking screws of the primary mirror.
Turn the primary mirror screws until the laser beam returns to the center of the laser collimator.
It doesn’t need to be super accurate because after that you will use the Skylabs Cheshire Eyepiece, which is way more accurate than the laser collimator.
Skylabs Cheshire Eyepiece
Use the Skylabs Cheshire Eyepiece in order to achieve the required accuracy level.
Introduce the Skylabs Cheshire Eyepiece in your focuser without using the locking screws.
Point the telescope to a bright bulb or illuminate the main mirror using a flashlight. This will make the reflective ring of the Skylabs Cheshire reflect back and forth to the eyepiece.
Untighten the locking screws of the primary mirror.
Turn the primary mirror screws until the triangle center spot sits in the middle of the circle of the reflective ring of the Cheshire.
When tightening the locking screws, the mirror might move a bit, which may require another iteration but using the locking screws this time.
Important note: If you are using high-end and strong collimation screws, you don’t need to use the locking screws anymore.
View of a perfectly collimated Newtonian telescope.
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