Actual Step

Step 3c: High Point Scientific Collimation Approach

updated: 2025-10-09

Purpose

In the final stage of the process, the telescope's optics are refined using a real or artificial star in order to restore the well-balanced image of the stars to its original point-like form. This is achieved by eliminating any distortions or elongations that may have occurred during the previous stages of the process.

Previous Step

Step 2: Collimation of the Secondary Mirror

Next Step

2 alternative follow-up steps:

  1. Step 4: Correct the Focuser Tilt
  2. Gear Installation an Calibration (Northern Hemisphere)

Resource(s)


Credentials

https://www.highpointscientific.com


Primary Mirror Adjustments 

Step

What To Do

Illustration

1

Primary Mirror Adjustments

  • Center a moderately bright star, such as a constellation star, in your field of view. 
  • Using a live view or looping short exposure, center this star perfectly in your camera. 
  • Then, ever so slightly defocus the star. 
  • You’ll need to zoom in using your imaging software to see the details necessary. 
  • If the star is defocused too much, the detail necessary to perfect the collimation will not be visible. 
  • If the shadow of your secondary is not centered in the unfocused star image, you will see something like this:

2

This telescope is in poor collimation. The question now is which of the three screws do you need to adjust to bring the telescope into a better state of alignment. 

  • To find out, simply reach your hand around and place it in front of the telescope’s aperture. 
  • Then move your hand’s orientation so its silhouette is seen in front of the widest portion of defocused light between the shadow of the secondary and the edge of the star.

3

Once this is accomplished, look at the primary mirror collimation screws (at the back of the telescope) and see which one of those screws best aligns with your hand placement. 

  • If you are lucky enough to find a collimation screw in the same orientation, then you will need to adjust that particular adjustment screw. 
  • If, however, your hand is between two collimation screws, then adjust the screw on the opposite side. 
  • The goal is to find a screw that is closest to the imaginary line that your hand creates across the telescope.

4

Short focal ratio imaging Newtonians will have a secondary mirror that is physically offset from the center of the tube. It might be a small amount, but it is offset. This offset can show in a star test if the star is defocused too far, and because of that fact, it is important to work through the following final step.

  • For some extra precision, tweak your focus a bit so that the star is just slightly outside of perfect focus. 
  • If your telescope is perfectly collimated, you should see a tiny white dot centered within the shimmering diffraction pattern. 
  • This small dot is called “Poisson’s Spot or Arago Spot” and it determines the true center of your diffraction pattern. 
  • Only when this spot is precisely centered can you call your work done. 
  • If it is off slightly, just keep making small tweaks to one or more of your collimation screws while keeping the star image as well centered as you can. If you are patient and persistent, you will end up seeing something like this:


Secondary Mirror Adjustments 

Step

What To Do

Illustration

5

Balanced Image
Once the axis star looks good, you’re going to move on to the secondary mirror. Please be sure the small lock screws for the primary are secure before moving on to the secondary mirror.


  • Now that the primary has been adjusted, bring the scope back into focus. 
  • Point the scope at a bright starfield and take an image. 
  • What do the stars in the corner of the frame look like? Are they balanced? Meaning, do all the corners look similar? 
  • It is normal for the extreme corners to have stars that might be slightly oblong in either direction. This is astigmatism and field curvature and is a normal part of an RC telescope. 
  • The corners should look like one of the following two images:


6

Unbalanced Image

You might notice that one of the corners is quite different from its opposing corner. If the stars look like one of the following images, then you need to adjust the secondary mirror to balance the image:


  • In these instances, you’ll typically want to adjust the secondary mirror so it moves across this angle. 
  • Choose a collimation screw that corresponds with this axis and make an adjustment, noting if the corner stars get better or worse. 
  • If they get better, keep adjusting. 
  • If they get worse, move the screw in the opposite direction. 
  • After some careful iterations, you should wind up with a star field that looks balanced from corner to corner.


In the event you can not fully balance the image, get it as close as you can, and then go back and recheck the primary. It is likely that the system was far off and the primary needs a small adjustment before coming back to the secondary mirror. It is not abnormal to go back and forth from primary to secondary when really trying to perfect the star images.



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