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What is Collimation All About?

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First, I don't have a scope; therefore I can only discuss this subject in the abstract sense.

As I understand it, collimation is about getting the light reflected by the primary mirror to focus at the correct spot (or spots?) on the secondary mirror so that when reflected into the EP, the light is coherent instead of distorted beyond the pupil's ability to focus on it. Is this even close to what the process is about?

I've tried to make drawings to help my brain around the subject, but lose the ability to visualize what is happening. I approach the process by considering a hypothetical single ray of light striking the primary. The parabolic grind (and the mirrored back) of the lens cause the light to reflect to a ??? Does each ray of light get focused back to the center of the secondary? . . . to a 'sweet' spot that is not (theoretically) a single spot? . . . or a theoretical single spot?

This is where I get confused because it seems to me that having all the rays focused on a single spot would be like using a magnifying glass to burn a hole in paper. I guess I have difficulty understanding what the light rays between the secondary and the EP are


Also, considering how a laser collimator works, do I understand it correctly that the collimator sits in the focuser and emits coherent, collimated light onto the secondary? I visualize this as the process, with the laser beam sent toward the secondary with the goal being to adjust the secondary and primary such that the beam gets centered on a point in the primary. This would indicate that the reflective portions of the secondary and the primary would be aligned such that - considering the parabola that is the primary and the ellipse that is the secondary - any light striking the primary would be reflected as accurately as the the accuracy of the reflective surfaces could attain.

These questions come from my attempt to understand what the objective of the actual operations of collimation are intended to accomplish.

Anyone care to attempt to educate me? :)

Edited by rabbithutch
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Collimation consists of two different alignments:

1- Position the secondary mirror to optimally intercept all light cones for all stars in the FOV. This is a coarse alignment.

2- Fine tune the secondary mirror and adjust the primary mirror to ensure the focuser axis and primary mirror axis coincide. The objective is to align the focus points and focal planes of both the primary mirror and the eyepiece.

You can read the 2nd and 3rd posts of the following thread. Just these two posts.


OK, let me simplify things:

Collimation bottom line is about the eyepiece and the primary mirror. Light from a star enters the scope as a bundle or parallel light rays then get reflected off the primary mirror and merge into one point at the eyepiece – not at the secondary mirror. But not all stars seen at the eyepiece will merge at the same point. Light from each star will merge at a different and unique point at the eyepiece. Good collimation ensures all these points are clear and appear as dots. If collimation is off then light from each star will not merge at a single dot but rather will be smeared overall a small area mimicking the shape of small comets. Another manifestation of poor collimation is the inability to bring all stars to focus simultaneously. And another manifestation of poor collimation is a non-uniform brightness across the field of view. That is, the brightness of a star will change as you move it around the field of view.


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Thank you for taking on the onerous job, Jason!

OK. I now understand that the focusing plane is at the EP, not at the secondary. However, that makes the relationship between the primary and secondary less clear to me. Perhaps my not understanding reference to a 'cone' of light instead of a 'ray' of light is the fault.

It has long been my practice to try understand complex subjects concerning compound objects by conceptualizing the behavior of a single object within the group. In this case, I hypothesize a single ray of light, i.e., a single beam assumed to be coherent at its source and uncorrupted in its travels (to simplify things). This single bear or ray of light would enter the OTA, strike the primary, and be reflected back to the secondary then to the EP. Where or when would it have acquired the shape of a cone instead of the shape a a single straight line? Does the primary affect rays to cause them to become conical? Are they already conical when they enter the OTA? If so, why? Is conical shape a natural condition that one accepts as a given?

This one is difficult to get my old brain around.

Thank you again for the response. At least now I know that the objective is to finagle 2 variables (primary and secondary) in order to present the light to a third one (the EP). I am going to pore over the links you gave to see if any of this will come clear for me.

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I see it as a tad simpler than that. The light hits a conical (or preferably parabolic) mirror wich focuses it in the focal plane. In a Newt it hits a 45 deg diagonal that pushes the focal plane out the side of the scope. You put a magnifying eyepiece in the focal plane to see the object.

In a Mak or Schmitt the focal plane is directed by a secondary mirror back through a hole in the primary, so you stick your ep in the back (usually off a diagonal).

In a refractor the objective (front) lens puts the focal plane at the other end of the tube, with a similar diagonal/ep arrangement for viewing.

The are other issues like correction, reduction, flatenning, croma, etc, and the type of scope determines how these are dealt with . Collimation is just about getting it all lined up right. :)

Edited by brantuk
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Yes, that's almost what I was trying to describe. Thank you brantuk!

What I'm confused about, I think, is how rays are reflected from the secondary in a Newtonian. My concept is that light rays strike a point on the primary which - when properly aligned with the secondary - will be reflected to a specific spot on the secondary. Each ray, because it strikes a different place on the primary, will be reflected to a different spot on the secondary. The angle formed by the incoming and outgoing rays is controlled by the grind of the primary, but I don't understand how that works.

Because the secondary is a plane, not a concavity as is the primary, each ray will be redirected at 90° from its incoming direction toward the focal plane. It is the nature of how the angles control the direction that I'm struggling with. If I understand it correctly, this process reverses left and right, up and down in the EP.

Again, this is a bit much for a near 70 yo brain to conceptualize and understand. I think I might be able to actually achieve collimation following the instructions, but I can't quite get the full concept pictured in my mind. :)

Thanks for playing. :D

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I think your conceptual problem (with which I heartily sympathise!) arises from the inadquate and misleading ray diagrams beloved of school teachers and textbooks.

What the these ray diagrams do not show satisfactorily is that all parts of the mirror contribute to all parts of the image. But they do. So all parts of the primary must send the same message to the eyepiece. That, I think, is what collimation is about. The primary must, therefore, receive and reflect light orthogonally and this orthogonality must be preserved by the secondary.

BTW, I think Brantuk meant spherical rather than conical. Easily done!


Edited by ollypenrice
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To understand newt operation and collimation, you can't think in terms of a single light ray. You have to think in terms of a bundle of light rays. The incoming light of each star in the eyepiece will fill the whole OTA, touches every part of the primary mirror, and a good portion of the secondary mirror.

Many incorrectly think only a single light ray comes from each star striking a single point on the primary mirror and another point on the secondary mirror. This is incorrect.

Check the attached animation. It shows the incoming light for 3 stars as a bundle of parallel light rays striking the whole primary mirror. Then the primary mirror will convert the incoming bundle of parallel light rays to cones with their tips touching the focal place at the eyepiece. Stars in the middle of the view will have their light cones reflect entirely off the secondary mirror. Stars at the edge of the view will lose some of that cone and will lose some of their intensity at the eyepiece.



Attached animation is not working on this website

Click on the following link



Edited by Jason D
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Thank you, All!

I'm learning - even if it seems painfully slowly.

I've had a go at drawing a model of light in a newtonian. Apologies for its crudeness. I'm no artist and my tools are more powerful than my imagination.

Anyway . . . in the attached drawing, I show, in the white sno-cone shape, light being emitted or reflected by objects in the sky, represented by the different colored circles. I show the light coming to the scope and entering it as the same white light in the field of view.

As that "white" light strikes the primary, the spherical form of the lens causes the light rays to be reflected back toward the secondary. I showed this reflected ' light with a pinkish color. This reflective light is conical in shape. This 'pinkish' light strikes the secondary which is positioned at a 45° angle to the focal plane with focuser and EP shown.

In the 2nd pic, I show just the secondary and focuser, with the light reflected from the secondary toward the focal plane. In the left drawing, I showed the light from the secondary to the focal plane as conical and on the right, as a circular tube - or, perhaps. an elliptical tube.

I am known to over think problems. Perhaps this is one of those times. It just seems to me that if the light does not reflect back in a cylindrical - or perhaps elliptical - manner, that it would be too distorted (if that's the term) to allow it to be focused through an EP.

I appreciate the fact that my attempt to simplify to a single light ray might be too much of a stretch. Our brains want to impose an order to light in order to perceive the objects it reflects from. Brains are more focused on interpreting the objects instead of perceiving and interpreting the light; so perhaps to describe light "rays" only makes sense for a focused beam, as in a laser. But, perhaps, it is more appropriate to describe light as a "glow" that has differing intensities. We aren't well equipped to perceive only the light.

At any rate, I thank you all for undertaking to give this old fellow a physics lesson. It has been more than 50 years since I last had one. I would like to press for one more answer: Is it correct that a laser collimator mounted on the focuser emits a beam onto the secondary which is then reflected to the primary and, if properly collimated, the laser point will be in the center of the primary?

Thank you again.



Edited by rabbithutch
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Incoming light from all objects in the sky will strike all parts of the primary mirror as a bundle of parallel light rays -- not as a cone. In fact, they will strike 1/2 of Earth surface as a bundle of parallel light rays.

With regards to your question about the laser collimator, if the laser beam reflects off the secondary mirror, hits the primary mirror at the center, then retraces its path back to the emitter, the scope will be consider axially collimated.


Edited by Jason D
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