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Newtonian Collimation


What Is Collimation?



Telescopes, all types, consist of optical components that serve the purpose of gathering light and forming a quality image at an eyepiece, which presents it to an observer (or camera). Refractor telescopes do this with an objective made of lenses (2 or 3) which collect the light and create an image for an eyepiece to present to the observer. Reflector telescopes do this with an objective made of a parabolic mirror which collects the light and creates an image for an eyepiece. Catadioptic telescopes use a combination of a lens and a mirror as the light gathering and imaging components.

All of these types of telescopes need to have the optical elements properly collimated. What is collimation? It is the process of putting all of a telescope's optical elements into proper alignment so that an optimal image will result. It's sort of like aligning the sights on a gun. Obviously if gun sights are off, shooting is sub-optimal. If any of the optical elements of a telescope are not properly aligned, images are sub-optimal.

Some telescope types tend to hold alignment very well. If you have one of these types of telescopes, you seldom if ever have to engage in the collimation process. Refractors are instruments that tend to stay aligned very well. Objective lenses are mounted securely in a cell that fastens solidly to the end of the telescope tube, and the eyepiece holder likewise fastens securely to the opposite end of the tube. Only if you make your own refractor will you likely pay any heed to alignment. And if you make your own DIY refractor, the main issue is to simply make sure the main telescope tube ends are very square with the length of the tube. If this is done, the attached lens cell and focuser will likely be quite well aligned.

If you have a Catadioptic type of telescope, likely it's a commercial one, and in such case alignment is either rather easy, or very difficult and best done at an optical facility. Maksutov type telescopes usually offer the user little if any alignment capability. Schmidt Cassegrains, like my Celestron NexStar 5SE usually offer the user the capability of adjusting the alignment of the secondary mirror. The image below shows the adjustment screws of the secondary on my NexStar 5SE.

NexStar 5 Collimation Screws

Celestron NexStar 5SE Collimation

You might notice that the collimation screws on my NexStar look like thumbscrews. The original screws were Phillips head screws. The original screws were replaced with the much handier Bobs Knobs Celestron 5" SCT f/10 Collimation Knobs . These handy thumbscrews are available for most any size Celestron telescope.

Misaligned Optics Star Image

To collimate my NexStar 5SE, I usually just point the telescope at Polaris, rack the image out of focus in a high power eyepiece, then tweak the knobs to produce the most concentric set of diffraction rings. The above illustration is a depiction of how rings appear with an out of focus, high magnification star image, and how the pattern changes as the collimation screws are adjusted.

The alignment process with an SCT involves adjusting the secondary alignment screws to bring the diffraction rings into a centered, concentric configuration. You do that by tweaking the screw that is nearest in the direction off the offset ring pattern. If the screw needs to be tightened, you may have to loosen the other two screws to allow tightening. If it needs loosened, you will have to correspondingly tighten the other screws. In the end, you want all collimation screws snug, and a centered, concentric diffraction ring pattern.

If you happen to have a Newtonian reflector telescope, then collimation is something you need to get familiar and comfortable with. It's something that must be done often (some astronomers do it every time they use their telescope), and is more tedious than adjusting the alignment of an SCT secondary.

Tools To Collimate A Newtonian Telescope

Though Newtonian telescope collimation is more tedious, I'm still partial to Newtonian telescopes for their incredible capability versus cost. In spite of the inconvenience of needing occasional optical alignment, my favorite telescope is my Discovery 6 Inch Newtonian, which is basically the same as the Celestron 31057 Omni XLT - 150. It's a 6 inch short focus Newtonian on an Equatorial mount, capable of seeing a lot, small enough and light enough to be easily transported, and possessing a wide field of view to make finding difficult star targets easier. While not the biggest telescope I've ever owned, it's the one I've seen more with than any other. But -- it needs to be collimated a few times per year.

Newtonian optical path

Newtonian Telescope Optical Path

The image above illustrates the optical components of a Newtonian telescope. Note that Dobsonian telescopes have the same optical design. Dobsonian refers to the mount style rather than the optical design. The diagram shows that light from some celestial target travels through an open tube to the parabolic objective mirror, which reflects the beam forming the image back through the open tube. The converging light beam encounters a diagonal flat mirror that is placed just beneath the eyepiece. The diagonal reflects the beam to the eyepiece, which is mounted to the side of the telescope tube.

If the objective mirror isn't properly aligned, the image won't reflect precisely back through the optical tube, and will result in a poor image. If the diagonal isn't properly centered in the optical tube and set at a precise 45 degree angle, again a poor image will result. And finally, if the eyepiece isn't properly centered over the diagonal and mounted precisely at 90 degrees to the optical tube, a poor image will result.

So I admit that the collimation aspect of Newtonian and Dobsonian telescopes can put people off. And short focus models like my 6 inch f/5 make the collimation process even more of a challenge. The shorter the focal ratio of the reflector telescope, the more precise the alignment has to be.

However, with a bit of setup and an alignment tool, collimating a telescope need not be all that difficult.

There are a couple of tools than can be applied to significantly aid in collimating your Newtonian telescope. One popular tool is the collimation laser, like the Orion LaserMate Deluxe Telescope Laser Collimator . Another is the Cheshire eyepiece, like the Celestron Collimation Eyepiece 1.25" .



centering template

To Aid In Alignment, Prepare the Primary

If you've not done it yet, you need to carefully glue a notebook reenforcement ring on the center of your primary. If you're going to use a Cheshire Collimation Eyepiece, you can just paint a dot on the center of the primary instead of using the notebook re-enforcement ring. Don't worry, the secondary blocks off light from the center of the primary anyway. So we can make best use of the primary's center as an aid to alignment.

A simple way to get your re-enforcement ring centered is to use a compass to draw a circle on some thin cardboard the diameter of your primary. I usually use a piece of poster board. Then draw about a one inch diameter circle at the center of the primary-sized circle.

Cut out the primary-sized cardboard circle, then cut out and remove the circle in the center.

A trick I've read about is to fold the cardboard cutout in half. Then unfold and fold in half 90 degrees from the first fold. Unfold again, and you have a cardboard circle with a cutout in the center, with fold lines that help to identify the precise center of the hole.

Carefully place this cardboard circle gently on the top of your primary to rest only on the outside edge of the primary. Don't push the cardboard down onto the primary's surface.

Use the fold lines to help identify the center of the cutout circle, and glue your re-enforcement ring on the exposed primary at the center of the hole.

This glued on ring can remain on your primary as a handy tool to aid in subsequent collimation procedures. Again, it won't damage your telescopes images, because the secondary mirror blocks off the center portion of the primary mirror anyway.





How To Collimate With Laser Collimator

Shown at left is a Laser Collimator for collimating a Newtonian telescope. It is designed with a low power laser in a barrel that slides into the eyepiece focuser, replacing the eyepiece during the collimation process. As a caution, Don't Look Directly Into The Laser Beam.

Laser Collimator Diagram

The diagram above illustrates the basic technique. The diagram assumes that the focuser is already properly mounted square to the telescope tube and directly in line with the secondary mirror. It also assumes that the secondary is properly centered in the telescope tube. If you have a commercially produced telescope, these assumptions are probably true. If you made your own DIY Newtonian, then you need to be sure that these assumptions are true.

The 7 step gif display shows that the procedure is to adjust the secondary mirror (Align Secondary labels), as the diagram illustrates, until the laser strikes the primary precisely in its center. The notebook ring re-enforcer helps identify this spot. The next and final procedure is to adjust the primary mirror alignment (Align Primary labels) until the return beam of the laser falls back on the laser beam source. When that is done, the optics are properly aligned.

Newtonian Secondary Alignment
Secondary Alignment Screws

You can find the screws used to adjust the Newtonian secondary by looking in the open end of the telescope tube at the center hub of the secondary support. The screws must all be snug. To further tighten one to change the tilt of the seconary mirror requires reducing tension on one or more of the others. If the necessary adjustment requires a screw to be loosened a bit, one or both of the others must be correspondingly tightened. Once the desired adjustment is found, test to be sure all three screws are snug.

laser collimation step 1

Laser Collimation -- Aligning Secondary

The illustration above shows an animation of what you might see as you look into the open end of the Newtonian telescope at the main mirror when the laser collimator is operating. You should see your notebook re-enforcement ring at the mirror's center, and you should see a spot where the laser beam is striking the main mirror. The laser spot will likely not be striking precisely at the center of the mirror. This indicates that the secondary mirror is not properly aligned.

To correct the situation, use the secondary adjustment screws to move the laser spot to the center of the main mirror, using the notebook re-enforcement ring as an aid. Once the laser spot is striking the center of the mirror, the secondary is properly positioned.

Laser collimation, step 2

Laser Collimation -- Aligning Primary

Now if you look into the open end of the Newtonian telescope tube at the bottom of the focuser and laser collimator, you should see the exit port of the laser where the collimation beam comes out, and likely a laser beam spot somewhere off center on the bottom of the laser collimator. This off center spot is the return beam being reflected by the primary mirror. If the return spot is not falling precisely on the exit port, then the primary is not properly aligned.

To correct this, which finishes the collimation, adjust the primary mirror alignment screws to bring the return spot in conjunction with the laser exit port. When both of these procedures are complete, you are done with the laser collimation.

Newtonian Mirror Alignment
Newtonian Primary Adjustment Screws

You can find the primary mirror adjustment screws on the backside of the primary cell. Some telescopes use 3 spring-loaded screws, and you just turn the one or ones that brings things into alignment. Some, like that pictured above, have 6 screws. These are push-pull screws. On this model you have to loosen the outer tensioning screw to be able to turn the actual adjusting screw next to it. When the proper adjustment point is found, re-tension the outer screw. Do this to each screw that needs adjustment.

Again, be careful not to look directly into the laser beam itself.



How To Collimate With A Cheshire Eyepiece

Celestron Collimation Eyepiece top view

Above you see a commercial Cheshire Eyepiece, the Celestron Collimation Eyepiece 1.25".

A Cheshire Eyepiece is a handy device for helping align optics. Since Newtonian design telescopes often need alignment, aligning Newtonians is a common use for a Cheshire Eyepiece. As you can see, it looks similar to a regular eyepiece except for the cutout at the side. The top and bottom cutouts are at 45 degrees to the optical axis of the tube. There are no optics in the tube, just a peephole at the top and a reticle at the bottom.

Celestron Collimation Eyepiece bottom view

Here's a look at the Cheshire eyepiece from the bottom. Notice the cross hair just inside the bottom rim.

Looking through the peephole and aligning what you see with the cross hair will put things into alignment.

Celestron Collimation Eyepiece side view

Above is the side view of the Cheshire, showing the details of the cutout. Notice the light colored surface with the center hole. This surface makes a reflective surface that allows light to reflect down onto the reticle, and also make the reflected image of the Cheshire eyepiece visible on the primary mirror.

Prior to owning a Cheshire eyepiece, I used to use the old idea of a old gutted eyepiece. Removing the optics give me a peephole device. But it was hard to use because once my eye got close to the peephole, all light was blocked off and I couldn't easily see all I needed to for good alignment.

That's where the Cheshire eyepiece works better. It's a peephole with cross hair that also allows enough light into the system to let you see what you're doing.



Misaligned Secondary

The Initial View

The Cheshire eyepiece has a cross hair and a cutout in the side to reflect light down the eyepiece tube as you're looking through it.

Chances are, you'll see something like the image above on your first peek if you've never aligned your optics before, or if you've re-installed mirrors after a cleaning.

In this and the following images, the black circle and black cross hairs represent the visible parts of the Cheshire eyepiece.

The secondary should be centered in the Cheshire eyepiece view before starting alignment..

The primary mirror may initially be quite off center and not even entirely visible.

The black dot represents the notebook ring on the primary.

The reflected image of the Cheshire eyepiece is an indicator used to align the primary.



Collimation -- Secondary

The first step (see above image) involves adjusting the secondary alignment until:

  • the secondary appears centered in the view

  • the reflection of the primary appears centered in the secondary

  • the reflected image of the re-enforcement ring on the primary is centered on the Cheshire eyepiece cross hair.
  • This animation shows the results of completing the secondary alignment. The secondary is centered in the view, the reflected image of the primary appears concentric with the secondary, and the center dot (or re-enforcement ring) appears centered on the Cheshire eyepiece cross hair.

    Chances are, the reflected image of the Cheshire eyepiece (the red cross hair) will not be centered.



    Collimation -- Primary

    To center the reflected image (see illustration above) of the Cheshire eyepiece, adjust the primary alignment using the adjustment screws on the back of the primary mirror cell, as shown in this animation.

    Note that some mirror cells simply have 3 spring-loaded adjusting screws, while others have 3 pairs of screws. When in pairs, usually one of the pair is a tensioning screw that must be loosened, and the other is the actual adjusting screw.

    If you have the pairs of screws, loosen all three tensioning screws, do the adjustment with the adjusting screws, then re-tighten the tensioning screws.

    Whichever method your primary cell uses, tweak them until you can co-align the Cheshire reflection with the cross hairs built into the Cheshire eyepiece.

    If you are successful in performing the previous two steps, you should see something like the image above when you look through your Cheshire eyepiece.

  • Note that the Cheshire, the secondary, and the image of the primary all appear concentric.

  • The re-enforcement ring you placed on the center of the primary appears centered on the cross hairs of the Cheshire eyepiece.

  • The reflected image of the Cheshire eyepiece also appears centered on the cross hairs.

  • If you see this, you will have a well collimated telescope that should provide good images. Also, subsequent alignments will be less dramatic unless you've had to remove the mirrors for cleaning.

    If you wish, you can do a final tweak of the collimation on your next outing by examining an inside and outside of focus star image. You might need minor adjustments of your primary to get an out of focus star image that looks like preciously centered concentric rings.