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Newtonian Collimation


Collimating a Newtonian Telescope

I'm 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. While no longer available from Discovery, it's basically the same as the Celestron 31057 Omni XLT - 150, a 6 inch short focus Newtonian on an Equatorial mount.

But I must admit that one aspect of Newtonian and Dobsonian telescopes that can put people off is the collimation process. And short focus models like my 6 inch f/5 can be a challenge.

However, with a bit of setup and an alignment tool, the process 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, described on this web page, is the Cheshire eyepiece, like the Celestron Collimation Eyepiece 1.25" .



What Is A Cheshire Eyepiece,You Ask?

Cheshire Eyepiece, Top View Celestron Collimation Eyepiece top view

On the above left you see my effort to make a Cheshire or collimation eyepiece. On the right you see a much better version, 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.

I would suggest you purchase your Cheshire eyepiece, but you can do as I did and make one. Mine is a bit crude, but definitely works. I was having a devil of a time getting my 6 inch f/5 Newtonian optics properly aligned until I cobbled together this 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.

Cheshire Eyepiece, Bottom View Celestron Collimation Eyepiece bottom view

Here's a look at the Cheshire eyepiece from the bottom. Again on the left you see my homemade version, on the right the Celestron version. Notice the cross hair just inside the bottom rim. The cross hair must be aligned with the peephole at the top of the eyepiece.

If this alignment is correct, then looking through the peephole and aligning what you see with the cross hair will put things into alignment.

Cheshire Eyepiece Reflector
Celestron Collimation Eyepiece side view

Above is the side view of the Cheshire, showing the details of the cutout. On the left is my homemade version, on the right the Celestron version. Notice the white 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.

I used to use an 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.

While my efforts show that you can certainly make one of these devices yourself, and for me at least I found the experience illuminationing, you may do better to simply buy one. It's likely more accurate, and that's a good thing. On the one I made, I found after I purchased the commercial version that I did two things poorly. I made mine too short, and the peep hole was too big. The commercial version is about 6 inches long versus my homemade version at 4 inches. And I made the peep hole nearly 1/4 inch, but the commercial version has a peep hole perhaps 1/16 inch.



centering template

To Aid In Alignment, Prepare the Primary

If you've not done it yet, you need to place a notebook reenforcement ring on the center of your primary -- or paint a dot on the center of your primary. Don't worry, the secondary blocks off light from the center of the primary anyway. So we can make best use of it as an aid to alignment.

A simple way to get your reenforcement ring centered is to use a compass to draw a circle on some thin cardboard the diameter of your primary. 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, the fold lines helping 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 reenforcement 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 collimations.



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 green circle represents the image of the secondary, which should be centered in the Cheshire eyepiece view.

The blue circle represents the reflected image of the primary mirror, which may initially be quite off center and not even entirely visible.

The black dot represents the notebook ring on the primary.

The red circle with cross hair represents the reflected image of the Cheshire eyepiece.



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 reenforcement 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 reenforcement 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 reenforcement 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.