the Basics of Collimation
by Tom Clark
Reprinted from Amateur
collimation? Simply the alignment of the optics.
The mirrors or objective lenses must be lined up properly for the telescope
to deliver a good image to the eyepiece.
- How critical
- Have you ever noticed that one scope
will sometimes outperform another identical scope? Why? The difference
is usually in the collimation. Even if the optics in one are slightly better
than the other, usually the one with the best collimation will be the best
- How often
do you have to collimate?
- Every time you use your scope. Even
if it is mounted in an observatory, you can easily see the collimation
change with these tools, just from temperature changes.
- How long
does it take?
- Once you have your optics well aligned
the first time, it takes only a couple of minutes to fine tune your collimation
each time you set up.
- Do you have
to spot your optics?
- A tiny spot is required on Newtonian
primary mirrors. The spot lies in the shadow of the diagonal mirror, and
causes no diffraction. Some observers have experimented with using a white
reinforcement ring (like you use on notebook paper), but personally I still
prefer the 3/8" square piece of black tape. I guess, like anything else,
you like whatever you get used to. Do not spot your diagonal mirror!
If I have heard it once, I have
heard it one-thousand times. At star parties all over, astronomers have
looked into the eyepiece of my telescopes, and then say "Wow! What a great
mirror!" The article "The Performance of Large Mirrors" in Amateur
Astronomy issue #12 went over what it takes to achieve good performance
from your optical system, and touched briefly on collimation. Since then
many requests has been received for simple instructions on how to properly
collimate a telescope.
Every time I go to a star
party I am bombarded with requests for collimation demonstrations. After
I show them how to collimate a Newtonian they are always amazed at just
how simple it is. Just how close do you have to collimate? If you have
a long focal ratio of over f/6 your collimation is much more forgiving.
If your scope is faster than f/6, collimation is much more important. Once
you get below f4.5, collimation is very critical and must be done with
The collimation tools came
from several different articles in Telescope Making Magazine. I first built
a sight tube and then, after Tippy D'Auria showed me a Cheshire eyepiece
that belonged to Don Parker, the design was greatly simplified and I built
a couple of them to try out. The results have been phenomenal, and the
performance of our telescopes so drastically improved that the tools have
been in constant production since. Sky and Telescope did an article about
our collimation tools back in 1987, and now they are in use around the
To explain just how important
collimation is, try to understand this: If you have two similar mirrors,
and one is 1/20th wave, and the other is 1/8 wave, the one that will easily
outperform the other is the one with the best collimation.
The Sight Tube is the basic Newtonian tool. Many amateurs make the mistake of collimating
the primary mirror of their telescope until they are blue in the face......without
ever obtaining images to their satisfaction. The culprit is almost always
the diagonal mirror, and that is where the sight tube comes into it's own.
The sight tube's main purpose is to collimate the diagonal mirror first
by centering the diagonal mirror under the focuser. This automatically
adds the necessary offset of the diagonal mirror towards the primary mirror
by moving the diagonal mirror closer to the primary. The sight tube is
slid into the focuser until the outline of the end of the tube is just
slightly larger than the outline of the diagonal mirror (left side of illustration).
Then the diagonal is adjusted until it is centered in the sight tube.
Next, the tilt and rotation of the
diagonal mirror is adjusted until the reflection of the spot on the primary
is under the crosshairs of the sight tube. First, loosen the diagonal bolt
and rotate the diagonal until the dot is located on the horizontal cross
hair. Then loosen the three (or four) screws that control the tilt of the
diagonal. Tighten and loosen these screws as necessary until the center
spot on the primary mirror is under the site tube's cross hairs. This tilts
the diagonal mirror and aims it at the center of the primary mirror.
The Cheshire is the most used collimating tool in the world, and it collimates the Newtonian
telescope so simply that it almost needs no instructions. When you look
into the Cheshire, you usually see something like the left side of this
illustration. Just move the primary mirror adjusting screws until the image
looks like the right half of the illustration -- a circle with four ears
on it. That's it! You can check your collimation just as easily in full
darkness simply by shining a diffuse red light into the side cut out. Note:
The 45_ angle on the inside of the Cheshire is not a mirror. It is lightly
polished until it reflects light evenly, and hot harshly like a mirror
would. This helps diffuse the light that is shining into your eye to make
it easier to see. Tip: When using a red flashlight at night use one that
has a diffuse red light rather than a harsh point source such as a bare
LED light. This diffuse red light provides a soft red illumination that
makes using the Cheshire at night much easier.
If you collimate at dusk, be sure
to check it again after a few hours. You will find that temperature and
humidity changes have changed the collimation of your primary. Fine tune
it and your collimation will be set for the rest of the night. With my
scopes I have found that if the collimation is fine-tuned a couple of hours
into the first night, I rarely have to change it for the rest of the star
party! During the day the images will be slightly out of collimation, but
they will be back in place that night. Try it. . .
The Cheshire easily collimates refractors
also. Cap the front of the tube and put the Cheshire into the focuser.
Looking into the Cheshire, you will see two or more ghostly images that
look like doughnuts. Adjust the cell until they merge into one.
The Autocollimator is best used in the dim light of twilight, and only after the Sight Tube
and Cheshire have been used to get the alignment very close. The Autocollimator
has its own mirror, and changes the collimation system from a mechanical
system of aligning images to an optical one of aligning three mirrors.
This is much more accurate than the other two tools, and we use it as a
checking system to see that the previous steps have been properly taken.
If you have used the sight tube and
Cheshire accurately, the Auto will confirm that by showing a darkened field.
If the three mirrors are aligned there is no way for light to enter the
view. You should only see the result of many reflections of the mirrors
bouncing images back and forth. If the mirror is white, usually the diagonal
needs fine tuning as stray light is sneaking past your diagonal mirror.
This can usually be accomplished by hand, moving the diagonal mirror a
very small amount, by either rotating it or tilting it. If that doesn't
do it, you have to go back to the Sight tube and Cheshire and try again.
Our booklet "New Perspectives on Newtonian Collimation " (see link above) has 80 pages of detailed instructions, photos and CAD drawings.
It answers any questions you may have about collimation.