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
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.
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.
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.
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
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 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"
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
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
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.
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
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 -- 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
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.
Again, be careful not to look directly into the laser beam itself.
How To Collimate With A Cheshire Eyepiece
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.
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.
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
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.
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.
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 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
If you have the pairs of screws, loosen all three tensioning screws,
do the adjustment with the adjusting screws, then re-tighten the tensioning
Whichever method your primary cell uses, tweak them until you can co-align
the Cheshire reflection with the cross hairs built into the Cheshire
If you are successful in performing the previous two steps, you should see
something like the image above when you look through your Cheshire
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
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