PART VIII. COLLIMATION OF THE TELESCOPE
Collimation is the alignment of the optical parts of a telescope. It's
intuitively obvious that these must be lined up to produce high-quality
images, but the necessity of doing so becomes more severe the shorter
the focal ratio of the scope. On the other hand, the same techniques for
collimating the short focal ratio can be equally well-used on a longer
focal ratio, so the techniques you learn for one will be used on the other.
Of the telescope types we've discussed, the one most likely to need collimation
is the Newtonian reflector (which includes Dobsonians). The Schmidt-Cassegrain,
though easy to collimate, is relatively unlikely to need it, so instructions
for SCT collimation will not be found here.
The most common tools that can be used to collimate the Newtonian reflector
--Combination Sight Tube/Cheshire
What follows is a step-by-step method of how to collimate the reflector,
and which tool to use for each task. What is described is the Modern Partial-Offset
Collimation Protocol, which is described many places in the literature.
In this protocol, all optical surfaces are aligned to the focuser axis,
so we will start out assuming that is OK. If the focuser is tight on the
tube, its alignment will be good enough for excellent collimation. The
entire procedure can be done in daylight, and it helps to point the telescope
at a bright sky for this, BUT DON'T POINT CLOSE TO THE SUN.
COLLIMATION OF THE NEWTONIAN TELESCOPE
1. Aligning the Secondary mirror in the tube. The tool used here is the
Sight Tube, a longish cylinder with a set of crosshair wires at one end
and a peep-hole on the other. The Sight Tube is inserted in the focuser
until the outline of the inside diameter of the sight tube appears to
surround the outside diameter of the secondary mirror, and then is fastened
tightly with the setscrew. If you can't tell where the edge of the secondary
mirror is, hold a piece of white paper against the inside of the tube
opposite to the focuser. This will surround the secondary mirror with
a white background, making it easy to see where the edge of the mirror
is. What we're trying to do here is to make the outside edge of the secondary
mirror appear concentric with the inside diameter of the end of the sight
tube, as seen through the peep hole. If the secondary appears too far
up the tube, away from the primary, its center bolt will need to be loosened
and the secondary lowered in the tube toward the primary. Do this with
the tube nearly horizontal to avoid the possibility of dropping a tool
on the primary mirror.
Once the mirror is in the center in the up-down tube direction, it will
need to be rotated until it is centered in the side-to-side direction.
Start out by rotating the secondary on the center bolt until it appears
round to the eye. Then, carefully use the screws on the secondary to make
the round image of the secondary appear concentric with the inside diameter
of the sight tube.
This will only have to be done once, so take your time to make it right.
2. Aligning the Secondary mirror to the primary. The tool used here is
the Sight tube or the point-source red laser collimator. The laser must
itself be collimated or using it will misalign the scope. Many low-cost
lasers come out of the box miscollimated. It is for this reason we recommend
the sight tube instead. Insert the sight tube and fasten the setscrew
tight. Look at the crosshairs through the peep-hole and note the distant
center dot on the primary mirror*
[*if the primary mirror does not have a center dot, you will have to
remove it and put one on the mirror. If you don't know how to do this,
have a local shop or astronomer friend help you out. You cannot successfully
collimate without one.]
You will carefully move the collimation screws on the secondary until
the center dot is lined up exactly behind the crosshairs intersection.
What you're doing here is to tilt the secondary mirror to point directly
at the center of the primary mirror. That's why we're adjusting the secondary
mirror. This may seem to move the round image of the outline of the secondary
out of concentricity with the inside diameter of the sight tube. If so,
repeat step one and step two again. Each iteration brings the secondary
closer to exact alignment. If you have to choose between the two, pick
the dot line up with the crosshairs. The centering of the secondary mirror
in the focuser is only to provide even illumination of the image all the
way around the edge of the field of view-less important that correct pointing
of the secondary mirror.
3. Aligning the Primary mirror. The tool used here is the Cheshire eyepiece,
or the combination sight tube + Cheshire tool. A Cheshire eyepiece is
a cylindrical tool with a hole in the side of it and an internal 45 degree
mirror to reflect light from the sky down onto the primary mirror and
back. Be careful when you insert this tool that you do not cover this
hole with the bill of a cap.
What this tool provides is a reflected bright ring with a dark center.
We will use the collimation screws on the Primary mirror to move the reflected
image of the center dot into the dark center of our bright ring of reflected
light. It may be necessary, if a lot of movement is required, to repeat
step two, and then return to step three. In a properly collimated telescope
both steps will agree at the same time. If a combination tool is used,
the crosshairs, center dot, and dark center will all line up at the same
[If you have a lot of light in the tube, a distant reflection of the
underside of the crosshairs will be visible, but a lot smaller than the
near-to-the-eye crosshairs in the tool. When the telescope is collimated,
this distant reflection of the crosshairs will be hidden behind the near-field
If your reflector is f/6 or longer, you can stop here. Indeed, if you
have been careful, any Newtonian will be well collimated by now. But,
for the users of f/5 scopes and shorter, some improvement can yet be had:
4. Eliminating all residual errors. The tool used here is the Autocollimator.
No, it doesn't do the work for you, but it allows you to see tiny misalignments
of the mirrors and correct them to a high degree. On short f/ratio scopes,
where the allowable tolerances on miscollimation are in the thousandths
of an inch, such a tool comes in quite handy. This tool is so sensitive
to misalignment that simple mechanical flexure will be visible. If your
secondary spider vanes are too loose, the telescope will not hold collimation
to a few thousandths through all the altitude changes the telescope goes
through, so the first thing to do here is to tighten the screws that hold
the secondary spider to the tube. Be careful! Though they should be quite
tight, most of them are small screws that can be stripped if applying
too much force. Most commercial scopes come with these screws quite loose,
however, and tightening is almost always called for.
The Autocollimator is now inserted and tightened. This is a tool with
an internal mirror that faces the secondary mirror. As you can guess,
when alignment is achieved, there is no light in the peep-hole and the
whole field goes dark. Because 4 reflections of the mirror's center dot
will be visible, you are essentially collimating the telescope exactly
on a point 8 times the focal length of your telescope away. If done carefully,
collimation will be accurate to a small number of thousandths of an inch.
This accuracy rewards the observer with the best possible images that
can be had with the telescope.
When looking in, there will be 4 images of the primary's center mark.
If your previous steps were carefully done, these 4 images will overlap
or nearly "stack" on top of one another. But, if all 4 are seen, better
alignment can be achieved. By carefully moving the secondary collimation
screws, the four reflected images of the center mark will stack tightly
on top of one another. It is desirable to return to the Cheshire at some
point and check alignment of the primary. If it's off, correct it in the
Cheshire (turning the primary collimation screws), and return to the autocollimator.
After a couple times back and forth, all four images of the center mark
will "stack" tightly on top of one another, and your telescope is perfectly
The next time the telescope is taken out, step one will not need to be
done. It's probably not a bad idea to check the alignment of the secondary
with the sight tube, but it is unlikely that more than a minor tweak of
one screw will be necessary. Then, proceed to steps two and three. If
you use an autocollimator, skip step one and start with step two. The
autocollimator will perfectly align the secondary.