Orion XT4.5 Appendix A, Collimation, Aligning the, Mirrors, scope or the image quality. That

Appendix A:

optical performance of the tele-

Collimation

scope or the image quality. That

might seem counterintuitive, but

(Aligning the

it’s true!

Mirrors)

Aligning the Secondary

Collimation is the process of adjust-

Mirror

ing the mirrors so they are perfectly

With the collimation cap in place,

aligned with one another. Your tele-

look through the hole in the cap at

scope’s optics were aligned at the

the secondary (diagonal) mirror.

factory, and should not need much

Ignore the reflections for the time

adjustment unless the telescope is

being. The secondary mirror itself

handled

roughly. Accurate

mirror

should be centered in the focuser

alignment is important to ensure the

drawtube, in the direction parallel to

peak performance of your telescope,

the length of the telescope. If it isn’t,

so it should be checked regularly.

as in Figure 16b, it must be adjusted.

Collimation is relatively easy to do

It helps to adjust the secondary mir-

and can be done in daylight.

ror in a brightly lit room with the

To check collimation, remove the eye-

a.

telescope pointed towards a bright

surface, such as white paper or wall.

piece and look

down the

focuser

Placing a piece of white paper in the

drawtube. You should see the sec-

telescope tube opposite the focuser

ondary

mirror

centered

in

the

(in other words, on the other side of

drawtube, as well as the reflection of

the secondary mirror) will also be

the primary mirror centered

in

the

helpful in collimating the secondary

secondary mirror, and the reflection of

mirror. Using a 2mm Allen wrench,

the secondary mirror (and your eye)

loosen the three small alignment set

centered in the reflection of the pri-

mary mirror, as in Figure 16a. If

screws in the center hub of the 4-

vaned spider several turns. Now

b.

anything

is off-center, as in

Figure

c.

16b, proceed with the following colli-

hold the

secondary

mirror

holder

stationary

(be careful

not to

touch

mation procedure.

the mirror's surface), while turning

The Collimation Cap and

the center screw with a Phillips head

Mirror Center Mark

screwdriver (See Figure 17). Turning

Your SkyQuest XT4.5 comes with a

the screw clockwise will move the

secondary

mirror toward the front

collimation cap. This is a simple cap

opening of

the optical tube,

while

that fits on the focuser drawtube like

turning the screw counter-clockwise

a dust cap, but has a hole in the

will move

the secondary

mirror

center and a silver bottom. This

e.

d.

helps center your eye so that colli-

toward the

primary

mirror. When

making these adjustments, be care-

mation is easy to perform. Figures

Figure 16. Collimating the optics. (a) When the mirrors are

ful not to stress the spider vanes or

16b-e assume you have the collima-

properly aligned, the view down the focuser drawtube

they may bend.

tion cap in place.

should look like this. (b) With the collimation cap in place, if

In addition to providing the collimation

the optics are out of alignment, the view might look

When the

secondary mirror is cen-

something like this. (c) Here, the secondary mirror is

tered in the focuser drawtube, rotate

cap, you’ll notice a tiny ring (sticker) in

centered under the focuser, but it needs to be adjusted

the secondary mirror holder until the

the exact center of the primary mirror.

(tilted) so that the entire primary mirror is visible. (d) The

reflection of the primary mirror is as

This “center mark” allows

you to

secondary mirror is correctly aligned, but the primary mirror

centered in the secondary mirror as

achieve a very precise collimation of

still needs adjustment. When the primary mirror is correctly

possible. It may not be perfectly cen-

the primary mirror; you don’t have to

aligned, the “dot” will be centered, as in (e).

tered, but that is OK. Now tighten the

guess where the center of the mirror

is. You simply adjust the mirror posi-

three small alignment screws equally

to secure the secondary mirror in that position. This adjustment will

tion (described below) until the reflection of the hole in the collimation

rarely, if ever need to be done.

cap is centered inside the ring. This center mark is also required for

best results with other collimating

devices, such as Orion’s

If the entire primary mirror reflection is not visible in the secondary

LaserMate Laser Collimator, obviating the need to remove the pri-

mirror, as in Figure 16c, you will need to adjust the tilt of the second-

mary mirror and mark it yourself.

ary mirror. This is done by alternately loosening one of the three

NOTE: The center ring sticker need not ever be removed from	alignment set screws while tightening the other two, as depicted in
	Figure 18. The goal is to center the primary mirror reflection in the
the primary mirror. Because it lies directly in the shadow of the
	secondary mirror, as in Figure 16d. Don’t worry that the reflection of
secondary mirror, its presence in no way adversely affects the

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