How to Collimate a Reflector Telescope
This document describes the
process for collimating (aligning) the optics of a reflector telescope
so that the maximum photons reach the eyepiece, at the best
focus. It starts with the "Twitter version
" and "short version
and then expands to
the "long version
" that includes the
"HOWs" and "WHYs".
Adjust 2 mirrors in 3 steps to align eyepiece and primary mirror centerlines.
This is the basic description of
how to collimate a reflector telescope. Go to the "long version
" for more detail.
|Step 1 - Adjust (up-down, side-side, rotation) the diagonal (secondary) mirror so it is centered in the eyepiece.
||Step 2 - Adjust the angle of the diagonal mirror so the primary mirror center mark centers on the eyepiece crosshairs.
||Step 3 - Adjust the angle of the primary mirror so the eyepiece center hole centers on the primary mirror center mark.
The long version describes the
collimation process in detail and explains both the HOWs and the
WHYs. It is intended for individuals who are new to collimating a
Reflector Telescope Image:
Light enters the open end of the
telescope, reflects off the parabolic primary mirror
to the diagonal
, and to the eyepiece
in the focuser
The interiors of most telescopes
are flat black. Color is used in the following images to make it
easier to distinguish items. Starting with the eyepiece, items
are identified in clockwise order:
- Eyepiece with
Horizontal and Vertical Crosshairs - The eyepiece is blue.
Notice the vertical and horizontal cross hairs.
- Diagonal Mirror
- The diagonal mirror is shown in red. The diagonal mirror
diameter appears slighly smaller than the inside diameter of the
- Primary Mirror and
- The primary mirror is shown green. The primary mirror typically
has three hold-down tabs. The center mark is shown. If the
mirror does not have a center mark, it is worthwhile to add a center
mark (Instructions for adding a center mark are described at this
link.). (Note: Pay special attention to the primary mirror center mark during collimation.)
- Diagonal Mirror with
Diagonal Spider Vanes - The reflected image of the diagonal
mirror is shown.
- Eyepiece Center Hole - Notice the diagonal mirror center
hole. (Note: Pay special attention to the eyepiece center hole during collimation.)
Concentricity - Centered:
Look for concentricity while
collimating a telescope.
- For the image above the items are NOT concentric
(centered). This image is of a telescope that is NOT collimated.
- For the image below all circles and crosshairs are
concentric (centered). This is how a collimated telescope appears.
- All collimating eyepieces have a small hole on the end of
the eyepiece. This hole is in the center of the eyepiece.
The user's eye is at the centerline of the eyepiece when looking
through the hole.
- Some eyepieces have crosshairs at the open end of the
eyepiece. The crosshairs are on the centerline of the eyepiece.
- The eyepiece shown is a "Cheshire" eyepiece. It has
an angled reflective surface with a black spot in the center to make
the center of the eyepiece more apparent when collimating a
telescope. See the left image below.
Screwdriver(s) and or Allen wrench(s) (depends on spider)
- Mounting hardware and adjusting tools for primary and
diagonal mirrors may vary from telescope to telescope.
- Some primary mirrors have the center marked with a ring,
dot or by some other means
Prepare for collimation:
Put the Telescope Tube At 45
- When the tube is vertical, gravity is in line with the
light path. Gravity pulls the diagonal mirror straight down from
the spider. It pushes the primary mirror straight down into the
- When the tube is horizontal, gravity is perpendicular to
the light path. The diagonal mirror and primary mirrors are
cantilevered from their mounts. Gravity tries to deflect the
mirrors from the light path. The mirrors push down on the ends of
the tube and the altitude bearings push up, trying to bend the
tube. Therefore, there may be a
slight change in collimation when
the tube moves between vertical and horizontal.
- 45 degrees is in the middle of the observing range.
Collimating the telescope at 45 degrees minimizes any difference in
collimation that may result from the effects of gravity.
- There is another advantage of collimating with the tube at
45 degrees. If a tool is dropped while adjusting the diagonal
mirror, it will slide down the inside of the tube and NOT drop on the
- WARNING - Be
careful to NOT point the tube at the sun when collimating outside.
Insert The Collimating Eyepiece
- The eyepiece shoulder should be against the focuser
shoulder so the centerline of the eyepiece will be the same as the
centerline of the focuser.
- Move the focuser to the maximum out position, away from the
diagonal mirror, so the outline of the diagonal mirror will be easier
to see when looking through the eyepiece.
Add Paper Behind And Below The Diagonal Mirror (Recommended)
- Without Paper (First Image) - This is the view through the
eyepiece with no paper.
- With Paper Behind Diagonal Mirror (Second Image) - The
paper behind the diagonal mirror makes it easier to define the outline
of the diagonal mirror when looking through the collimating eyepiece.
- With Paper Below Diagonal Mirror (Third Image) - The paper
blocking the path between the diagonal and primary hides the reflected
image of the primary mirror.
There are many different spider
designs. Many spiders have a "push-pull" design where three
adjust screws push DOWN, and the center screw pulls UP. The three
screws "push" against the diagonal cylinder while the center screw
"pulls". "UP" is the directon toward the open end of the
telescope tube, and "DOWN" is the directin toward the primary mirror.
- UP - To adjust up,
loosen the three adjusting screws, maybe
a half a turn. Keep track of how much each screw is turned.
Don't overloosen the screws. Tighten the center screw.
- DOWN - To adjust
down, loosen the center screw, maybe a
half a turn. If the center screw is loosened too much the
diagonal cylinder (that mounts the diagonal mirror) may fall out of the
spider. Therefore, hold the diagonal cylinder. DO NOT touch
the mirror surface.
- SIDE to SIDE - To
move the spider to the side, loosen the
screw on one spider vane, maybe one turn. Tighten the screw on
the opposite spider vane. Some spider designs do not have SIDE to
- ROTATE - To rotate
the diagonal cylinder, loosen the center
screw, maybe a half turn. Rotate the diagonal cylinder, and then
tighten the center screw.
- ADJUST DIAGONAL -
To aim the diagonal mirror toward the
primary mirror, loosen one of the three adjusting screws, maybe a half
turn, and tighten another of the adjusting screws.
After making adjustments, be sure to tighten ALL screws that were
Step 1: Adjust the diagonal
mirror (up-down, side
to side, rotate) so it aligns with the eyepiece (focuser):
- For this example the open end of
the telescope is at the left and the primary mirror is at the right.
- IMPORTANT: The up-down, side-to-side position of the diagonal mirror (with respect to the eyepiece)
is NOT critical. Some spiders have no up-down or side-to-side
adjustment. The amount of light reaching the eye may be slightly
smaller if the diagonal mirror is not perfectly centered with respect to the eyepiece. However, sharp focus can be achieved with the adjustments in steps 2 and 3.
Looking through the hole in the
collimating eyepiece, the image of the diagonal mirror
concentric with the inside of the collimating eyepiece
images below show the diagonal
(red outline) viewed through the collimating eyepiece
(blue): (Your images will not look EXACTLY like
the images below.)
- Image A - diagonal
mirror is to the left – diagonal mirror needs to move to the right.
Image B - diagonal
mirror is to the bottom – diagonal mirror needs to move up.
- Loosen center screw on end of the diagonal mirror holder
- Tighten the three alignment screws on end of the diagonal
mirror, and check to see if mirror is centered, left and right, when
looking through the collimating eyepiece.
- If the diagonal mirror is to the right, loosen the three
screws on end of the diagonal mirror holder, and then tighten the
Image C - diagonal
mirror is rotated – diagonal mirror needs to be rotated
- Adjust the spider screws that mount the diagonal mirror
to adjust the diagonal mirror as required.
Image D - diagonal
mirror is concentric – diagonal mirror is correctly aligned. Go to Step
- Loosen the three screws at the top of the diagonal mirror
holder, and rotate the diagonal mirror holder as required.
Repeat items 2-4 as required until the diagonal mirror is
concentric per image D.
Note: It is typically NOT necessary to perfectly center the diagonal mirror in the eyepiece. "UP-DOWN"
and "SIDE-SIDE" adjustments assure that the maximum photons reach the
eyepiece. Sharp focus can be achieved with Step 2 and Step 3 even
if the diagonal mirror is not perfectly centered in the eyepiece.
If perfect centering is desired, it may be necessary to adjust the focuser to align it with the diagonal mirror.
Step 2: Adjust the angle of the diagonal mirror so it aligns
with the primary mirror:
- Remove the paper that blocks the path between the diagonal
and primary mirrors.
Adjust the screws on the end of the diagonal mirror support
to center the cross hairs on the little reinforcing ring in the center
of the primary mirror. When the cross hairs are centered on the
reinforcing ring**, the image of the primary mirror is concentric with
the diagonal mirror. (Your telescope will not look EXACTLY like the
- The edge of
the diagonal mirror is shown in red. The edge of the primary mirror is
shown in green.
- The diagonal lines
are the vanes of the diagonal spider.
- The image on the LEFT is BEFORE alignment for a primary
mirror with a reinforcing ring.
- The image in the RIGHT is AFTER alignment for a primary
mirror with a reinforcing ring.
**If there is no reinforcing ring or other center marking, align the
image of the primary mirror so that it is concentric with the diagonal
Typical Primary Mirror Cell
- The primary mirror typically has a "push-pull"
design. The three small screws push against the mirror
cell. The three thumb screws pull the mirror cell against springs.
- To adjust the angle of the primary mirror, loosen the three
small screws. Then turn the three thumb screws to adjust the
angle of the primary mirror. After the mirror angle is adjusted,
tighten the small screws.
Note: The three thumb screws are typically in the middle of their adjustment range.
If a thumb screw reaches the end of adjustment BEFORE the primary
mirror can be adjusted, readjust all three thumb screws to the middle
of their range, and then proceed with mirror adjustment.
Step 3: Adjust the angle of the primary mirror so it aligns with
the eyepiece (focuser):
- Loosen the three tensioning screws at the bottom of the
- Adjust the alignment screws at the bottom of the primary
mirror until the small black circle at the center of the reflected
image of the diagonal mirroraligns with the cross hairs.
- Turn a mirror adjustment screw and watch what happens with
the image. If the diagonal image gets closer to the cross hairs, good.
If not, turn the screw the opposite direction.
- Turn the mirror adjustment screws one at a time, as
required, until the image is concentric as shown.
- After everything is concentric, snug down the tensioning
screws. Your image will be similar to the image below with EVERYTHING
Without A Collimating Eyepiece:
The collimation process without a
collimating eyepiece has the same three steps with a collimating
Mark To Primary Mirror:
This is how to add a center mark
to a primary mirror:
- On paper, draw a circle the diameter of the primary mirror.
- Cut out the circle and fold the paper in half, and then in
half again to make a “pie-shaped” folded paper.
- Cut off the point of the “pie-shaped” paper 3/8' (10 mm)
from the point.
- Unfold and lay the paper on the mirror.
- Attach an adhesive backed reinforcing ring (from office
supply store) to the center of the mirror using the paper as a template.