How to Collimate a Reflector Telescope

home:  dbpeckham.com

Overview:

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".

Contents:

Twitter Version:

Adjust 2 mirrors in 3 steps to align eyepiece and primary mirror centerlines.

Short Version:

This is the basic description of how to collimate a reflector telescope.  Go to the "long version" for more detail.

Image
Image

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.
Image
Image
Image

Printable collimation document

Long Version:

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.

Reflector Telescope Image:

Light enters the open end of the telescope, reflects off the parabolic primary mirror to the diagonal mirror, and to the eyepiece in the focuser.

Reflector Telescope

Identification:

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 eyepiece.
  • Primary Mirror and Center Mark - 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.)

Image

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.

Image

Required Tools:

Collimating eyepiece
  • 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.

Cheshire
Cheshire 2

Screwdriver(s) and or Allen wrench(s) (depends on spider)

Disclaimers:

  • 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 Degrees
  • 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 mirror cell.
  • 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 primary mirror.
  • WARNING - Be careful to NOT point the tube at the sun when collimating outside.
45 degree angle

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.

Eyepieces

Add Paper Behind And Below The Diagonal Mirror (Recommended)
Paper
  • 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.

Image
Image
3rd

Typical Spider:

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 SIDE adjustment.
  • 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 adjusted.

Spider

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 should be concentric with the inside of the collimating eyepiece.  The images below show the diagonal mirror (red outline) viewed through the collimating eyepiece (blue): (Your images will not look EXACTLY like the images below.)
ImageStep 1
  1. Image A - diagonal mirror is to the left – diagonal mirror needs to move to the right.
    • 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 center screw.
  2. Image B - diagonal mirror is to the bottom – diagonal mirror needs to move up.
    • Adjust the spider screws that mount the diagonal mirror to adjust the diagonal mirror as required.
  3. Image C - diagonal mirror is rotated – diagonal mirror needs to be rotated
    • Loosen the three screws at the top of the diagonal mirror holder, and rotate the diagonal mirror holder as required.
  4. Image D - diagonal mirror is concentric – diagonal mirror is correctly aligned. Go to Step 2.
  5. 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:

  1. Remove the paper that blocks the path between the diagonal and primary mirrors.
    • 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.
  2. 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 images below.)
    • 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 mirror.
Step 2
Image Image

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.
Mirror Cell
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 primary mirror.
  • 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 concentric.

Step 3
Image Image

Collimate Without A Collimating Eyepiece:

The collimation process without a collimating eyepiece has the same three steps with a collimating eyepiece.
telescope
Image
Image
Image
Image

Add Center 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.
Center mark

References: