How to Collimate a Celestron C-90

Initial Star Test

• 1

  Take your scope to a location where you have an unobstructed view of the sky on a clear, dark night. Allow the scope to reach the air temperature of the environment; this usually takes 30 to 60 minutes.

• 2

  Sight your scope on a bright star and make sure the image is in the exact center of the field of view.

• 3

  Bring the image in and out of focus so that it changes from a bright pinpoint -- the in-focus image -- to a large nest of concentric, less bright circles -- the out-of-focus image.

• 4

  Examine the out-of-focus circles to see if they are concentric or asymmetrical. If the out-of-focus circles are symmetrical, your C-90 is properly collimated, and no adjustments are needed. Your scope is ready for use.

Collimation

• 5

  Move your C-90 indoors to a well-lighted room if it is out of collimation. Allow at least 30 minutes for it to come to the room's temperature.

• 6

  Point the scope at a white, unmarked wall and remove the eyepiece.

• 7

  Look into the rear opening of the telescope tube with your eye centered on it as closely as you can.

• 8

  Examine the image -- which will include a reflection of your eye in the center of your field of view -- to see whether the reflection of the inside of the telescope tube on the periphery of your view is uniformly dark and thick all around or forms a crescent that is darker and wider on one side. An asymmetrical crescent indicates that the collimation needs adjustment.

• 9

  Locate the adjustment screws surrounding the rear opening; these adjust the tilt of the mirror cell. Very slightly (no more than a quarter turn) adjust the one(s) closest to the darkest section of the crescent. Turn the screw(s) -- using the appropriate size Allen wrench or screw driver -- right or left depending on which direction narrows the dark section and makes it a thinner line.

• 10

  Adjust the screws on the opposite side of the opening in the opposite direction to equalize the pressure on the mirror cell and stabilize your realignment.

• 11

  Repeat steps 3 through 6 as necessary to achieve a concentric and even image of the inside of the C-90 telescope tube. When the image you see through the rear of the telescope tube is even and concentric, repeat the Initial Star Test as a final test of collimation.