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How to Collimate a Telescope That Has a Focal Length Doubler

The optics of any telescope can go out of alignment, but some types of telescopes are more prone to misalignment than others. Catadioptric telescopes are carefully checked before leaving the factory; they rarely need adjustment, and can only be adjusted at the factory.

Refractors rarely need collimation (the alignment of optical components) because their lenses are permanently mounted and do not move; any need for collimation indicates damage. However, Newtonian telescopes are notorious for optical misalignment and require frequent collimation. A focal length doubler, which you can remove or leave in place during collimation, adds no difficulty to the realignment process.

Things You'll Need

4-by-4-inch piece of cardboard
Small screwdriver (the type depends on the adjustment screws on the telescope flat)
Large screwdriver (the type depends on the adjustment screws on the primary mirror)
Assistant

Instructions

- 1
  Remove the eyepiece from the drawtube, leaving the focal length doubler in place if you desire. Use the focus knob to move the drawtube out as far as possible, but do not allow it to fall out. Remove the tube cap (the cover at the end of the telescope).
- 2
  Place a piece of cardboard over the open end of the drawtube. Perforate the piece of cardboard to form a central hole measuring about 5mm in diameter, and center this hole over the drawtube. Point the open end of the telescope toward a light-colored, brightly illuminated wall.
- 3
  Look through the hole in the cardboard so that the flat (i.e., the small mirror positioned inside the telescope tube) is visible. If the flat is not exactly centered within the field of view, adjust its position using the adjustment screws on the flat support. You can access these screws by reaching into the open end of the telescope.
- 4
  Adjust the tilt of the flat so that the image of the large primary mirror (i.e., the large mirror at the base of the telescope) appears centered within the flat. The screws that adjust the tilt of the flat are located near the first set of adjustment screws. Do not change the first set of adjustment screws, or you will have to return to Step 3.
- 5
  Locate the three adjustment screws for the primary mirror, which you'll find on the outside bottom of the telescope. Do not confuse these adjustment screws with the screws holding the mirror in place within the telescope. Examine the six screws at the base of the telescope carefully to identify which ones hold the mirror in place, and which ones adjust the mirror's position.
- 6
  Direct an assistant to look through the hole in the cardboard while you make primary mirror adjustments at the base of the telescope. Slowly adjust the screws on the primary mirror until the flat, as viewed by your assistant, appears centered in the reflection of the primary mirror.
- 7
  Set up the telescope as you would for deep-sky viewing. Focus on a fairly bright star, and then slowly defocus the eyepiece so that the star begins to blur.
  
  Check the expanded view of the star for symmetry; it should appear circular, but out of focus. If the star appears to be irregular in shape, make very minor adjustments to the primary mirror to bring the image into a perfect circle. (You may wish to have your assistant view the star while you make these adjustments.)
  
  When the star image is circular, the telescope is properly collimated. Removing or replacing the focal doubler will not change the collimation.

Astronomy