Home >> Science & Nature >> Astronomy

The Best Way to Collimate a C8

The popular Celestron 8-inch Schmidt-Cassegrain Telescope (SCT), or C8, has three optical elements: one front-mounted transparent lens called a corrector, a rear-mounted 8-inch primary mirror and a smaller secondary mirror mounted in the corrector's center. The elements must be in precise alignment, or collimated, in order for the C8 to perform as designed. Collimation requires adjusting the angle of the secondary mirror relative to the primary mirror by moving three screws at the front of the telescope. The easiest and most common method to check a C8's collimation is to observe a defocused bright star through the telescope.

Things You'll Need

Telescope mount
90-degree star diagonal
10mm eyepiece
Phillips or Allen screwdriver

Instructions

- 1
  Assemble the C8 and telescope mount at a location that provides an unobstructed view of the area of sky along the celestial meridian and near zenith. Steady star images are required for best results. Stars appear steadiest near zenith, and the collimation adjustment screws are easiest to reach with many mounts when the C8 is pointed toward the celestial meridian.
- 2
  Follow the specific instructions of the mount's manufacturer to align and orient the C8. Because the star image must remain well centered in the C8's field of view throughout the star-test process, accurate tracking is essential and relies on accuracy of physical alignment and/or electronic orientation of the mount.
- 3
  Install a 90-degree star diagonal in the C8, and then install a 10mm eyepiece in the star diagonal. The 90-degree star diagonal ensures that the eyepiece is within comfortable reach throughout the collimation process.
- 4
  Allow sufficient time for the temperature of the C8's optics to match the ambient temperature. That may take as long as one hour. Optical components that are just a few degrees from ambient temperature may degrade star images enough to render them useless for a collimation test.
- 5
  Aim the C8 at a bright star near zenith and along the celestial meridian, and engage the mount's tracking function. Center the star precisely in the C8's field of view, and adjust focus until the image of the star is smallest. Observe the star for one minute or longer to confirm that the mount is properly aligned and oriented and that the star remains well centered. Observe the star also to determine that viewing conditions provide a steady image.
- 6
  Defocus the star image until it transforms into numerous alternating bands of light and dark rings of expanding diameter, called "Airy rings." If these rings are not perfectly concentric and the dark shadow of the secondary mirror is not precisely centered within them, then the telescope is out of collimation and the secondary mirror must be adjusted until the rings are precisely concentric and the secondary shadow is perfectly centered.
- 7
  Hold the appropriate screwdriver in one of the three collimation screws at the front center of the C8 while viewing the defocused star through the eyepiece. The stock collimation screws have either Phillips or Allen heads, which signifies which kind of screwdriver to use. Adjust the screw no more than 1/4 turn in either direction while viewing the defocused star and observing the effect on the rings and central shadow.
  
  Repeat this procedure with each screw until you find the appropriate combination of adjustments, the alternating rings are perfectly concentric and the central shadow is precisely centered.

Astronomy