Out of collimation | Collimated |
| |
Figure 12. A star test will determine if a telescope’s optics are properly collimated. An unfocused view of a bright star through the eyepiece should appear as illustrated on right if the optics are perfectly collimated. If the circle is unsymmetrical, as in the illustration on left, the telescope needs collimation.
Star-Testing the Telescope
When it is dark, point the telescope at a bright star and accu- rately center it in the eyepiece’s field of view. Slowly de-focus the image with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle (Figure 12). If the image is unsymmetrical, the scope is out of collimation. The dark shadow cast by the secondary mirror should appear in the very center of the out-of-focus circle, like the hole in a donut. If the “hole” appears off-center, the tele- scope is out of collimation.
If you try the star test and the bright star you have selected is not accurately centered in the eyepiece, the optics will always appear out of collimation, even though they may be perfectly aligned. It is critical to keep the star centered, so over time you will need to make slight corrections to the telescope’s position in order to account for the sky’s apparent motion.
Appendix B:
Cleaning the Optics
Cleaning Lenses
Any quality optical lens cleaning tissue and optical lens clean- ing fluid specifically designed for multi-coated optics can be used to clean the exposed lenses of your eyepieces or finder- scope. Never use regular glass cleaner or cleaning fluid designed for eyeglasses
Before cleaning with fluid and tissue, blow any loose particles off the lens with a blower bulb or compressed air. Then apply some cleaning fluid to a tissue, never directly on the optics. Wipe the lens gently in a circular motion, then remove any excess fluid with a fresh lens tissue. Oily fingerprints and smudges may be removed using this method. Use caution; rubbing too hard may scratch the lens. On larger lenses, clean only a small area at a time, using a fresh lens tissue on each area. Never reuse tissues.
Cleaning Mirrors
You should not have to clean the telescope’s mirror very often; normally once every year or so. Covering the telescope with the dust cap when it is not in use will help prevent dust from accumulating on the mirrors. Improper cleaning can scratch mirror coatings, so the fewer times you have to clean the mir- rors, the better. Small specks of dust or flecks of paint have virtually no effect on the visual performance of the telescope
The large primary mirror and the elliptical secondary mirror of your telescope are front-surface aluminized and over coated with hard silicon dioxide, which prevents the aluminum from oxidizing. These coatings normally last through many years of use before requiring re-coating, which is easily done.
To clean the secondary mirror, it must be removed from the telescope. Do this by holding the secondary mirror holder sta- tionary with your fingers (don’t touch the mirror itself) while unthreading the Phillips head screw in the center hub of the 3- vaned spider. Completely unthread the screw from the holder, and the holder will come loose in your fingers. Be careful not to lose the spring on the Phillips head cap screw.
Handle the mirror and its holder carefully. You do not need to remove the secondary mirror from its holder for cleaning. Follow the same procedure described below for cleaning the primary mirror.
To clean the primary mirror, carefully remove the mirror cell from the telescope. To do this, you must loosen the three screws from the end of the optical tube that are flush with the end of the tube. Completely loosen all three of the flush screws (do not loosen the other three screws) until the mirror cell comes out of the telescope.
Now, remove the mirror from the mirror cell by removing the three mirror clips that secure the mirror in its cell. Use a Phillips head screwdriver to unthread the mirror clip anchor screws. Next, hold the mirror by its edge, and remove it from the mirror cell. Be careful not to touch the aluminized surface of the mirror with your fingers. Set the mirror on a clean, soft towel. Fill a clean sink, free of abrasive cleanser, with room- temperature water, a few drops of liquid dishwashing deter- gent, and if possible, a capful of rubbing alcohol. Submerge the mirror (aluminized face up) in the water and let it soak for several minutes (or hours if it is a very dirty mirror). Wipe the mirror underwater with clean cotton balls, using extremely light pressure and stroking in straight lines across the surface. Use one ball for each wipe across the mirror. Then rinse the mirror under a stream of lukewarm water. Any particles on the surface can be swabbed gently with a series of clean cotton balls, each used just one time. Dry the mirror in a stream of air (a “blower bulb” works great), or remove any stray drops of water with the corner of a paper towel. Water will run off a clean surface. Dry the bottom and the edges with a towel (not the mirror surface!). Cover the mirror surface with Kleenex, and leave the entire assembly in a warm area until it is com- pletely dry before reassembling the telescope.
