Optical Inspection

Okay, good work. While resting up from these exertions, take a look at the telescope's optics. Remove the corrector's dust cover and look down the tube (Plate 34). The primary mirror at the bottom of the OTA should be bright and shiny, and there should not be any obvious dust, dirt, or packing debris on its surface. If there is any of that visible, call the dealer. A little dust will not hurt anything, but there have been reports recently of new scopes arriving with numerous pieces of Styrofoam packing on the mirror, and even of a case or two where there were human hairs inside the

Plate 34. (Corrector End of a CAT) Looking down the front of a Celestron NexStar11 GPS. Credit: Author.

optical tube. CATs are not assembled in clean rooms, but you should not accept one with an obviously dirty mirror.

On the other hand, beware of the "flashlight test." Examining a mirror by the light of a flashlight or other bright-light source, especially one pointed at the mirror at an oblique angle, will make it look horrible. Don't worry. The primary is not the welter of pits and dust it appears to be. Even the most reflective mirror coatings cannot reflect 100% of the light that strikes them, and any light that is not reflected, especially light striking the mirror at an angle, is scattered across the surface of the primary. That causes small particles of dust and other nearly microscopic flaws to stand out in dramatic and frightening relief. What is the solution? As the doctor said: Do not do that. Inspect the mirror in normal room light. If it looks reasonably clean, move on.

The corrector is almost as important as the primary mirror, and its exterior surface should be clean and free of blemishes and streaks of any kind. Do not fret over a few specks of dust. They will not hurt anything, so leave them alone. If there appears to be excessive dirt clinging to the corrector, follow the cleaning procedures in Chapter 9 of this book. Sure, you could call the dealer and complain and even demand a new scope, but the replacement telescope's corrector would probably look the same. It is hard to assemble and ship an SCT without getting a little dust from somewhere onto the lens.

There may even be some specks of dust visible on the inside of the corrector plate. As with the outer surface, a little dust or a few tiny flecks of paint dislodged from the tube interior due to the tender mercies of delivery people will not hurt a thing. Naturally, if the interior surface is excessively dirty or scratched, get on the phone to the folks who sold the scope. If you are not sure what is "excessive," the best advice I can offer is to call on a fellow CAT-owning astronomy club member for assistance. If there is no local club, try one of the Internet CAT groups, like the SCT user group listed in Appendix 2. There is also more information on optical cleanliness issues in Chapter 9.

What is visible looking down the front of the telescope tube? There is the secondary mirror holder mounted in the center of the corrector, and there is the reflection of the secondary mirror's surface in the primary. What about the secondary? Its holder should be firmly attached. If this holder moves when gently turned, call the dealer immediately. If this is a C14, be aware that some of these telescopes are equipped with Fastar-compatible secondary mirrors that are removable for use with the Fastar optical system, and all that may be wrong is that the retaining ring that secures the secondary assembly in the holder is loose. Check the manual to see if the scope in question has a Faststar-compatible secondary mirror holder. Like the primary, the secondary's surface should be reasonably clean and shiny bright. If it is hard to tell what its condition is from looking at its reflection in the primary, move around to the rear of the scope and take a look through the rear port(remove the plastic cap first, of course).

If the primary, secondary, and corrector receive a clean bill of health, move to the CAT's rear and check the focus mechanism. Before doing that, however, check the manual. There may be "shipping bolts" that need to be released before focusing. Or, the scope may have a mirror lock that needs to be disengaged first. Failure to do either thing before fooling around with the focuser control can cause damage. The manual should have an illustration of the telescope's rear cell that identifies the focus knob and other controls. Exercise the focus knob by turning it several turns in either direction. Movement should be smooth (exactly how easy it is to turn will depend on the brand and model of the scope), and nothing should be obviously bent or out of alignment. If the focus control will not turn in one direction, gently turn the knob several turns in the opposite direction and see if it will then turn the opposite way. (There are, by the way, about 35 to 40 turns of the knob from one end of focus travel to the other.) If so, all is well. If not, get on the horn to the dealer.

If the focuser checks out, take a look up the rear port next. Do that by removing the plastic cap that should be present. Note that the cap that seals the rear port usually just snaps off. It is threaded into place on a few models, but in that case the cap will usually be aluminum rather than plastic. Take another look at the secondary mirror and also observe the interior of the baffle tube that extends from the rear port and up into the tube. It should be free of dirt and grease. Occasionally, a new SCT will have grease from outside the baffle tube (used to lubricate the mirror-tube interface) that has migrated into the interior. If there are globs of grease on the inside of the baffle tube, call the dealer.

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