My first Magazine Sky & Telescope - 03.2019

contact the surface.) This part is optional, but it’s worth doing if you can work up the courage, because your ﬁ ngertips are so sensitive that you can feel any grit particles that might still be stuck to the mirror. Gently feel around with your soapy, wet ﬁ ngertips to make sure the mir- ror is truly clean. If you ﬁ nd a grit particle, gently knock it free and rinse it off the mirror so you won’t drag it around. If you see a stub- born stain, it’s okay to rub it a little — lightly — with your ﬁ ngertips to clean it off. A fresh cotton ball would work, too, but unless you’re a guitarist and you’re using your fretting hand to wash the mirror, your ﬁ ngertips are gentler than the cotton ﬁ bers. When the mirror is clean to your satisfaction, rinse off all the soapy water with warm tap water, then rinse off all the tap water with distilled or deionized water. This last step ensures that no mineral deposits will be left on the mirror when it dries. Water will sheet off a truly clean mirror, so odds are you’ll only have a few drops remaining. You can dry these drops by blotting them with a soft towel, but there’s no need. The distilled/deionized water will dry perfectly clean. coating is going bad. Pinholes in the glass overcoating let oxygen and other contaminants reach the aluminum layer, and that can eventually lead to corrosion. These spots usually look like an irregular patch of algae on a rock or an ice cream cone splat on the pavement, and they won’t clean off. When more than a small per- centage of the mirror is affected with this corrosion, it’s time to have it recoated. (See below for a list of coating vendors.) Assuming your mirror is in good shape, though, you’re done! Put it back in its cell, remember- ing not to overtighten the mirror clamps. You want them to just hold the mirror from tipping forward when the scope is tilted horizontal, not push down on it (not even a little bit). Too much pressure will pinch the optics. Now reassemble the scope and collimate it. We’ll cover the ﬁ ner details of collimation in the next installment. When to Recoat a Mirror ¢ Contributing Editor JERRY If you notice a lot of splotches on a clean mirror, it’s possible that your OLTION loves cleaning mirrors, t ROUND TWO Top: Rinse off the soapy water with lots of warm tap water. Middle: Flood the mirror with distilled water to prevent mineral stains. Bottom: The cleaned mirror may still have some de- fects. This one is beginning to have coat- ing issues but is still perfectly useable. especially other people’s. Majestic Optical Coatings 152 Willow Way, Clark, NJ 07066 888-278-8308 majestic-coatings.com Optical Mechanics Inc. 2224 Heinz Rd., Iowa City, IA 52240 319-351-3960 opticalmechanics.com Precision Optical 320 Kalmus Dr., Costa Mesa, CA 92626 949-205-7346 precisionoptical.com Spectrum Thin Films 135 Marcus Blvd., Hauppauge, NY 11788 800-815-8184 spectrumthinﬁ lms.com Optiques Fullum 1111 Rt. Harwood #6, Vaudreuil-Dorion, QC J7V 8P2 Canada optiquesfullum.com Ostahowski Optics P.O. Box 390440, Anza, CA 92539 951-763-5959 ostahowskioptics.com Reynard Corporation 1020 Calle Sombra, San Clemente, CA 92673 949-366-8866 reynardcorp.com Spectrum Coatings 1165 Ring St., Deltona, FL 32725 386-561-9779 spectrum-coatings.com sk yandtele scope.com • M A RCH 2 019 39

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