What to do with an Old Reflecting Telescope
What if you have a an old reflecting telescope and would like to get it up and working to see the planets? It’s really dusty and the mirror looks a little “spotty”. What can you do to get the scope back in order? For the sake of this article, the telescope was a fairly common Tasco 4.5” mirror, f/8 with a 900 mm focal length.
In this instance, I would not recommend just cleaning the mirror per se - (read more below), but I'd like to offer you a couple of courses of action that you can follow that will lead to excellent views of the sky.
Option 1: Mirror Replacement
Switching out your existing mirrors for new mirrors is your easiest and certainly lowest cost solution. It's not something to be afraid of at all since very little mechanical ability is needed to do this task. Check out Orion Telescope's website (http://www.telescope.com) for replacement mirrors. A replacement set of mirrors in this instance (4.5" f/8 Primary Mirror) would be $39.95. Your secondary mirror will have to be measured before you order it. It is a small elliptical flat mirror on the back side of the spider near the front of the telescope. More than likely, you will need to take off the spider to remove this mirror. Often they were glued onto the spider. If so, measure the mirror (in mm and inches) BEFORE you try to pry it off the spider. It's real hard to measure something in 100 pieces if it should break! The new secondary mirror from Orion will cost $15 - $20.
(Orion's replacement mirror is listed as a D=4.5" = 114.3 mm, and since 114.3 mm x f/8 = F = 914.4 mm which is very close to the example’s telescope’s focal length of 900 mm. The Orion replacement mirror should work very well.) Total cost for this option including shipping will be about $65.00.
Option 2 – Re-silvering
With a reflecting scope older than 10-15 years, at the very least it’s a good bet that the primary and secondary mirrors need to be re-silvered. The aluminum oxide silvering oxidizes over time and typically lasts between 7-10 years before its reflectivity is sharply reduced. If this is the case, the good news is that you won't have to run the risk of scratching the primary mirror by wiping the dust off. Just take the mirrors out of their mirror cell mounts and rinse them thoroughly with distilled water. The idea is that without physically touching the mirror surface to get as much of the dirt off so they won't get scratched when shipping them back to be re-silvered. After dry, use fresh newspaper and masking tape to cover the entire mirror, covering the optical surfaces carefully. Wrap each mirror up in several layers of soft paper towel, using more tape on the outside, THEN package up the mirrors for shipping with plenty of bubble wrap to the re-silvering company (See "Mirror Coating Services below). The cost of this including shipping, insurance, and telephone calls will be about $100. It will take about 2-3 weeks after they receive your mirrors. Upon receipt of the freshly re-silvered mirrors, carefully reinstall, re-collimate and enjoy your "new" telescope!
Option 3 - Entire Telescope Replacement
Used 6" reflectors in excellent shape are advertised quite often on Astromart (http://www.astromart.com/) for prices around $125 - $175. You can get a "dobsonian" mount reflector and +77% more light gathering area with an optical surface that has been most likely configured by a modern optical grinding machine. Alternatively, you could make your own 6" reflector in about 2 - 3 weekends, have the mirror silvered for about $150, but you would enjoy the scope even more because you made it. Telescope Making Websites: http://www.atmsite.org/ , http://www.starastronomy.org/TelescopeMaking/Links/ , http://tie.jpl.nasa.gov/tie/dobson/index.html#TOC.
Mirror Coating Services
P.A.Clausing, Inc. 847-676-0330 Voice, 847-676-2930 Fax, e-mail: email@example.com. Beral, Silver, Gold, Aluminum, Copper, Rhodium, (most other evaporable metals) and overcoatings, Fast turn around available.
Majestic Optical Coatings 732-388-5604, Fax 732-388-5826. firstname.lastname@example.org
Research Service Co. 781-284-0393. Specializing in recoating of telescope mirrors. Charge amateurs $4.00 per inch up to 18". One week turn around typical. COATERRSC@aol.com
Spectrum Coatings 904-789-6662. email email@example.com