Many telescopes use mirrors instead of lenses to focus light. A telescope mirror will tarnish over time, reducing the amount of light it reflects. Removing and resurfacing a telescope mirror will restore a tarnished hunk of glass to a high quality astronomical reflector and return the telescope to service. Resurfacing a telescope mirror requires some chemical preparation and application that deposits a silver coating on a clean glass surface. After resurfacing, a properly protected mirror will last for many years.
- Skill level:
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Things you need
- Pure alcohol
- Silver nitrate, 60g
- Distilled water, 3000ml
- Ammonium hydroxide (concentrated), 1000ml
- Potassium hydroxide, 60g
- Table sugar, 100g
- Nitric acid (concentrated), 40ml
- Eye protection goggles
- Rubber protective gloves
- Glass mixing container
- Glass stirring rod
- Glass container
- Small wooden plank
- Pitch, 56.7gr.
Remove the telescope mirror from the optical tube and then remove any supporting materials such as metal supports, nuts and bolts. Do not touch the mirror surface with your fingers or any tools.
Thoroughly clean the mirror with detergent, rinse with distilled water and allow to dry. When dry, rinse the mirror in 100 per cent pure alcohol and allow to dry. The mirror is now ready for resurfacing which should begin immediately, to reduce the chance of dirt or dust accumulating on the mirror.
Determine the surface area of the mirror by using the formula, pi (3.14) x radius squared. For example, an 8-inch diameter mirror has an area of 3.14 x 4x4 (16) = 50.24 (round off to 50) square inches. The grams of silver nitrate solution needed for a mirror is the surface area divided by 5. For an 8-inch diameter mirror this will be 50/5 = 10g of silver nitrate.
Prepare the silvering bath in a glass container by adding 17ml of distilled water to every gm of silver nitrate. Using the 8-inch diameter mirror example, this will be 10 x 17 = 170ml of distilled water. Add concentrated ammonium hydroxide to the silver nitrate solution while stirring, until a brown precipitate forms in the solution. Continue adding ammonium hydroxide, drop by drop, until the brown precipitate just redissolves and stop.
Prepare a mixture of potassium hydroxide and distilled water using 1g of potassium hydroxide to 17ml of distilled water. Make an amount that is 1/2 of the volume of the silver nitrate solution. Using the example from above, this would be 85ml. Slowly add this solution to the silver nitrate solution. A brown precipitate will again form.
Add enough silver nitrate solution to this solution, drop by drop, until the solution clears. Add several drops of the potassium hydroxide solution until the brown cloudiness reappears. If the brown cloudiness disappears, continue to add silver nitrate or potassium hydroxide to the solution until it just reappears. The solution is now ready and will precipitate metallic silver immediately when you add the reducing agent.Pour this solution into the glass bath container in which the mirror will be placed.
Prepare the reducing agent in a glass container by mixing 100g of table sugar, 40ml of concentrated nitric acid and 1000ml of distilled water. Bring this solution to a boil and allow to cool to room temperature.
Pour the reducing agent into the silver nitrate solution and stir vigorously with a glass stirring rod for a few seconds, then lower the mirror into the resurfacing bath. The solution must be kept in motion across the surface of the mirror by continually moving the mirror or by gently tilting the bath container from side to side.
When the silver film has formed on the whole surface of the mirror and black specks begin to settle in the solution, remove the mirror and rinse it in distilled water. Do not touch the newly formed silver finish. If you decide the silver film is too thin, prepare a second bath and repeat the resurfacing procedure.
After rinsing in distilled water, allow the mirror to dry. Replace any supporting structures, bolts and nuts and then return to the telescope. The mirror is ready for use and should last many years.
Tips and warnings
- The glass container must be large enough to allow the telescope mirror to lay flat without touching the sides.
- Temperature is not critical but the solutions are best made at room temperature.
- Mirror may be silvered face up or facedown. Generally small mirrors, less than 8 inches in diameter, are silvered facedown and larger mirrors are silvered face up.
- For facedown resurfacing, attach a narrow plank of wood across the back of the mirror with pitch and use this wooden handle to move the mirror within the solution.
- Wear eye and skin protection when mixing chemicals.
- Nitric acid is a dangerous corrossive and must be handled with extreme care. Do not allow it to come into contact with skin or eyes.
- Prepare all chemical in a well-ventilated area.
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