Many of you out there clean the mirrors for your Newtonian 'scopes
regularly without qualms. Others, fearful of the damage you could
potentially cause to this crucial, expensive, and delicate
possession, have never done so, or hold off until you think it's
really bad.
I classed myself closer to the second camp than the first, until
early this past June when I decided to give a non-traditional
technique a try. I read whatever I could find about cleaning
mirrors. I looked for accounts of people's mishaps, and for their
tips learned from unexpected problems. I found very little of that,
but I did find one web page <http://www.ghg.net/cshaw/clean.htm >
about cleaning mirrors by applying liquid adhesives that, once dry,
pull dirt away with them.
I ordered a couple of cans of Universal Photonics
<http://www.universalphotonics.com/> "PRE-COTE 33 BLUE SPRAY" for
$13.95 each. This package arrived within a week. After trying it on
a couple of square-inches of a rear-surface mirror in my house --
just to find out how it behaved -- I applied it to the primary of my
12.5" F4.5 Nova mirror without further hesitation.
I took the mirror, in its cell, out of the 'scope and put it on a
newspaper. I thoroughly swept it with puffs of air from a "hurricane
blower". I then taped a cylinder of newspaper around the mirror. I
applied the stuff fairly thick, because something I read said that
worked just as well as several thin coatings. It was a little too
thick, unfortunately. It ran down the sides of the mirror, catching
on some rather rough-surfaced portions of the glass. I let it dry
for about an hour, and then used a strip of masking tape to lift the
plastic-like film off of the reflective surface. This proved
difficult because of the spillage over the sides, but I eventually
got it all off. What I was left with was a stunningly clean mirror.
With one lesson learned about applying the stuff in thinner coats, I
decided that this was easy. I wouldn't hesitate to clean it again
whenever the mirror looked significantly dusty.
Fast-forward to yesterday (Saturday, October 13th): After a number
of summer observing trips to rather dusty places like Lake San
Antonio and Grant Ranch, my mirror had accumulated enough of a
coating of dirt that it looked "a little milky" when looking at
myself in it a few yards away. So I set aside some time before going
to a star party that night to clean it again.
I removed the mirror and cell, placed it shiny-side-up on some
newspaper, blew off the loose dust, taped a cylinder of newspaper
around it, and applied three very light coats of the blue spray.
This time it didn't run down the sides. After an hour, I used
masking tape to peel it off.
A lot of dust and large particles came off the mirror, but what was
left was fuzzy and pale blue haze over most of the mirror! I tried
to pick this residue off with masking tape from near the obstructed
dead-center of the mirror without success.
The horror! What have I done!
I remembered reading a vague comment that the blue spray would not
remove oils or liquids. This blue haze seemed to resemble the oily
layer that condenses on automobile windows from diesel exhaust and
other airborne pollutants -- I speculated that some of the blue spray
had dissolved into an oily film on the mirror and that the blue spray
wasn't able to adhere to this oily film.
I happened to have all of the supplies on hand for a more
conventional mirror cleaning: Distilled water, sterile 100% cotton
balls, a 6oz. squeeze-bottle, and 97% ethanol alcohol. To the first
squeeze bottle of distilled water I added a few drops of the
detergent "Simple Green". I placed the mirror shiny-side-up in a
clean plastic wash-basin, and covered it with the detergent-water
solution. After a few minutes of soaking, I used cotton balls to
gently wipe the mirror in an inward-to-outward radial direction, only
using each cotton ball for one pass. Each cotton ball came up blue,
and took the blue cast off the mirror as it went.
After that "scrubbing", I rinsed the mirror in two more bottles of
distilled water, and then a final rinse of ethanol. I then set the
mirror on end to dry.
After about 5 minutes or less of ambient-air drying, I was left with
a mirror as pristine and unblemished as the day it was delivered,
almost a year-and-a-half ago. A really clean front-surface mirror is
a magical thing to behold!
So another lesson learned, in the future I should make a habit of
using a detergent bath on the mirror before applying the blue spray.
That night, I proceeded to take that mirror, in my 'scope, fearlessly
out to the notoriously dusty Henry Coe State Park. I had an
excellent evening of observing. But that's a separate report.
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