Sunday, December 30, 2012
Mirror Cleaning Madness
Do you
frequent the Cloudy Nights bulletin boards, muchachos? If you do, you've
probably noticed Unk dispensing mirror cleaning advice. That advice, for those
of you who don't do the CN forums, usually consists of one word: "Don't."
Oh, I amplify on that a wee bit, telling the questioner that every time you
clean a first surface mirror you run the risk of doing more harm than good.
That's what Unk was taught as a novice astronomer, and it is still somewhat
true, but if pressed the Rodster will now admit cleaning a telescope mirror is
not always an idea that springs from the depths of Hell.
One thing
that's changed in the last half century is that most telescope mirrors now have
pretty good overcoatings. I don't mean to say you can get out a Brillo Pad and
give your primary a good scrub to remove that misbegotten spot of squirrel poop.
But at least you don't have to worry as much about destroying your mirror's
coating or leaving a bunch of "sleeks" (small scratches) as you did in
the day when it wasn't unusual for a mirror's reflecting surface to be formed by
a thin and fragile chemical deposition of silver that had no protective coating
at all. Sometimes, following unexpected disasters like the depredations of Mr.
Squirrel, you just gotta clean a Newtonian reflector's mirror.
Cleaning is maybe also more needed these days because,
unlike when Rod was a little feller, we are much more prone to do all-night
observing runs (3 a.m. in the case of your aged Uncle) than we were back in the
hallowed Day. A few hours in a tree-shielded backyard meant it was unlikely the
primary would attract dew. Today, go all night on an open star party field with
the scope left out till dawn and the primary will probably wind up a soggy mess.
Why does dew make mirror cleaning more needful? Air pollution, campers.
It ain't
like we didn't have air pollution in the 1960s and 1970s—the air is undoubtedly
cleaner now—it's that dew, which can be loaded with airborne pollutants, didn't
often get on the mirror for the above reasons. The picture up top shows Old
Betsy's mirror after a few nights at the Deep South Regional Star Gaze. When it
dried, the dew bath left behind nasty streaks aplenty. Did these streaks affect
images? No. Not to any noticeable extent. If it were just a matter of the
mirror not looking clean and pretty I wouldn't have cared pea turkey. The
reason I cleaned Betsy's primary was that I was worried about damage to the
coating from unknown contaminants. It's possible dew includes a ration of pollen as well, which is abrasive, and not much of a Good Thing.
Yes, today's
mirrors are comparatively well protected, but those overcoatings are porous on
the microscopic level, and if the stuff the dew leaves behind on the mirror is
nasty enough it might eventually do damage, especially after it is soaks in
following repeated wettings by more dew. Is this a huge concern? Probably not, but I paid a significant number of
buckeroos for my super-duper coating from Spectrum
Coatings (recommended), and I want to preserve it for a while. So, I
cleaned my mirror and I am going to show you how to clean yours in relative
safety.
Eventually. Before
we get to that, let's do one other thing—Uncle
Rod's Year in Review. That, like the Christmas Eve blog, is a tradition 'round
here. I thought about skipping it this year and getting straight to the
distilled water and Kleenex, but it won't take long to clean Unk's 12-inch
mirror and there is something to be said for tradition.
January 2012
Unk had got
the camera a couple of months previous, but, naturally, the weather gods had
seen to it that I hadn't been able to do much with it till we got to the CAV.
In other words, Unk didn't know a cotton-picking thing about running the
Mallincam. My images a year later are miles better, but from the first I was
blown away by what this little camera could do. I mean, the dadgummed Horsehead
in living color? In 28 seconds? Get out!
February 2012
The February
blogs included everything from Unk getting his 12-inch Dobbie, Old Betsy, out
of mothballs (high time) to one of his patented nostalgia trips; this time to
ALCON 2003. The standout, though, was the entry on the last two nights of Unk and Miss D's January Chiefland adventure That trip
was memorable for a couple of things in addition to the new Xtreme. We discovered
the beautiful Manatee Springs State Park,
and—the real biggie—Unk was beginning to see the light at the end of the long,
long Herschel Project tunnel.
March 2012
April 2012
The bad weather
and Unk's trips down memory lane continued, with me tending to agree with Mr
Eliot that April is indeed the cruelest month. Did get some semi-clear skies, though, and I ticked-off some of the relatively few
remaining Herschel objects, if not as many as I would have liked.
Night 32 of The Project, the second run of the month, was plagued both by gear problems and that consarned skunk ape and his pals, The Little Gray Dudes from Zeta Reticuli 2.
May 2012
May was a
busy month, with me and Miss D. journeying to one of our fave vacation
destinations, Rocket City, Huntsville Alabama. Chipped away at the remaining
Herschels that month too, whittling away the stragglers, which were mostly
little Coma – Virgo galaxies. The coolest thing for Unk, howsomeever, was
finally getting off his rear and putting his Ultima C8 back together. Sorta, anyway.
June 2012
With The
Project nearing its end, Unk was rewarded with a real good night that included
not just another supernova to add to his collection of images of exploding
stars, but a supernova in a Herschel object. "Synchronicity," I calls it. It was
also the first outing for the little (tiny) Orion digital video recorder that
replaced the humongous home DVD recorder and the monstrous inverter I formerly used
for recording video. Other than that, the most notable event in June was our observation of the Venus transit
from Fairhope Pier. We saw it, yeah, if only barely.
July 2012
July was
quite the month. Unk had a fantastic birthday
and he knocked off the last of the Herschels in Chiefland just in time for the
publication of his big Herschel Project
article in Sky and Telescope
magazine. Not unexpectedly, I was at first ecstatic to declare victory over the
aitches, but was soon feeling let down. Oh, there would still be fuzzies to
re-image and sketch, but the thrill was undeniably gone. The cure? A new
project that you will likely hear about next month.
Anyhoo, it was a good trip despite rain and high
temperatures. Unk not only got the last of those consarned galaxies, I got the
pictures, terrestrial and celestial, I needed to go with another S&T piece I
was working on. Only bummer? For the first time in years Dorothy wasn't with
me; she was up in DC visiting daughter Beth. The best thing about this year's
CAV trips was not the Mallincam Xtreme, wonderful though it was; it was having
D. with me.
August 2012
September 2012
September came and saw me and D. back on the
road to the Chiefland Astronomy Village, but this was to be a different sort of
CAV expedition. No cameras. No big SCTs. Just a good old C8 and our Atlas
mount. I wanted to do a little Herschel object sketching (if The Project ever
does turn into a book, I'll need those) and test the Atlas' new firmware. This run
also allowed me to (finally) get under dark skies with my enormous Zhumell binoculars and the mount D. and I built
for them. Conclusion? I am not obsessed by giant binoculars, but I had a great
time with them Down Chiefland Way and saw quite a few deep sky wonders in an
entirely new way.
October 2012
The October
blogs were, once again, a diverse lot, with the winner probably being the story
of Unk's never-ending quest to take the perfect (within his modest
expectations) image of M13. Otherwise? A
little on the current states of ham radio and amateur astronomy, and, in
recognition of the coming of fall star party season, a primer on the "how
you gonna act" part of star partying.
November 2012
December 2012
In addition
to the yearly Christmas Eve blog you-all just read, December featured the
revival of a small observing project of mine, "Unk's Messier Album," wherein I observe the objects in old Chuck M's
catalog and compare what I see with what John Mallas saw and recorded in his
landmark book, The Messier Album. There
was also an account of our trip to the U.S. Space and Rocket Center for our
Thanksgiving getaway. All in all, a good if mostly quiet month to put the cap on
yet another year of The Little Old Blog from Chaos Manor South.
Now, finally, on to the purported subject of this blog entry, cleaning your
pea-picking mirror. Before we get started, I will say it one last time: Despite modern mirrors being protected by
overcoatings, you are well advised to
leave your mirror alone if it doesn't need cleaning. Stains and streaks
left by dew are one thing; motes of dust are another. Dust will not harm your
images. It will always be there. Any mirror will look horrible if you examine
it under the oblique light of a bright flashlight. Unless there is a specific
reason for cleaning, as in Unk's primary's repeated dew baths at DSRSG, confine
yourself to, at most, a once-a-year mirror washing. In other words: LEAVE IT ALONE.
After those
scare words, the actual process of cleaning a mirror is going to be disappointingly
anticlimactic. First step, natch, is removing the mirror from the telescope. If
you have a truss-type Newtonian, that usually involves nothing more than
releasing/removing some safety clips and carefully lifting the mirror off the
sling and out of the rocker box. Solid tube telescope? Usually, it will be held
in place by three clips. Loosen or remove them and carefully lift the mirror out of its
cell. You'll probably have to remove
both mirror and cell from tube first, natch. Whatever kind of scope you've got,
it might not be a bad idea to mark the mirror's rotational orientation on the
cell and on the mirror's side to make collimation quicker, at least, after
cleaning.
What if,
like more than a few imported telescopes, your mirror is glued into its cell by silicone stuff? You have two choices: clean the mirror in the cell or break the
silicone loose (using a wire to cut it is usually the best way to get the
mirror out). I prefer to clean the mirror in the cell if possible. Some
imported truss tube scope mirrors are glued into their cells, and these cells
cannot easily be removed from the tube. What then?
It is
perfectly acceptable to clean the mirror in
situ in the mirror box. A spray bottle of distilled water can make the
process a little less messy, but it is not a big deal. When you are done, just
dry the rocker box. Dob guru Dave Kriege cleans mirrors this way, I
believe. Frankly, this is probably the easiest and safest way to clean a large
mirror, even if it can be easily removed from the scope.
Mirror out
of telescope, the main thing is DON'T DROP IT AND DON'T BUMP IT ON ANYTHING. Trot
it into the kitchen where you've laid out your supplies. Said supplies are a
gallon of distilled water, a box of Kleenex (original Kleenex, white, no lotion
or scent), paper towels, detergent, and a bath towel.
The perfect
place to clean a mirror of 12 – 16-inches aperture or less is in one-half of a
double kitchen sink. Big dog? If you are going to remove it to clean it, I
guess a bathtub would work, but, as above, cleaning in the scope is probably
the way to go. Anyhow, spread a towel out in the sink bottom/side leaving the
drain free, and rest your mirror against the towel padding the side of the
sink, arranging it so it is not likely to slip (the towel with help prevent
that) at an angle great enough for water to drain off its face freely.
Time for the
scary part, scrubbing the mirror. Actually, you don't scrub, you swab, with
Kleenex dampened with the Dawn solution. Some folks will tell you to use USP
cotton balls, but I think Kleenex is better. It is softer and easier to get. Pour
cleaning solution across the mirror surface and, APPLYING NO PRESSURE, JUST
DRAGGING THE KLEENEX ACROSS THE MIRROR'S FACE, swab it from center outward,
rotating the tissue as you go and changing tissues frequently.
When the
mirror is cleaned to your satisfaction, begin final rinse number one. Use tap
water just as with the initial rinse. Keep going until you are certain the last of the soap has gone
down the drain. You could actually stop here, but in most places tap water has
enough minerals in it to cause spotting when it dries. That's where the
distilled water comes in and is why you MUST use distilled, not bottled,
water. Use plenty of distilled water, maybe even the whole gallon; you want to
be sure you rinse away any minerals or salts from the tap water bath. When you
are done, it's time to dry.
While waiting
for the mirror to dry, clean/dust the interior of the mirror box and perform
other adjustments or housekeeping as required. When the primary is water-drop
free, carefully move your pride and joy from sink to scope, place mirror in
cell, fasten everything down as 'twas before, and check the collimation. Tweak
that and you are done. Thank god.
And that is
it. Fun is fun, but done is done. Cleaning a mirror is not hard and it is not
scary. I go out of my way to underline the caveats because some folks just
can't leave well enough alone or keep from going too far with optical cleaning.
You would have to hear him tell it to get the full effect, but my old friend
Doc Clay Sherrod tells the story of a dude with a brand new LX-200 who notices a
speck of dust on the mirror. The denouement is that the scope winds up under water in the bathtub. I know none of
y'all would let the great god Murphy lead you so far astray, but I still want
to be on record, when it comes to optical cleaning, as saying TAKE IT EASY ON
YOUR BEAUTIFUL MIRROR.
Next Time:
More CRAZY computers...
Comments:
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Always enjoy your blog. One question... Let's say you do mess up and scratch something. Can it all be fixed with a re-coating?
# posted by 
: 8:49 PM
: 8:49 PM
It will take many, many scratches before performance will be affected. If the scratch is not so deep that it goes into the mirror's glass, yes, recoating will fix it.
Happy New Year! And thanks for all the astro insight and comic relief you provide here and elsewhere. Two questions (1) Where do use the paper towel you mention? Some of the pictures above actually looks like you dabbing water off the mirror with a piece. (2) does the collimating washer shaped dot on the mirror tend to move or come loose, and then what? Thanks from a fellow Navy civilian (a Rickover guy) and NOVAC member (hope to see you at the AHSP this year).
Hi Jim:
I use the corner of a paper towel to "wick" droplets off the surface of the drying mirror without touching the paper towel to its surface...
I use a self-adhesive paper reinforcer. If it comes loose, just replace it. Don't know the exact center? Cut a circle of paper the size of your mirror, fold into quarters, put a hole through the center of the "crosshairs" that will form from the folds, and mark with a soft pencil or water based marker. ;-)
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I use the corner of a paper towel to "wick" droplets off the surface of the drying mirror without touching the paper towel to its surface...
I use a self-adhesive paper reinforcer. If it comes loose, just replace it. Don't know the exact center? Cut a circle of paper the size of your mirror, fold into quarters, put a hole through the center of the "crosshairs" that will form from the folds, and mark with a soft pencil or water based marker. ;-)
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