you want the good news first, or the bad news first? The good
news, first? Ok. I'll give it to you.
I FINISHED MY MIRROR!!!
Immediately afterward, I killed it!
THAT needs some explaining. It's not really dead. In a
nutshell, I got my mirror so that it was about 1/8 wave, a good mirror,
and then we tried to make it just a little bit better, and overshot,
which effectively killed the figuring work I'd done during the
day. It's comparable to cutting a board a little too short.
I stepped back, took a deep breath, asked myself what lesson I could
learn from that, which kept me from getting really upset and
frustrated. Then, I asked Dave what we do to start fixing
A little background
One of the quality ratings for a telescope
mirror is in wavelengths of light. The math actually works out to one
wavelength of helium light is 587.56 nanometers (or
0.0005876 mm or 0.00002313 inch). That's 23.13 millionths of an
inch. Put another way, if you cut an inch into a million pieces,
you would only stack up 23 of them. That's small.
1/8 wave, is
1/8 of a wavelength of light or about 3 millionths of an inch. So,
basically, I had my mirror accurate to about 3 millionths of an inch,
and we decided that wasn't good enough. What was I thinking?
Well, it's not uncommon to get a mirror to 1/12 wave. Even 1/20
wave isn't out of the question. That's 50% to 150% more
accurate. As this isn't uncommon, then it was only natural to say,
"I'll just go a touch more."
Unfortunately, a touch was
about 2 minutes of polishing.
when I went to far?
The center of the mirror went too
deep. By about 3 millionths of an inch. The problem is that
it's much more of a problem to be 3 millionths too low, instead of 3
millionths too high. There isn't an easy way to keep the proper
shape, and reduce the outside edge of the mirror. As soon as you
start lowering the outside edge of the mirror, it throws off the shape
of your parabola, and it's a downward spiral. To get the right
depth, it really throws off the shape of the mirror. (In
perspective, the mirror is still within a few millionths of an inch.)
do I do from here?
Well... this time I've got a little
homework. I've got about 3 hours worth of polishing to bring the
outer zones of the mirror down to a point where they are in correct
relation to the center zone. I might try to figure the mirror on
my own again, but we'll have to see how things go. I think I
understand the process of determining what strokes to use to 'erase'
errors in different zones of the mirror. If it doesn't go well, I
can always return to spherical, and go from there, so it's not quite an
'all is lost' scenario.
Next class, I could easily have the mirror
finished (again) but I've got to leave early to go to Samantha's dance
recital. Dave said that he'd be willing to get together with me to
finish it up if we run out of time at that class. That was great
of him, and made me feel much better that I could still have a working
scope to use this summer.
Like I said, those 'Springfield Telescope
Makers' at Stellafane are a great bunch of guys!