A Pitch Hardness Tester
After spending 25 hours parabolizing a 10" f/3.8 mirror, it finally
dawned on me that my pitch might be too hard. Hard pitch won't conform
to the steep parabola required of a fast mirror, so it's much more
difficult to maintain contact and much more difficult to get consistent
results. I had finally gotten a decent parabola, but only after a lot of
frustration. So I decided to see if too-hard pitch was my problem.
Up until now I've been testing my pitch with a thumbnail. If it left a
dent after a few seconds, I figured it was good enough. That's not much
of a test, though, so I looked around for a standard test that would
actually give me some numbers and let me compare my numbers with other
people's. Turns out there's a great design for just such a testing
apparatus in Jean Texerau's How to
Make a Telescope. You have to get the second edition; it's not in
the first. In the second, it's appendix J on page 393.
It's a pretty simple device. You need a point that's ground to a
14-degree angle, with a 1mm flat bottom. You need a framework that will
support that point over a pitch sample. You need 1 kilogram of weight on
that point. You need some way to measure the distance the point drops in
five minutes. And you need a timer to measure five minutes.
I bought a cheap dial indicator at Harbor Freight Tools and mounted it
as you see here. The arm I mounted it on was too loose by itself, so I
glued that triangular wedge to it so the wedge would rest against the
dowel and the weight of the dial indicator would hold it snug. I
discovered that the dial indicator's probe spring would lift the dial
indicator upward if the spring was compressed too far, and that would
spoil the measurement, so I have to use the lower half of the scale
(from 0.0 to 0.5"). That's plenty of travel, though, so no worries.
The lower arm holds a nylon bushing, through which my 1/4" metal rod
slides freely. I've driven the rod through a circular disk on which I
place a bottle of lead shot. I rested the point of the tester on a scale
and added shot until the combined weight of the point, disk, bottle,
shot, and spring pressure of the dial indicator came to exactly 1
kilogram.
Here's a closeup of the point and the end of the arm it goes through.
It's free to slide in that nylon bushing, so there's essentially no
friction resisting its drop. The pitch is the only thing holding it back.
The angle here looks more like 17° than 14°, but that's an
illusion of perspective. It's actually ground to 14 degrees, and the
bottom is ground flat with a 1mm circular end.
This photo shows it after it has sunk into the pitch for a couple of
minutes.
After five minutes, this is the hole the tester made in my pitch tool.
It fell 0.087" in five minutes' time.
Obviously, you'll need thick enough pitch that the tester won't bottom
out in five minutes. You can't measure for 1 minute and multiply by
five, either, because the rate of drop isn't linear. It slows down
dramatically as the point moves deeper into the pitch.
I've plotted the results of several different pitch
samples on the chart at left. The X is the pitch lap pictured above,
which tests nearly ideal according to Texereau, but then he was basing
his recommendations on f/6 to f/12 mirrors. At f/3.8, my pitch should
have been way softer than this.
The other samples are Gugolz #64 still in the can at three different
temperatures (G), Acculap "standard hardness" (A), and some ancient
Burgundy pitch off an old lap (B).
The chart is a little difficult to figure out until you realize that
the diagonal lines are there just to give you an idea of how any
particular pitch will behave with temperature differences. For instance,
if I tested the new Gugolz #64 at steadily increasing temperatures, the
row of G's would stay in that diagonal bar as the results moved upward
to the right. Note how at 60 degrees the Gugolz hardly flowed at all.
Even a smidgen more drop would have put it in the same diagonal bar as
the upper two measurements, so I think the diagonal bars are pretty
accurate. And that 60° measurement illustrates one point very
clearly: Don't try to polish a fast mirror (or any mirror, really)
in a cold shop!
That also leads to the complementary insight: if your pitch is too
hard, you could warm up your work space and achieve the same thing as
softening the pitch with turpentine or mineral spirits.
That's all I can think of for design notes. Feel free to email with
questions if you have any.
How to contact me
I'd
love to hear from people who are interested in this pitch tester. Please
feel free to email me at the address on the right. (Sorry you can't
click on it or copy and paste it; it's a graphic file to thwart spambots
that search the internet for addresses to send junk mail to.) I
have no idea how much mail this idea will generate, so I can't guarantee
a response, but I'll do my best to answer everyone who writes with a
genuine question or comment about the design.
