Class #6
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  2005-2006 Mirror Making Class & Workshop 'Official Page'.

-- More Figuring --
(Shaping a parabola)

Class #5 Pics and Info
During Class


Photo by Ken Slater

Telescope Making Lecture
We started class off this month with a good instruction about the different parts of a telescope.  Everything from top to bottom.  Ken gave us a lot of good information, but still ended every discussion with something like, "Now you just have to decide exactly what you want to use for your scope."  You know, helpful information, but leaving you with a lot of decision making to do on your own afterward.  Click picture to enlarge


Photo by Ken Slater
Dave's giving us some instruction
Here you can see Scott getting some instruction from Dave on polishing, while I'm pressing my pitch lap to improve contact between the mirror and the tool. Click picture to enlarge
NOTES:

Do you want the good news first, or the bad news first?  The good news, first?  Ok.  I'll give it to you.  

The good news:
I FINISHED MY MIRROR!!!

The bad news:
Immediately afterward, I killed it!

Now 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.

So, 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 it.  

A little background information:

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.

What happened 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.)

What 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!

 

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