Class #3
Randolph Center Sky Clock -


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

Class #3 Pics and Info
During Class

Ken Slater's photo taken from here

Fine Grinding

This picture shows making a pitch lap (in this case it's Scott making his).  We take the tool I made (from the Mirror Class 2 page), and wrap masking tape around the top, and actually going about 1" above the top of the tool, and then pour in about 3/8" of synthetic pitch (they used to use real tree pitch, boiled and filtered.).   
Click picture to enlarge

Pressing time.  Once the pitch lap has cooled, we need to cut the grooves into it.  The grooves main purpose is to give the pitch room to flow as it conforms to the shape of the mirror.  A side benefit is that it allows excess materials (polishing solution or glass bits) to flow out of the way.  Pressing forces the tool to follow the shape of the mirror, so that we have good contact, and the mirror will (hopefully) become spherical.
Click picture to enlarge

Here's another picture of my newly completed pitch lap.  The orange colored material in the middle is the 3 micron cerium solution that we use to polish. 
Click picture to enlarge

Ken Slater's photo taken from here
Here's a picture of Scott and I working on our mirrors.  We're both making 8" mirrors, but he's making an f/8 and mine is an f/6.25 so his finished scope will be about 5'4" and mine will be 4'2".  The main difference is that he'll get slightly more magnification out of the same eyepieces that I'll get slightly more wide-angle.  Both have their own benefits.
Click picture to enlarge

Ken Slater's photo taken from here
Another shot of me pressing again, before I polish.  To the lower left of the picture, you can see Glenn working on the final stages of his 10" mirror.

Ken Slater's photo taken from here
Here's just a quick picture of a couple mirrors that the club is working on.  In the front, you see a 16" mirror.  In the back, you can see a 27" mirror with a 16" grinding tool on the top.  Both mirrors are in the grinding stages.
Click picture to enlarge

Class went great again, with a definite feeling of accomplishment when I left.  

We started off with a class on making a pitch lap and polishing, moving to making the pitch lap, and finally to polishing for about 2.5+ hours during the afternoon.  

There were lots of short down times during the day, due to waiting for the pitch to soften, waiting for it to harden, pressing, etc. all added up to a general feeling of not doing much, but things kept progressing further and further.

Polishing is going really well.  Just before I left, Dave Kelly put my mirror in the tester, and told me that it has a focal length of 100", making it an f/6.25.  He also told me that it's currently an Oblate Spheroid, which just means that it's not quite deep enough in the middle to be perfectly spherical, yet.  He's also guessing that I'm somewhere near half way polished.  This was nice to know as it showed me that I was doing it correctly.

Rick had to keep getting on my case about slowing down, though.  I tend to try to speed right along with my polishing tool, and I need to keep that in check.  The tool needs to go slower, so that the pitch has time to follow the contour of the mirror and polish.  Too fast makes the tool skip, which causes an imperceptible 'dog biscuit' texture that shows up very well in the Foucault Tester..

I've also brought one of the Foucault Tester loaners home to use.  After I'm done polishing, I can use this to make certain that I'm perfectly spherical before I head back for class #4.  It's not required, but since we have a 2 month break before the next class, I'd like to make the most use of the time that I can.

Can't wait to keep moving on it, though.  It's really a lot of fun.

Post Class Ronchi Test:
I've done some more polishing on my mirror, and am almost done.

After hurting my back just after Christmas, I decided to lay low for a bit, and hold off on the polishing, until I was sure that my back was ok again, but after getting antsy about not doing anything on my mirror for a few nights when I had free time, I finally decided to try out the Foucault Tester.  This is an image from the Ronchi test, which uses a screen of lines that the light shines through.  If my mirror was perfectly spherical, these lines would be straight up and down.  This gives the appearance of a mirror that is slightly parabolic.
Click picture to enlarge

Here's a look at a shadows on my mirror while doing the knife-edge test.  When I looked at this one, at first I said, "That looks parabolic."  I'm normally an optimist, so I had to slow myself down and say, "What's it really showing here."  After a while, I had determined that it's showing a mirror that is mostly spherical, but with a sunken center.  It's kind of like heading toward a parabolic shape, but not there yet.  Flat-looking areas represent a spherical shape, and bulges represent mounds and holes.  Note that, due to mirror magnification, a mound that looks like it's 0.2" low (similar to this picture) is actually only about 2 millionths of an inch low.  (The dark areas on the mirror are where fingerprints on the mirror are not reflecting back as well as the glass itself.)
An 'enhanced' version of the Knife-Edge Test pic, to try to show the 'hole' a little better.  (Same fingerprints... Sorry!)
NOTES:  As I'm not yet fully polished, these 'figures' don't really mean anything, other than that it appears that I'm headed in the right direction, which is nice to know.  I've got about 2 hours of polishing left to go, then I have to work toward making it spherical, even though I'm on the right side of it already.  Better to go back and do it right, than to wish I had later on.
LATER NOTES:  I sent a the pictures above to Ken, and he says it looks like I've got a turned down edge.  Will have to wait until class to find out for sure, and to see what to do with it.
Next class... Start figuring (making my mirror a parabolic shape)


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Site was last updated on: 02/19/2006