Class #4
Randolph Center Sky Clock -


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

Class #4 Pics and Info
During Class

Photo by Ken Stevens

Rebuilding My Pitch Lap
Another beautiful day, even though we had snow.  It was around 45 degrees out that day, so I'm only outside for a few minutes.  Here, I'm removing the old worn pitch from my pitch lap.  The procedure for doing this is simply this:  Put the pitch lap upside down in the snow for about 10 minutes, then chisel the pitch off the tool base.  The pitch gets brittle when it's cold, so it's easy to work with.   Click picture to enlarge

Photo by Ken Stevens
After you heat up the new pitch and pour it on, you get to play.  Here I am poking at the bubbles with my finger, to pop them.  Jay says jokingly that he's not sure if it's really needed or not, but it's fun to pop them.  I think it's a mirror-maker's anti-stress activity.  Must be working, cause I'm smiling!  Click picture to enlarge

Photo by Ken Stevens
After the pitch cools enough, you coat it with cerium oxide (CeO), to keep the pitch from sticking to the tools.  Then we work on pressing groves into the pitch with an angled piece of aluminum.  Click picture to enlarge

Photo by Ken Stevens
After the groves are pressed in, you put on plenty of Cerium Oxide.  The only step I haven't done here is the trimming.  It needs to be trimmed to get it to the same diameter as the tool, and to put groves on top of the facets for extra pitch flow-ability.  The pitch has to be able to move as needed to allow it to conform to the contours of the mirror.  Remember, pitch isn't actually solid.  Click picture to enlarge

Photo by Ken Stevens
Watching a movie on new specialty production equipment.
We watched a movie out in the telescope room.  Don't let the fact that I was in a T-shirt, and looked like I'd been sweating fool you.  We left the 75 degree polishing room, for the more spacious 40 degree telescope room, for  a 10 minute movie.  We wound up being out there for about 30 minutes with informational talk afterward.  Click picture to enlarge

Photo by Ken Stevens
Back to the Grind (or Polish as the case may be)
Here's a shot of Scott, Fran and I all polishing.  Does it seem like you've seen this one before?  It's probably because 98% of making a mirror looks like this, either in grinding or polishing!  :-)  Click picture to enlarge

As I had feared, I got to class and found that I had a really bad case of Turned Down Edge (TDE).  TDE is when the edge of your mirror starts to get a longer focal length than the rest of the mirror, which in the tester appears like it's turned down, hence the name.  It's kind of like having a half of a $100 bill.  No  one wants it, and you don't really know what to do when you get it.  It's pretty frustrating.  Dave Kelly gave me some pointers which I started working on during class.


Post Class

I spent quite a long while polishing, and two days before class, I retested and found that I still had a nasty TDE (see NOTES above).  I did some research online that night, and came to the conclusion that I had a problem with my pitch lap getting too warm.  This causes it to get smushy in the middle, and polish on the outside.  If you click the image and look at it, you can see that all the way around the edge, the lines bend in.  That's the TDE.  Also, there is a faint bending of the lines around the middle, giving the impression that the middle is lower than the rest of the mirror.  This is due to a little bit of the center not being spherical yet.  Click image to enlarge.

After polishing for a while with a cooler pitch lap, I was finally able to make progress on the TDE.  Here you can see that the TDE is almost gone.  It's progressed from about a 3/4" band around the outside, down to approximately 1/16".  Almost there.  Also, you'll notice that the lines have straightened out a lot in the middle.  That part looks very spherical.  I think I'm ready for class.    Click image to enlarge.



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Site was last updated on: 02/19/2006