Ten Inch Porthole Glass Project

Oct 01, 1999:

• A fellow named Mark Zemanek gave me a 10 inch porthole glass recently at a Seattle Astronomical ATM meeting.
• Mark got the glass from a boat salvage yard in Fife Washington for $5.00 (what a deal!) I just started hogging it out.
  

• Here's a picture as I'm about to start hogging it out with 60 Grit
• The thing on the right is some plumbing that I used to hog out one of my 8 inch mirrors.
• It works great. I use a fast swirling stroke stroke with it. Sort of like beating an egg.
• Click here to see it in action on my 8 inch F 5.2 mirror.

October 7th, 1999 After about 2 hours of hogging I've got the mirror depth (sagitta) down to 0.08 inches. To reach my target focal length of 60 inches, the sagitta must be 0.10 inches.

October 15th, 1999 After another 30 minutes (total time of 2.5 hours) the sagitta is about 0.95 inches. I've decided that this is deep enough to go to a plaster-tile tool for the rest of the job.

 

Me hogging out the mirror.	Grind grind grind...	Making the plaster tool.

Oct 17th - 22nd After taking the plaster tool out of its mold I placed it under a bright light for 5 days to dry out. After that I placed it in a 170 degree oven for 6 hours to get it really dry. I then glued a hex tiles down with glue adhesive. The last thing to do is to seal the tool with Varathane. Oh boy, pretty soon I can start grinding again.

 

Gluing the tiles	Sealing the tiles with Varathane.

Oct 23-31 Spent about 3 hours grinding with 60 grit bringing the tool to the same shape as the mirror. There were about 6 titles that were a bit lower than the rest and it took a while to bring the rest down to their level. I suspect that I could have just forgotten about them and had the grinding done in about 1/2 hour!

Grinding mirror on top.

Nov 1-5 About an hour with 80 grit, then realized that the focal length was too long so went back to 60 for about and hour. Back to 80 grit for an hour or so and then about 2 hours with 120 grit. Now it's on to 220 grit. Begining to think that the Varathane should've gone on before the titles. No real problems, but between grits I'm spending a lot of time with a tooth brush getting the old grit out.

Nov 6-8 1 hour 220 grit and 2 hours on 320 grit. Doing all Mirror on top (MOT) to avoid scratches from grit falling out of the cracks between the tiles. Probably not neccessary. The finer grit is a lot easier to clean out. Starting to get a nice shine at low angles. Oh boy! On to 500 grit.

Nov 9-10 1.5 hours on 500 grit. Picked up a small scratch, but 20 minutes of work took it out. On to 12 micron.

Nov 11-20 3 hours 12 micron, 3 hours 5 micron. On to polishing!

Grinding Times Summary

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