16.2-Inch-Mirror-Blank-Project

I originally started with a 16.2 inch diameter mirror blank with circular web back pattern. This was a mistake due to stress being concentrated in the narrowest part of the webbing. This being said a 16.2 inch disk was worked and a good figure attained in spite of what might be considered a risky anneal. The picture on the right is a jpg conversion of the original dwg format file.

Original AutoCad file:Media:16-inch-blank.dwg

What does this tell us? Because my anneals have gotten much better and the circular webbing moved away from that our end product is workable. Whether picky anal astro types wince or not is not the issue. The issue is not emotion but rather IF something can be done and with what level of difficulty. The answer is YES it can be done and at reasonable and comparable effort levels for the mirror design at hand.

The ronchi test of the finished processed mirror is shown below left. This was a test done by the guy who finished the processing job. I myself then tested the mirror using the focault test with an eye towards detecting astigmatism. I could detect none. Astigmatism is the most common problem resulting from poor annealing.

Master Mold Generation Process Notes

Below is an overview of the mold making process. It will be altered somewhat for larger diameter pieces but is representative of the process and is thus presented: The method for this mirror was as follows

• 1) Circular nub biscuits
  

---a)make a tapered circular nub on a milling machine. In the photo a hex nub is shown because I now longer have any round ones.
---b)use the nub to make an elastomer nub master mold
---c)mold numerous plaster of paris nubs using the rubber nub negative mold
---d) sand each nub to exact fit

• 2) case a dog bowl out of plaster of paris around a suitable diameter circular object and sand to finish to exact dimensions
  
• 3) Glue nubs into dog bowl using full sized paper template printed out on Kinkos gigantic printers. This insures VERY good placement. Its pretty hard to do with out this template. This forms a plaster of paris negative of the mirror. It is a positive of the refractory mold desired
  
• 4) Make a rubber mirror positive mold using the plaster of paris hand made mirror neg / refractory pos. This is done in a vacuum using a relatively expensive two part stiff silicon.
  
• 5) Use the rubber mirror positive to case refractory material against in a tube to retain material from without and the rubber mirror within to shape the desired.
  
  

Drawbacks: The all in one piece makes for a heavy and hard to separate situation. It is manageable for 16 inches but even in the case of 20.5 inches another method was required.