Click for larger image.
Click for larger image.
The graph shows the relative fall distances for a pitch tester constructed according to the instructions in Appendix J of ”How to Make a Telescope“, Second Edition by Jean Texereau. In this test a 1 kg. weight is loading an indenter for 5 minutes and the fall distance is measured. The further the indenter penetrates, the softer the pitch, so in the graph, soft is up. The pitch was equilibrated for at least 24 hours at the indicated temperature before measurement. This is required because pitch is a very poor heat conductor and I found very large variations in the data without a very long equilibration time.
As the graph indicates, my data shows that the equivalence stated by the manufacturer between Gugolz and Acculap recommends an Acculap grade that is at least one grade too hard. I have only tested one batch of each of the Gugolz grades but since they are softer than the recommendation suggests, I don't think my Gugolz has age-hardened. I have tested several different lot numbers of Acculap soft (the one I use the most) and have found them to be consistent. I use mostly Acculap medium for polishing and Acculap soft for figuring and have had predictable results. I would like to point out that the Texereau limits on the graph are for an F/6 to an F/12 mirror. Shorter F ratio mirrors would need softer pitch for figuring.
I found that the Acculap pitch behaved very similar to pitch in terms of melting and cutting with a razor, it chips like Gugolz and channels like Gugolz. I did find that you can spoil it if you overheat it to the boiling point, it discolors and the properties change dramatically. It hardens to an unusable state so keep an eye on it when heating.
When pressing I found the Acculap slightly less sticky which was an advantage when getting the lap off of the mirror, but it is also doesn't seem to adhere to my aluminum lap backing quite as well either. Every so often I'll bump the edge of the lap and a square will pop off. You can put it back on by heating with a match though. For polishing and figuring I found it produced a very nice surface, similar to Gugolz. Water does seem to bead up on the surface a bit more than Gugolz and I have heard that this can cause faster or uneven drying. I use a very thin cerium mixture and keep things very wet so I have not run into this problem.
Acculap also has the advantage for the ATM that it doesn't change properties over time or over many remelts so even if you only make a mirror once a year your Acculap will still be OK. Unless you really like the smell of Gugolz, I recommend giving Acculap a try.
Acculap can be ordered from:
Carl Anderson reports on making and using a lap made with Acculap.
Copyright © 2005
Eric J. Shrader
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