Quote:
Originally Posted by dave brock
Hi Steve.
Stockholm tar can be bought from most vet supply stores or saddlery/equestrian supplies. I use 7 parts rosin to 1 part S/t for 20° temp. An extra teaspoon of tar will soften 1 ltr mix a reasonable amount.
I'm sure it would work with pitch.
Dave
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Hi All,(and special thanks to Dave Brock
),
The stockholm tar tip has provided me with a real breakthrough in my being able to continuing figuring my 22" and 25" mirrors at my current 4-10deg C temps,(working outside in my back yard). Previously with #55 Gugolz I was unable to successfully make any headway with figuring, due to the pitch laps being too
hard.
Now with the right mix of pitch/stockholm tar have produced three very complient sub-diameter laps,(7",12.25" and 14"). With the use of these, I have brought my second 22"er, to a point where star testing, using the Ronchi eyepiece (133lpi) is the next step. I found with these large very (very) thin mirrors, that astigmatism during figuring presents a real problem,(if stoke is not quite right, one can produce what appears to be major
"stig" within 3-5minutes). The good news is that the defects polished in can be polished out, (though corrective therapy takes considerable longer).
During the final 1-2 hours of figuring, while I was working with the larger 14" lap to remove some residual stig, and improve overall smoothness to the figure, I was greatly aided by the climatic
conditions, gently raining, (greatly helped to lubricate the action between mirror and lap-was just using the minimum amount of CeO), temperature at 8.5deg C. The result, a very smooth finish. As I mentioned in a previous post I estimate the pitch hardness as equivalent to about #40G.
Also making progress with the 25"er, again coming in and out of stig problems but overall I am making headway and expect this optic to be ready for star testing soon.
Best regards
Stephen.