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Lonnie Robinson
of Sacramento California
Comments on Using TSIT

Accurate repeatable Foucault testing proved very time consuming and difficult for me. What I needed was a test that could be done quickly, accurately, and with repeatable results. In 2010, while heading up a telescope workshop for our local club, I met ATMer Bill Thomas. He was hawking his new test, the TSIT, which promised to cure my Foucault issues. He even offered to supply the specialized tester, and all I had to do was purchase a digital SLR camera body. Learning this completely new and easy test was great fun, and Bill graciously helped me through every step. With the TSIT, it isn't uncommon for me to accomplish six to eight figuring sessions in a day. It's incredible to see the results of figuring so quickly, with a single accurate test, and have precise information to plan the next solution.

Many years ago, I made a 16" f/5 by hand working around a barrel. It was tested and figured using many sets of averaged Foucault readings. No two readings were ever the same and judging the center and far edge shadows proved extremely difficult. Not surprisingly, a recent retest with the TSIT showed a high center. Last winter I had two back to back chances to put TSIT through its paces, both 16" f/4.5 refigures for club members. Refiguring went well with the first mirror, and I was able to quickly bring it down to near perfection. Initial testing of the second mirror, at several orientations, raised a warning flag suggesting there were some serious surface variations. I decided it was prudent to go back and regrind starting with 220 grits. Careful attention to centering the mirror on the machine during regrind through polish, and rotating the mirror often on the turntable, eliminated the surface irregularities. For a sanity check I continued with an occasional multi-axis test routine, and each time they showed consistent results all around. These tests reaffirmed my decision to regrind, and indicated the current precautionary measures were successful. Figuring by hand with small tools has the potential of uneven correction and creating zones. Using a machine to figure the curve, laps sized for each zone, and varying the stoke length and quill offsets often, one can attain a smooth surface of revolution. This was an amazing experience, the fast and accurate TSIT was simply a joy to use. My confidence in the test was bolstered each time a figuring solution produced the exact planned result.

The TSIT's accuracy is dependent on a high tolerance zonal mask. Bill and I used a plunge router mounted next to a guide rail, a dial indicator reset at each hole location, and cut our mask holes in sheets of .060 thick ABS plastic. The resulting hole centers varied about +-.007, which we deemed excessive. Our ATM machinist friend, Jack Sales, suggested using a Digital Readout Scale (DRO), eliminating potential errors from moving and resetting the dial indicator. The DRO's progressive digital numbers were a great help accurately reading and keeping track of hole locations. I installed a " brass sleeve bearing below the router plate, and a close-fitting guide bearing attached to the cutter. This arrangement eliminated any plunge router movement and produced acceptably accurate hole centers varying less than +-.001. When cutting the holes, a touch of 3-in-1 oil kept the ABS cool and cutting clean. This highly accurate zonal mask is centered with a pre-aligned laser mounted on the test stage, first aimed at the exact center of the mirror, and then the mask is centered on the laser.

Ronchi and knife-edge examinations are very useful for polish condition and figuring, providing needed information about surface quality, turned edge, and a smooth curve. I use FigureXP for final figuring, it displays a quantitative edge-on surface profile to accurately plot my next figuring solution. Bill's TSIT deserves serious consideration, it makes exceptional mirrors possible and our ATM lives a lot easier. Remember, you can only make a mirror as accurate as your test.

Lonnie Robinson DRO Mask Cutting System

Lonnie Robinson
Hourglass Telescope Mirrors

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