April 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - April 1999 Issue

The Telescope Nut
by Jeff Baldwin

The Tornado Graph

In the February and March issues I talked about the tornado graph, so I'm going to extend that conversation a bit.

When the mirror is spherical and hasn't been parabolized yet, all the centers of curvature are at the same place. This means that all four zones on the Foucault test have the same readings, and when plotted on the tornado graph will graph as a horizontal line.

When you do parabolizing strokes, check your progress on the tornado graph: the right side stays put in the center of the skinny part of the tornado, while the left side starts to move downward. (See illustrations in the February issue). The far left side of the tornado is the widest part, and will allow more error than anywhere else, while the tornado gets thinner as you move towards the right. This means that an error on the micrometer readings doesn't do as much optical damage on the left side as it does on the right side. However, it would be nice if all four zones made it to the center of the tornado at the same time.

If you are correcting the mirror and one part of the data readings is getting to the tornado quicker than another part, you need to change your strokes so that a) this is corrected, and b) the zones start moving towards the tornado equivalently. If the middle is getting there too fast, then the parabolizing strokes are preferring the center too much, and they need to be off center more. If the center of the mirror is not moving towards the center of the tornado as fast as the other parts, then the strokes need to be working more towards the center. Modify and adjust as you work.

As this work progresses, the horizontal line of the sphere pivots about the upper right hand corner of the graph and slowly swings down on the left and eventually is a nice smooth curve within the tornado.

When you are nearing final corrections you will probably have a few readings in the tornado and a few above it. This is when you do your work very slowly and carefully. I usually attempt to correct the zones that are above the tornado. This can be done by doing chordal strokes that end in the zone you need correcting. If your 71% zone is under-corrected, strokes with a sub-diameter lap that end in the 71% zone on the other side of the mirror and off to the right, as well as your side of the mirror and off to the right, will correct the 71% zone, while at the same time slightly raise the 50% zone and the 95% zone.

When you are very close you will use far less pressure, polish extremely slowly, and use thinned out polishing compound in your water. To keep the work very smooth, you will have to take lots of trips around the barrel or you will see individual strokes in your final check on the knife-edge test. For this reason we need to thin the mixture and work slowly and lightly.

Another important thing to keep check on is the smoothness of the glass. It is possible to get all the readings in the proper places, but have a rough surface or striations or microripple. If all the zones are correct, but examining the mirror's surface with the knife-edge shows any texture, then the mirror needs to be smoothed out. A typical W stroke with very slow motion should do the trick without changing the correction by very much. You will probably have to go back and adjust the correction, but a rough surface is absolutely not to be left rough. The rough surface will probably do more optical damage than mis-correction.

If the zones are showing the correct readings, and the mirror appears smooth, there is no T.D.E., the laser shows no pits, and there is no astigmatism, then you have pretty much finished the show analysis of the mirror.

Most people will consider this a finished mirror, but there are still a few more things to do to it.

It is highly recommended that the mirror be star tested. We will discuss this visual analysis next month.

Another thing to do before sending the mirror off to be coated is to remove the edge of the mirror. This will remove any itty bitty T.D.E. that might get by, and it will produce a nice circular perimeter that will guide you in painting the edge of the mirror black when it comes back from the coating service.

To remove the edge of the mirror, place a piece of plate glass (not one that you care too much about) on the barrel. Place a circle of 12 micron or 5 micron grit the same size as the mirror onto the glass. Now place the mirror face down onto the slurry. Make small circular strokes with the mirror for about a minute. Slide the mirror off the slurry and glass and hose it off. Don't wipe the slurry off the mirror or it will scratch the optical surface. This process grinds only the very edge of the mirror. Since the mirror is concave and the plate is flat, the optical part never comes into contact with the grit. It will leave a beautiful, round edge to the mirror that is non-reflective.

When the mirror comes back from the coaters, carefully paint this rough circle with flat black paint for a perfect baffling edge to the mirror. This makes a significant contrast increase in the performance of your mirror for two reason. First, the last bit of T.D.E. is gone, and T.D.E. is the #1 killer for sharp focus. And secondly, any stray light off the edge of the mirror is history.

Next month: Star Testing!

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.