Mirror
and tool after being pressed.
When polishing a telescope mirror it is common for the edge of the mirror to exhibit either a turned down or a turned up edge. Once the edge has turned up or down it is often difficult to make it return to "flat". In general, TDEs are more difficult to eliminate since they require the removal of much large amounts of material (glass) than do mirrors with TUEs. Even so, TUEs can be a problem. When attempting to figure a telescope mirror, one wishes to have a mirror which is "smooth" and does not suffer from edge defects.
Here I outline a simple explanation as to why edge defects persist and often grow. This explanation only applies to full size pitch laps. I do not attempt to explain the initial cause of the edge defect.
It is useful to think of a spherical mirror/tool as "flat" and any defect away from a sphere as being none flat. If a mirror is spherical except it has a turned down edge, then the central portion of the mirror can be thought of as flat and the edge as having a downward slope going away from the center. A turned up edge is the opposite. Notice that a flat mirror is a degenerate case of a spherical mirror where the radius center of curvature is infinite.
The key to the explanation is what happens to the pitch lap tool after it is pressed...
When a mirror with turned down edge is pressed on its tool it will look like this in cross section.
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Mirror
and tool after being pressed.
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Notice that the edge of the tool is higher than the rest of the tool. When the mirror is pushed back and forth across the tool it will look like this in mid stroke:
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Mirror
and tool in mid stroke.
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Notice that the mirror rests on the tool on two points and as the mirror is worked, the edge is increasingly worn down! Thus causing the TDE to get worse.
When a mirror with turned up edge is pressed on its tool it will look like this in cross section.
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Mirror
and tool after being pressed.
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Notice that the edge of the tool is lower than the rest of the tool. If the tool is made of soft pitch, the action of the mirror going back and forth will tend to cause the area of the tool which is lower to enlarge.
After a soft lap has been worked, the mirror and tool will look like this in mid stork
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Mirror
and too in mid stroke after tool has been worn.
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Notice that the central portion of the mirror receives even wear while the edge of the tool receives tapered wear. This will tend to cause central portion of the mirror to deepen with respect to the edge.
Pitch Lap Hardness
The raised lip of tool due to a TDE should get pressed out as the mirror is wears against it. If the tool is hard, this may not happen for a while and thus hard laps should tend to exacerbate TDE's.
As I have attempted to demonstrate above, soft pitch laps should exacerbate TUE's
Pressing Frequency
The frequency at which the tool is repressed can also be taken into consideration.
According to the ideas presented here it follows that frequent pressings should exacerbate TDE's since the raised lip of the tool will be recreated more often.
Conversely, frequent pressing should help TUEs since the zone of pressed down tool will be, on time average, decreased (see above drawing).
Well, hope that makes some sort of sense. Happy glass pushing.
Send comments to brucew@noperiod.com