Mechanism conventions

This is an attempt to come to some consensus regarding zero points for the various mechanisms.  Meant to foster discussion and record what we finally decide.  Comments required!

For wheels - slit, decker, filter, lens, cal - we are limited by the 10 turn pots, not the hardware.

For echelle, need to go from around -30 degrees to around 70 degrees.

Flip mirror has two stops that may or may not be set right at desired position.  Depending on vibration performance, may choose to allow motion beyond warm alignment position.

Paraboloid (not yet attached) will likely be limited by 10 turn pot, depending on gearing and focus position.

For each wheel, we want to have a zero point that we feel confident about.  I guess I've been less than happy about the open/open/open on slit/decker/filter, but that's a different blog post.

Here are my suggestions for the zero points for different mechanisms:

Echelle - zero point is defined such that when in hi-med mode with start of the Q branch of C2H2 is in the middle of detector echelle angle = 55.94 degrees.

Slit - zero point is defined so angle 0 has pinhole grid centered.

Decker - zero point is defined so that when large edge of central decker arc is cutting through center of pinhole grid, the angle is 58 degrees (TBC).

Filter - zero point is defined so angle 0 is center of open filter position.

Lens - zero point is defined so angle=0 is center of open.  Note that we may decide to put fiducials in the lens wheel to define a zero point.

Parab - echelon paraboloid zero point will be when echelon order is centered on dectector.

Calwheel - calibration wheel will be at 0 degrees when retro mirror on wheel is centered.

Flip mirror - zero when in bypass (echelle) mode.  This gives positive voltage and positive steps when moving toward positive angles.