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4m f/8 Secondary Mirror Control System

J.Baldwin, G. Schumacher, 14 November 1995

The f/7.8 secondary mirror is controlled by a CTIO "Smart Motor Controller". The mirror can be both focused and tilted by the operation of three computer-controlled jack screws which are spaced 120 degrees apart on the back of the mirror cell. Each screw is driven by by its own servo motor, which includes an incremental encoder. In addition, a Futaba linear encoder is mounted next to each of the jack screws, and gives an independent reading of the position of the jack screw to an accuracy of nominally 1 micron.

1.0 Menu Commands.

Control commands are normally issued from the "Tilt Secondary" and "Focus Secondary" menus on the TCS screen. The commands are:



  TILT SECONDARY
    INIT TILT
    RELATIVE TILT          (tilt to new value)
    ABSOLUTE TILT          (tilt to new value)
    LAST LOG ENTRY         (tilt to last value in IMAN log file)
    OLD LOG ENTRY          (select any value from IMAN log file)
    DISPLAY TILT   

  FOCUS SECONDARY
    INIT FOCUS             (reset zero points of encoders, 
                            then return to present focus position)
    MOVE TO VALUE
    STEP FOCUS

2.0 Command Mode commands.

Commands for the mirror can also be typed into the TCS using the Command mode:


sec encoder 
		Return readings of Futaba encoders A, B1 and B2, and
		differences, in the order A B1 B2 (B1-A) (B2-A). Units
		are microns of motion at the secondary mirror. All
		other commands dealing with focus motions use units of
		microns of movement of the focal plane.

sec tilt [tilt amplitude] [tilt azimuth]  

		Tilt secondary to specified absolute position.

sec focus [value]

		Change focus by specified DIFFERENCE from present focus.

sec afocus [value]

		Change focus to specified absolute value.
sec display 

		Show present values of tilt and azimuth.

sec fast [value]

		Set fast focus speed, in arbitrary units. Default = 120.

sec slow [value]

		Set slow focus speed, in arbitrary units. Default = 50.

sec init

		Moves mirror to fiducial position and rezeros
		encoders.  Does NOT restore previous focus value
		(unlike the menu command).


sec reset

		Zeros out all registers and hardware; leaves smart motor
                controller ready to receive commands.

3.0 Units, etc.

The three jack screws and their accomapnying Futaba encoders are labelled A, B1 and B2. A is on the South side of the secondary mirror when it is in its observing position; B1 is on the NW side and B2 is on the NE side.

Focus motions are achieved by driving all three jack screws by the same amount. Focus units are microns of travel of the focal plane, = 9.56819 times the motion at the secondary mirror (but Beware!, the command "sec encoder" returns the values at the secondary mirror). Positive focus changes move the focal plane upwards.

Tilt is produced by driving the three jack screws by differing amounts, so as to tilt the mirror about a point located 4.68 inches behind its vertex. This center of tilt was chosen because it is in the plane defined by the three rollers which provide the lateral support beween the inner mirror cell (which moves) and the outer mirror cel (which doesn't move).

Tilt units are microns of wavefront error for coma3 at the edge of the pupil (the units returned by IMAN), and the azimuth of the error. The mirror will then tilt so as to remove that amount of coma. This is to maintain consistency between values pulled from the IMAN log and values entered manually. The conversion to the actual angular tilt of the mirror is:

        .0133 degrees of tilt = 1 micron of coma3.
The tilt azimuth is defined as 0 deg azimuth to the west, and then increasing as you go around to the north. Including the fact that the mirror is moved so as to remove the entered coma value, a tilt request containing a positive coma amplitude will cause the side of the secondary mirror in the specified PA to move closer to the primary mirror while at the same time the opposite side moves away from the primary mirror. Therefore, relative to the tilt=0 fiducial position:


   sec tilt 1 0      Lowers W side, raises E side, 
                              through a 0.0133 degree tilt. 

   sec tilt 1 90     Lowers N side, raises S side.
The above tilt changes cause the absolute tilt (read out using the DISPLAY TILT menu command) to change by the requested amplitude but with a position angle which is the requested PA - 180 degrees. This will be added vectorially to the previous absolute tilt. For example, a relative tilt of: 
   sec tilt 1 90
will produce an absolute tilt of 
   1.0 micron  PA 270,  if the starting point was abs. tilt = 0   0.
   1.4 micron  PA 225,  if the starting point was abs. tilt = 1 180.
   etc.