Echelle Spectrograph Slit

The slit width is read from a manually operated micrometer on the outside of the spectrograph. The micrometer should always be set toward higher numbers. The micrometer reading is in units of .01mm, and repeats after each 50 units. The scale is 6.56 arcsec/mm, or 152 microns/arcsec, or 15.2 micrometer-units/arcsec.

Decker

The decker is used to define the length of the slit. It is manually controlled by a knob on the outside of the spectrograph. The knob has an indicator on it to show the current setting. The scale at the slit is 6.56 arcsec/mm. There are two decker plates available.

The "Normal" Decker

This is the default option. The decker openings are in discrete steps. Positions 1-8 have double holes for taking focus plates and are not normally used with CCD detectors. Positions 9-23 and "open" are used with the CCDs. All decker dimensions are given in arcsec at the slit. The scale at the slit is 6.56 arcsec/mm.

Decker Lengths for Stellar Positions 

    Indicator          Length           Indicator      Length 
    Position          (arcsec)          Position      (arcsec) 
      
        9		 3.3   		   17		52.5
       10		 6.6   		   18		65.6
       11		 9.8   		   19		78.7
       12		13.1		   20 		91.8
       13		16.4		   21	       105.
       14		19.7		   22          118.
       15		26.2		   23          131.
       16		39.4		  open         341.

Separation Between Holes for (unused) Calibration Positions

We list these for completeness. 

    Indicator        Separation         Indicator    Separation
    Position          (arcsec)          Position      (arcsec) 

        1	    central hole	    5		28.2	
        2 		 8.5		    6		34.8
        3		15.1		    7		41.3
        4		21.6		    8      multi-aperture

Wedge Decker

There is also a wedge-shaped decker, in which the decker length gets linearly larger with higher indicator positions. This is almost never used because there are enough choices available with the normal decker.