The decker is a long reflective plate which overlies the slit. The observer may select a variety of options including slits of varying length, rectangular blockers of varying widths, circular blockers of various diameters, a set of non-symmetric slots for bizarre sky subtraction schemes, or the decker may be withdrawn completely for unobstructed direct imaging. One positon called "the finger" places the end of the rectangular blocker just off of the slit for use as a reference for positioning objects at the center of the slit length (it's usually a convenience in the reduction if everything falls along the same rows).

Twenty named positons are available, of which 0-3 are protected and may not be renamed by users.

Notice that there are two full length positions (0 and 2). Position 0 may be used for either spectroscopic or direct. Position 2 will allow a full length slit for "normal" slit sizes, but will be paritally obstructed by adjacent portions of the decker if the slit is fully open, as for a direct window. Position 2 is closer physically to the normally used finger (position 3), so if you are switching between large and small spectroscopic slits, you may wish to use these adjacent positions. In fact the decker moves very rapidly, so 0 will work just as well. But don't use 2 for directs. A full-length decker on the Reticon 400x1200 chip (red side) with 27 micron pixels is 185 pixels = 145 arcsecs. A full-length decker on the Fairchild 2k x 2k CCD (blue side) with 15 micron pixels is 333 pixels. This corresponds, of course, to the full length of the slit itself.

The decker slide is presently configured as follows:

Position*	Name*	Contents	Step number
0	open	fully open	350
		asymmetric slots	560
		asymmetric slots	610
		asymmetric slots	660
		asymmetric slots	710
1	2 arcsec	2 arcsec long	815
		15 arcsec long	840
		30 arcsec long	865
		60 arcsec long	890
2	spect	full length, not for directs	940
3	finger	end of blocker near slit	980
		2 arcsec rectangular blocker	990
		4 arcsec rectangular blocker	1010
		6 arcsec rectangular blocker	1030
		8 arcsec rectangular blocker	1045
**		9 arcsec circular blocker	1160
**		6 arcsec circular blocker	1335
**		3 arcsec circular blocker	1507
*Named positions of the decker (positions 0-3) are not user changeable **The occulting disks are alumininized spots on a quartz plate which is not AR coated.

The aluminized slit opens bilaterally, and the smallest available step is a tiny fraction of a pixel. For convenience, the most commonly used slit sizes may be selected from the spectrograph controller simply by typing the desired size in arcsecs according to the table below. Alternatively, you may compute the step numbers for any size you wish and enter them directly, so the result is very flexible for the observer.

Note that the slit gets smaller for larger numbers, and is fully closed at 10288. Remember that you need at least a two pixel slit to minimize aliasing problems with narrow emission lines.

There are 20 named slit positions, of which 0-9 are protected and may not be renamed by users.

Scales and Conversions
scale at slit = 3.86 arcsec/mm
1mm = 3.86 arcsec = 4.936 pixels (red side) = 8.885 pixels (blue side) = 200 stepper motor steps
Red side: 1 pixel = 0.78 arcsec = 0.2026 mm = 40.52 steps Blue side: 1 pixel = 0.43 arcsec = 0.1126 mm = 22.51 steps
slit setting = 10288 - 200*(slit width(mm)) = 10288 - 52.82*(slit width(arcsec))

Position	Name	Step number
0	open	2250
1	.5 arcsec	10262
2	1 arcsec	10236
3	1.5 arcsec	10210
4	2 arcsec	10184
5	2.5 arcsec	10158
6	3 arcsec	10133
7	4 arcsec	10081
8	5 arcsec	10029
9	9 arcsec	9822

There are three stacked filter wheels called, from top to bottom, the upper, lower, and user wheels. One position of each wheel (position 0) is always empty.

Filters in the top three wheels will affect both beams of the spectrograph. If you want to use different filters on the two sides, they must be installed in the holders in front of the cameras. Since these later filters are in the collimated beam they must be bigger than 3.5 inches in diameter in order to avoid vignetting for direct imaging (a 2" square filter will reduce the effective aperture to about 1-meter; if this is your plan, it would surely be more politic to ask for one-meter time in the first place).

Order separating filters should not be necessary in typical double-beam use, except in the far red.

Upper Filter Wheel
Position	Name	Contents	Step number
0	open	empty	257
1	(n/a)	(not in use)	457
2	Spinrad NS	Spinrad night sky*	657
3	ND5.0	neutral density 5.0 mag	857
4	ND1.25	neutral density 1.25 mag	1057
5	ND7.5	neutral density 7.5 mag	1257
6	ND2.5	neutral density 2.5 mag	1457
7	calcite	calcite prism	22
*Spinrad night sky filter pass band is ~6100 - 7600 A

Lower Filter Wheel
Position	Name	Contents	Step number
0	open	empty	288
1	BG14++	BG 14++	488
2	OG570	OG 570	688
3	ND6.25	Neutral Density 6.25	888
4	GG455	GG 455	1088
5	CuSO4	CuSO4*	1288
6	GG385	GG 385	1488
7	GG495	GG 495	88
*This is a new, good quality, full slit length crystal.

The user filter wheel accepts up to four filters of your choice mounted in our 2" square holders. Filter may be up to 8mm deep. Someone will definitely need to show you how to mount the filters on the first occasion. There's a card in the control room on which you may note which filter is in which position. Please remove your filters at the conclusion of your run. It's definitely safest not to assume that if the card is blank there are no filters in place. We suggest always setting this wheel to the open position if you're not using it, just in case.

We have a fairly large library of narrow and intermediate band interference filters available, mostly on loan from astronomers within the UC system. It's clearly best not to assume that any specific filter will be here when you need it, as the owners are certainly free to remove them at will, and they may on occasion be loaned out. Since they're not really ours, we don't really control them. Any arrangement to take them elsewhere should be made with the owners, and duly recorded on the mountain as well.

User Filter Wheel
Position	Name	Contents	Step number
0	open	empty	230
1		(user determined)	550
2		(user determined)	870
3		(user determined)	1190
4		(user determined)	1510

Instead of the User Filter Assembly (described above) a Polarimeter Module can be installed (the current default Kast setup has the polarimeter installed instead of the user filter assembly). The polarimeter module contains two filter wheels and a waveplate. The polarimeter module will affect both beams of the spectrograph.

The upper and lower filter wheels accept up to four filters mounted in our 2" square holders. Filters may be up to 8mm deep. The lower filter wheel usually has B, V, R, and I filters installed, though this may not always be the case. If you install other filters, please remove them at the conclusion of your run. We suggest always setting these wheels to the open position if you're not using them. The tables below list the usual contents of the two wheels, though if wish to use them you should double check the contents.

Polarmeter Upper Filter Wheel
Position	Name	Contents	Step number
0	open	empty	464
1	filter	filter	147
2	polaroid	polaroid	1424
3	none	empty	1104
4	empty	empty	784

Polarmeter Lower Filter Wheel
Position	Name	Contents	Step number
0	open	empty	1155
1	B	B	835
2	V	V	515
3	R	R	195
4	I	I	1475

The waveplate can be moved in and out of the light path via the Kast motor control software. Be aware that in the software the waveplate is designated as the "User Filter Wheel".

Polarmeter User Filter Wheel
Position	Name	Contents	Step number
0	in	Waveplate In Position	1050
1	out	Waveplate Out Position	684

Waveplate rotation is controlled from the data taking software, rather than the Kast motor control software.

There are two separate carriers, either one of which (or neither, but not both) may be in the beam. Either carrier may contain one of the overcoated dichroics, or an aluminized flat mirror. The carrier called 1 is the one farthest away from the access door, and moves in an E-W sense with the TUB at its standard (90 degree) position angle (or left-right as seen from the access door); number 2 is closer to the door and moves N-S (or toward and away from the door). Crossover for the blue dichroic (D46) is about 4650 A, and for the red dichroic (D55) it is about 5500 A. About 200 angstroms of the spectrum are affected by the crossover of the dichroic. Insertion or removal of the D46 dichroic shifts the red spectrum by less than one pixel, but the D55 dichroic shifts the spectrum vertically upward by about 10 pixels. If you're only using one side at a time, in order to switch sides move the mirror in for blue or out for red. To get Type Side illuminated clear out red dichroic D46 or D55 both mirror mirror blue

Dichroic Transmission and Reflectance