User's Guide to the Kast Double Spectrograph

Table of Contents

Quick Reference
Hardware Overview
Common Path
Blue Side
Red Side
Detector Characteristics
Kast Controller
Data Taking System
Position Angle
Arc and Flat-field Lamps
Diagonal Mirror
Kast Focus
Telescope Offset
Setup and Observing Hints
Setup Procedures
Observing Hints
Calibration Lamp Spectra
Exposure Time Calculator

Data Archive
Mt. Hamilton Homepage

Common Path

Decker | Slit | User Filter Assembly | Polarimeter Module | Dichroics and Mirror


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blockers of various diameters, a set of non-symmetric slots for bizarre sky subtraction schemes, or the decker may be withdrawn completely for unobstructed direct imaging.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).

Notice that there are two full length positions (1 and 10). Position 1 may be used for either spectroscopic or direct. Position 10 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 10 is closer physically to the normally used finger (position 11), 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 1 will work just as well. But don't use 10 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:

PositionNameEncoder numberNotes
2Asymmetric slots 1-2942
3Asymmetric slots 2-2601
4Asymmetric slots 3-2259
5Asymmetric slots 4-1918
62 arcsec long-1201
715 arcsec long-1031
830 arcsec long-860
960 arcsec long-689
10Spect-348Full length, not for directs.
11Finger-75End of blocker near slit.
122 arcsec rectangular blocker-7
134 arcsec rectangular blocker130
146 arcsec rectangular blocker266
158 arcsec rectangular blocker369
169 arcsec circular blocker1154*
176 arcsec circular blocker2348*
183 arcsec circular blocker3522*
*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 selecting the size in arcsecs according to the table below. Alternatively, you may enter the desired slit width in arcseconds from the 'Other...' option in the Slit selection menu.

Minimum slit size is set to 0.4 arcsec. Remember that you need at least a two pixel slit to minimize aliasing problems with narrow emission lines.

Scales and Conversions
scale at slit = 3.86 arcsec/mm
1mm = 3.86 arcsec = 4.936 pixels (red side) = 8.885 pixels (blue side)
Red side: 1 pixel = 0.78 arcsec = 0.2026 mm
Blue side: 1 pixel = 0.43 arcsec = 0.1126 mm

PositionName Encoder Number
0Open -39800
10.5 arcsec 1190
21.0 arcsec 1058
31.5 arcsec 925
42.0 arcsec 792
52.5 arcsec 660
63.0 arcsec 527
74.0 arcsec 262
85.0 arcsec -3
99.0 arcsec -1065

Other... User enters desired slit size in arcsec

User Filter Assembly

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
PositionName*Encoder number
0 Home 0
11 Open 1275
12 None 2299
13 Spinrad NS** 3323
14 ND5.0 4347
15 ND1.25 5371
16 ND7.5 6395
17 ND2.5 7419
18 calcite prism251
*ND values are in magnitudes.
**Spinrad night sky filter pass band is ~6100 - 7600 A

Lower Filter Wheel
PositionNameEncoder number
0 Home0
11 Open 1435
12 BG14++ 2459
13 OG570 3483
14 ND6.25 4507
15 GG455 5531
16 CuSO4* 6555
17 GG385 7569
18 GG495 411
*This is a 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. Please have a staff member (telescope technician or support astronomer) update the Kast motor control software with the current filters. Please remove your filters at the conclusion of your run (and have a staff member update the motor software accordingly). It's definitely safest not to assume that if the motor control software says the wheel is empty that 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
PositionNameContentsEncoder number
0 Home N/A 0
0 Open Empty 1178
(user determined) 2816
(user determined) 4454
(user determined) 6093
(user determined) 7731

Polarimeter Module

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.

Polarimeter Upper Filter Wheel
PositionNameEncoder number
1 Open 2319
2 Filter681
3 Polaroid7234
4 None 5596
5 Empty 3957

Polarimeter Lower Filter Wheel
PositionContentsStep number
1 Open 5914
2 B 4275
3 V 2637
4 R 998
5 I 7552

The waveplate can be moved in and out of the light path and rotated via the Kast motor control software.

Waveplate Insertion and Rotation
PositionNameEncoder numberNotes
Out Out N/A Waveplate out position
0 Home0
1 0.0 deg 485
2 22.5 deg 673
345.0 deg 860
467.5 deg 1084
590.0 deg 1235
6112.5 deg 1423
7135.0 deg 1610
8157.5 deg 1798
9180.0 deg 1985
10202.5 deg 2173
11225.0 deg 2360
12247.5.0 deg 2548
13270.0 deg 2735
14292.5 deg 2923
15315.0 deg 110
16337.5 deg 298

Dichroics and Mirror

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 (or X) 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 (or Y) 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 Select Side illuminated
clear Clear red
dichroic D46 or D55 both
mirror Mirror blue

Dichroic Transmission and Reflectance

Support Astronomers (
Last modified: Mon May 9 14:55:20 PDT 2011