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

Red Side

Collimator | Gratings | Filter Wheel | X-Y Stage | Direct Imaging


A typical focus value is presently about -5 mm. The range is -24 mm to +23 mm. The focus does not appear to be affected by removing or inserting the dichroic. The off-axis paraboloid is silvered and overcoated.


Blue is left in the displayed spectra. The total range for the tilt is -3400 to +24900. A flat mirror may be inserted into the grating tray for direct imaging on the red side. The tilt to use for access to the grating tray is -3350.

Access to the grating tray is via a long black door just above the red side dewar. It is held closed by two captive screws, and it's hinged at the bottom. Each grating is clearly labeled on the end facing the door, so it's easy to see which ones are there without the necessity of taking them out. Be particularly careful of the fragile ion pump.

The grating positons in the tray are numbered 1, 2, 3, from left to right. A reinforcing plate blocks the position in the middle of the access door, so you may need to change the grating tray position (with the spectrograph controller or manual buttons) in order to check all three gratings. Select position 2 to see the two outside gratings (positions 1 and 3), and either end position to see the center grating.

We suggest that you always look for yourself (or ask a telescope technician) to verify that the gratings you requested are where you think they are.

It is strongly preferred that observers do not change their own gratings. The entire community depends on their good condition, and the replacement cost is thousands of dollars each, not to mention the time involved. If you must do it yourself, do not under any circumstances do so without being explicitly checked out on the procedure. The tilt to use for loading the grating tray is -3900, and you will need to move the tray in order to gain access to all three tray positions, as previously described. Never place a grating anywhere except in the spectrograph or in the grating file. Again, we prefer that the technicians make any changes required.

grating grooves/blaze A/pixrangetilt* useful range (approx.)
1 600/5000 2.36 2830 2.22(c)-4283 3800-10000
2 600/7500 2.35 2820 2.22(c)-4231 3800-10000
3** 830/8460 1.70 2040 3.15(c)-4338 3800-10000
4 1200/5000 1.17 1404 4.58(c)-4469 3800-7310
5*** 300/4230 4.59 5508 1.09(c)-3792 3800-11000
6*** 300/7500 4.60 5520 1.09(c)-3781 3800-11000
7 600/3000 2.34 2808 2.21(c)-3733 3000-????
* c is the desired central wavelength, in Angstroms
** 830/8460 grating in second order yields 0.85 A/pix, range is 1020 A, and the second order central wavelength is given by 6.49(c)-5107. The max tilt of 25,500 restricts lambda < 4816A in 2nd order. Longer wavelengths can be reached by manually moving the red x-stage.
*** Note that the 300/7500 is more efficient than the 300/4230 for all lambda greater than ~5400 A. Also, the 300/7500 in second order can be confused with the 600/7500 except the throughput is extremely low. We do not recommend using the 300/7500 in second order.

Grating PositionEncoder NumberNotes
00Home (not for observing)

Filter Wheel

There is provision for four round filters up to 5.5 inches in diameter, plus an open position. Notice that small filters here in the collimated beam will drastically reduce the effective aperture of the telescope.

Position NameContentsEncoder number
0 Home Home 0
1 Open empty 23900
2 GG455 GG 455 (3mm) 4700
3 GG495 GG 495 (3mm) 9500
4 OG550 OG 550 (3mm) 14300
5 NS Spinrad NS 19100

X-Y Stage

We assume that you have read the general description of the stages given in the section Blue Side - X-Y Stage, so we won't repeat that here.

Please be clear on the fact that the observer should not find it necessary to move the x-y stage on the red side. The two dials are labeled x and y, and the convention adopted is that x moves along the dispersion, and y moves perpendicular to it. The nominal y position is so that the spectrum falls near the top of the chip for most efficient readout. The red side x position is set to the middle of the range of good camera imaging. It should never be advisable to move this, since you can get the same effect by moving the grating tilt, and the grating tilt equations are derived for this particular setting.

For the red side, higher numbers in x move to longer central wavelength, and higher numbers in y move the spectrum higher on the chip; that is, to lower row numbers.

To summarize the red side x-y stage motions (scale 37 pixels/turn):

red side

nominal + moves to
x 10.00 higher wavelength
y 10.00 lower row number

Direct Imaging

A good window for red side direct imaging using no splitter and the tilted mirror in the grating tray (tilt = 8800) is: number rows (nr) = 200, number of columns (nr) = 200, start row (sr) = 130, start column (sc) = 500. Check it with the top lights, not the TUB lights.

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Last modified: Fri Aug 7 14:46:00 PDT 2009