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 Quicklook

Data Archive
Mt. Hamilton Homepage

Red Side

Detector | Collimator | Gratings | Filter Wheel | Direct Imaging


The Red side detector has a mask in front of it to prevent scattered light from illuminating unused portions of the chip. The spectrum covers rows 760 to 3310 (though more spectrum may be visible, it may be vignetted beyond these rows). The mask is sufficiently large in columns to never vignette the data. For more detector specific information, see Detector Characteristics.


A typical focus value is presently about 2 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 at the bottom 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.

Observers may 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. Contact a support astronomer or telescope technician if a grating needs to be changed. The tilt to use for loading the grating tray is -3350, 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.

grating grooves/blaze A/pixrangetilt* useful range (approx.)
1 600/5000 1.30 3470 2.23(c)-4687 3800-10000
2 600/7500 1.30 3470 2.23(c)-4687 3800-10000
3** 830/8460 0.94 2490 3.15(c)-5039 3800-10000
4 1200/5000 0.65 1720 4.70(c)-5818 3800-7310
5*** 300/4230 2.53 6760 1.09(c)-4413 3800-11000
6*** 300/7500 2.55 6785 1.09(c)-4388 3800-11000
7 600/3000 1.29 3445 2.20(c)-4372 3000-????
* c is the desired central wavelength, in Angstroms
** 830/8460 grating in second order yields 0.47 A/pix, range is 1245 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.

At grating tilts between 18520 and 24000, an extended, diffuse background feature appears near the blue limit of the resulting spectrum. Depending upon the grating tilt, this feature may also be accompanied by secondary (or "ghost") spectral lines/features superposed onto the primary spectrum at Y pixel values of circa 2100 and higher. These features possibly result from reflection(s) from a surface of the 830/8460 grating. The general trend of these superposed lines/features is slightly offset in the spatial direction from the primary spectrum. These features are most easily seen in wavelength calibration spectra using Neon calibration lamps. Because the most noticeable feature is extended/diffuse, its presence should not compromise wavelength determinations. Because of the spatial offset of the "ghost" features, they should not contaminate the features on the main trend in primary scientific spectra. However, if these features are of concern, observers should consider modifying the grating tilt to avoid these features.

*** 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

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) = 350, number of columns (nr) = 390, start row (sr) = 2020, start column (sc) = 424. Check it with the top lights, not the TUB lights. When the TUB is at 90 degrees, North is down and East is to the right.

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Last modified: Sat Mar 6 16:16:43 PST 2021