CSHELL Information


The CSHELL is a long-slit spectrograph which uses a 31.6 lines/mm, 63.5 degree echelle with narrow-band circular variable filters that isolate a single order (orders from 11 to 56). The spectrograph can achieve resolving powers up to 40000, or 7.5 km/s, over the 1-5 micron spectral region. The instrument also has a direct imaging mode, and it has been recently upgraded to use a single Hughes SBRC 256x256 InSb detector array, sensitive from 1-5.5 microns.. Further technical information can be found in Greene et al. (1993, SPIE, vol. 1946, p.313).


Array size:        256 spectral by 160 spatial pixels

Pixel Size:        0.20 arcsec; 2.7 km/s per pixel (at blaze angle)

Wavelengths:       1.08-5.5 microns

Spectral Range:	 ~ 2.40E-3 * wavelength (@ blaze angle)

Resolving Power (R)     43000   21500   13300   10800    5040
Slit width (arcsec)       0.5     1.0     1.5     2.0     4.0
Pixels/resol              2.5       5     7.5      10      20

R = lambda/(delta lambda)
Slit: 30" in length. Discrete slit widths of 0.5, 1.0, 1.5, 2.0 and 4.0". The 4" slit has been provided to permit spectrophotometry on point sources. The slit orientation may be changed by manually rotating the spectrograph with the instrument rotator on the telescope.


CSHELL has a direct imaging mode in which a plane mirror substitutes for the grating and provides an image of the 30" field-of-view on the infrared array. This mode is useful for viewing the field in the infrared, acquiring sources, and centering them in the slit. Images of the field may be recorded for reference with either narrow-band or standard photometric filters, though the system is not meant to provide high quality photometric images.


CSHELL has an internal Tektronix 512 x 512 CCD with nearly a 1' field. This device can be used for object acquisition and point source guiding to V=17 in good seeing with no moon.


Argon, krypton and xenon spectral lamps and a continuum lamp are provided for wavelength calibration and flat fields.


On-line quick look data reduction software is available for use with the instrument. It allows examination of the data by providing sky subtraction, flat fielding, and simple spectral extractions. Raw or reduced data is recorded in FITS format files.


The spectrograph sensitivity F in W/m**2/micron is given by 

F   = F0*(SNR)*R*SQRT[ N*(nr**2+nb*t)/(T*t) ]/(lambda**2)

F0	=	SBRC 256 InSb: < 3 microns: 2.4e-19 (16.5% throughput @ blaze)
		               > 3 microns: 2.9e-19 (12% throughput)
SNR	=	signal-to-noise ratio
R	=	resolving power
lambda  =	wavelength in microns
N	=	total number of pixels summed (spatial + spectral directions)	
nr	=	readout noise in electrons ( ~30)
nb	=	dark current and background in electrons/sec
t	=	chip exposure time
T	= 	total observation time

We present below the sensitivity for SNR = 10, R = 21500 (1.0" slit), and T = 1 hour. Point Source calculations are made for nodding the telescope along the slit such that the object remains in the slit in the sky beam location; for the surface brightness sensitivity, the telescope is nodded to blank sky. Pixels are summed 1" along the slit and over one resolution element along the dispersion direction; therefore, the results are per resolution element, not per pixel. The numbers below may be scaled to other cases using the above formula.

For wavelengths shorter than 2.5 microns, we take nb=5 elec/sec and t = 120 sec. At 3.4 microns, nb=28 elec/sec and t=120 sec. At 4.8 microns, we assume nb=1000 elec/sec and t=20 sec, which fills the wells to half capacity. We sum 5 pixels in the dispersion direction by 5 pixels in the spatial direction.

                      PREDICTED SBRC 256 InSb PERFORMANCE

Lambda  Continuum       Mag.       Line flux          Surface brightness
(um)    (W/m**2/micron)	           (W/m**2 / resol)   (W/m**2/sq. arcsec)

 1.25    9.6e-15        13.8       5.6e-19            7.9e-19
 1.65    5.6e-15        13.3       4.3e-19            6.1e-19
 2.2     3.2e-15        12.8       3.2e-19            4.6e-19
 3.4     3.2e-15        10.9       5.1e-19            7.2e-19
 4.8     8.2e-15         8.5       1.8e-18            2.6e-18

See IRTF Online (anonymous FTP to irtf.ifa.hawaii.edu) or the IRTF WWW site 
(http://irtfweb.ifa.hawaii.edu/) for more CSHELL info.

-Tom Greene

LAST UPDATE: February 1, 2010 (format only)