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CGS4 Sensitivities
JACH |JCMT |UKIRT |Computer Services |CGS4
Up: CGS4Handbook

CGS4 Sensitivity, Resolution, Wavelength Coverage and Pixel Sizes.

The 256x256 array has a read noise of about 42e- for 2-read NDR with multi-read NDR noise of about 23e-.
 

Sensitivity:  Some Very Important Notes - Please Read.

  • The values listed below are surface brightness sensitivities.

  • The sensitivities are calculated for the best part of the band (near 100% atmospheric transmission). They cannot always be used to interpolate sensitivites at other wavelengths within the band.

  • Sensitivities for point sources can be estimated from the following tables. Assuming 0.6" seeing and that the spectrum falls onto three rows of the array, the signal-to-noise will be approximately a factor 2.5 lower than quoted in the tables (about one magnitude).

  • CGS4 is less efficient when very bright sources are observed and it is difficult to obtain signal-to-noise greater than a few hundred. The sensitivities on very bright sources also do not scale with brightness.

  • In order to achieve the quoted sensitivities, you need to use long exposures. Typically > 30 seconds in H and K, and >75 seconds at J. Shorter exposures will result in a lower sensitivity. Please see  CGS4 Exposure Times.

CGS4 sensitivity per pixel with the 40 l/mm grating and the long camera

The 3-sigma 30-minute sensitivities per 0.6"x0.6" pixel for the case of nodding up and down the 1-pixel wide slit are as given in the following table. Sensitivities for very extended objects which require nodding to blank sky will be 0.4 mag poorer. The line flux assumes that the line is unresolved. Please remember to reduce the sensitivity by one magnitude for point sources.
 
wavelength (µm)  order magnitude  line flux (W/m**2)
0.9 2 19.2 2.0e-19
1.25  2  19.8  5.0e-20 
1.6  2  18.0  9.0e-20 
1.6  1  19.3  6.0e-20 
2.2  1  18.7  4.0e-20 
3.4 1 12.4 1.0e-18
3.8  1  12.4  1.6e-18 
4.9  1  10.9  2.0e-18 

 

Predicted CGS4 sensitivity per pixel for the 150l/mm grating and the long camera.

The 3-sigma 30-minute sensitivities per 0.6"x0.6" pixel for the case of nodding up and down the 1-pixel wide slit are as given in the following table. Sensitivities for very extended objects which require nodding to blank sky will be 0.4 mag poorer. The line flux assumes that the line is unresolved. In the J-K windows the predicted sensitivity is based on a model for the dark time inter-OH sky background and the throughput of CGS4. There will be variation in this background depending on lunar phase. If you observe an astronomical line which is not resolved from an OH line, then the limiting magnitide will be less, depending on the strength and variability of the line. The limiting magnitude is approximately one magnitude brighter on a typical OH line. Please remember to reduce the sensitivity by one magnitude for point sources.
 
 
Off OH line
wavelength (µm)  order magnitude  line flux (W/m**2)
1.25  3  19.4  1.1e-20 
1.65  2  18.6  1.2e-20 
2.2  2  17.6  1.1e-20 
3.8  1  11.9  6.1e-19 
4.9  1  9.9  1.5e-18 

On OH line


 
 
 
 
 
wavelength (µm) order magnitude lline flux (W/m**2)
1.25 3 18.3 3.0e-20
1.65 2 17.0 6.1e-20
2.2 2 16.5 3.2e-20
2.2 1 16.5 6.4e-20

CGS4 sensitivity per pixel with the echelle grating and the long camera

The 3-sigma 30-minute sensitivities per 0.6"x0.6" pixel for the case of nodding up and down the 1-pixel wide slit are as given in the following table. Sensitivities for very extended objects which require nodding to blank sky will be 0.4 mag poorer. The line flux assumes that the line is unresolved.  The values assume that the observations are read noise limited with sufficiently long exposure times for the multiple NDR to provide a read noise of about 25e-. The sensitivity will be less if an exposure time less than 100sec is used to avoid saturating a strong OH line close to a faint astronomical line. Please remember to reduce the sensitivity by one magnitude for point sources.
 
wavelength (µm)  magnitude  line flux (W/m**2)
1.25  15.0  9.0e-20 
1.6  14.6  8.0e-20 
2.2  14.0  6.0e-20 
3.8  10.6  3.0e-19 
4.9  8.9  7.0e-19 

 
 
 
 
 
 



 

Resolutions and wavelength coverage (with the long camera)

40 l/mm grating1:

 
order resolving power coverage (um)
2   (0.95 - 1.40 um) 800 x lambda 0.32
1   (1.30 - 5.50 um) 400 x lambda 0.64

150 l/mm grating1:

 
order resolving power coverage (um)
3   (1.07 - 1.30 um) 4700 x lambda 0.055
2   (1.25 - 2.50 um) 3100 x lambda 0.08
1   (2.10 - 5.50 um) 1550 x lambda 0.16

 

Echelle1,2:

  •  Resolving power is 37000 at the optimum order.
  •   Coverage is 0.006 x lambda.


1 These are the resolving powers for the 1-pixel wide slit (0.609"). Half the values when using the 2-pixel wide slit. They will be x4 lower when using the 4-pixel wide slit for extended object, but probably only x2 lower for guided point sources.

2  There is no 4-pixel wide slit available for use with the echelle.
 


 

CGS4 Pixel Sizes


The following table gives the pixel scales for CGS4 and the long focal length camera (300mm).
 
 
 

Grating Perpendicular to slit Along slit
40 l/mm 0.61" 0.61"
150 l/mm 0.595" 0.625"
Echelle 0.41" 0.90"

 

Notes:

  1. The 40 l/mm grating pixel sizes are independent of wavelength setting.

  2. The 150 l/mm grating pixel sizes are virtually independent of wavelength setting.

  3. The echelle pixel sizes were measured at a blaze angle of 64 degrees. The dimensions will change for angles greater or less than 64.



Last Modification Date: 1998/07/31 - Last Modification Author: Tom Kerr
Tom Kerr (tkerr)
 
 
Contact: Tom Kerr. Updated: Wed Oct 6 11:58:10 HST 2004

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