CISCO/OHS grisms

Available grisms and sensitivities

CISCO contains three grisms which cover the entire wavelength range over which the detector is sensitive. Each grism has its own order sorting filter. All spectra are properly sampled, even with the narrowest slits. The following sensitivities are 5 sigma detections per pixel in 1 hour of on-source background-limited spectroscopy, using a 0.5'' slit. See the important notes below regarding the effects of seeing and slit widths on the actual sensitivities. Examples of how to combine the figures in these tables can be found on the IRCS grism page.

range (um)
(0.5'' slit)
zJ 0.882-1.400 380 5.83 J=18.3
JH 1.056-1.816 400 8.23 H=17.6
K 1.850-2.512 600 8.63 K=17.3
JH 1.108-1.353
400 8.23 H=18.7

Note that the wavelength coverage of the JH grism is reduced by the OHS optics.

Corrections for finite seeing and slit width

Point sources

The sensitivity for point sources can be improved by extracting the spectrum along several spatial pixels. Apertures between 1 and 2 times the seeing FWHM are optimum. The following corrections should be added to the numbers in the above table to determine the point source sensitivity in the relevant seeing conditions.

Correction (mag)
Seeing 0.3'' slit 0.5'' slit 1.0'' slit
0.3'' +2.3 +2.3 +2.0
0.5'' +1.6 +1.8 +1.7
0.7'' +1.1 +1.3 +1.4
1.0'' +0.6 +0.8 +1.0

Extended sources

Increasing the slit width will improve the sensitivity of an extended source, if its surface brightness remains constant. Because a wider slit admits more sky flux, the improvement over the numbers in the first table above is 1.25 log (width/0.5'') magnitudes.

Slit sizes

Instrument Slit widths Slit length pixel scale
OHS 0.5'', 1.0'' 30'' 0.105''
CISCO 0.3'' - 1.0'' 2' 0.105''

Remember to double your exposure times if your source is too extended to nod along the slit.

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