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GNIRS Guideline Exposure Times for Standard Stars

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Because spectra in the 1.0-2.5 micron region are often obtained in better conditions than those specified in Phase2, it is recommended that exposure times on standard stars be set conservatively. The exposure and coadd settings in the following table will provide high S/N in poor conditions without saturating the detector in good conditions. It is assumed that the observations consist of 1 ABBA sequence and are made at airmass 1.2. Minimum S/Ns are for 2.35 microns, near the long wavelength end of the K window; near the centers of the XJHK windows the S/N will be considerably higher (without saturating).

GNIRS Mode A0V star magnitude no. coadds x single exposure time ABBA min S/N
IQ70 CC50
ABBA min S/N
IQany CC80
32 l/mm, SXD, 0.30 arcsec slit 6.0 5 x 2.0 sec 900 300
32 l/mm, SXD, 0.45 arcsec slit 6.0 5 x 1.2 sec 900 300
32 l/mm, SXD, 0.675 arcsec slit 6.0 5 x 1.0 sec 900 300
111 l/mm, longslit 0.30 arcsec slit, J-band 6.0 2 x 8 sec 700 225
111 l/mm, longslit 0.30 arcsec slit, K-band 6.0 2 x 15 sec 1100 400
10 l/mm, LXD, 0.05" pix, 0.10 arcsec slit, no AO 5.0 4 x 5.0 sec 1200 200
111 l/mm 0.05" pix, 0.10" slit, no AO, J,H, or K 5.0 1 x 40.0 sec 700 150

The above exposure times may be scaled by a factor of 2.5 per magnitude difference. E.g., for a magnitude 7.0 star the exposure times in the first two rows should be 5.0 and 3.0 seconds, with the same number of coadds to achieve the same S/N as in the table. For the first three rows in the table increase exposure times by a factor of three if the 111 l/mm grating is substituted for the 32 l/mm grating and SXD is used.

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