NIRI Grisms, Slits, and Order-sorting Filters

NIRI contains eight KRS5 direct-ruled grisms (prisms with transmission gratings on one surface) and slits of various widths and offsets from the center of the field of view. Their combinations permit low to moderate resolution spectroscopy through most of the 1-5um region. Table 1 below gives predicted spectral resolving powers and useful wavelength coverages for the f/6 configuration only.

Five of the grisms are for use with the f/6 camera optics and cover the 1.5.5um windows. The other three are for use at f/32 with the adaptive optics module Altair, and cover the J, H, and K windows.

The accessible wavelength ranges given in the first table are limited either by the array size, the order-sorting filters (whose properties are given in the second table below) or the atmospheric window. In general, full atmospheric windows are spectrally imaged onto the detector at f/6. Where this is not quite the case with f/6 center slits (slits centered in the focal plane), blue slits for the f/6 optics that are offset in the focal plane are available to provide the missing wavelength region. This is only an issue for the J and L bands (and a very minor issue for the H band).

The available slit widths at f/6 are approximately 0.23, 0.46 and 0.69 arcsec (2, 4 and 6 pixels) wide, with small variations (see the table). Slit widths for use at f/32 are ~0.09, 0.14, and 0.23 arcsec. The f/6 slit throughputs as a function of image quality are listed elsewhere. The listed resolutions and resolving powers assume that for all available slit widths the target overfills the slit.

The grism data are based on laboratory and telescope tests. Note that the resolving powers of many of the grisms do not match those expected for ideal grisms. For example, at f/6, except for the H band the resolutions using the 2 pixel-wide slits are not 2 pixels and, apart from H, these slits do not give twice the resolving powers of the 4 pixel wide slits. The tables will be updated as more accurate information becomes available.

Order-sorting filter transmission curves and data were measured at normal incidence and warm (~300 K) unless noted otherwise. The H-band order sorting filter has a red leak and is crossed with the PK50 long-wave blocker.

Grism Name	Useful Wavelength Range (microns)	Slit Dimensions (arcsec)	Estimated Resolution (pixels)	Estimated Resolving Power (l/dl)	Argon lamp or sky spectrum	Order-sorting filter transmission curve available	Order-sorting filter transmission data available
f/6 J	1.05 - 1.41	0.23 x 50	4.5	770
		0.47 x 110	5.7	610
		0.75 x 110	7.6	460
	1.00 - 1.36 (w. 'blue' slits)	0.23 x 50	4.5	770
		0.41 x 50	5.4	650
		0.70 x 110	7.2	480
f/6 H	1.43 - 1.96	0.23 x 50	2.0	1650
		0.47 x 110	4.0	825
		0.75 x 110	6.4	520
	1.36 - 1.89 (w. 'blue' slits)	0.23 x 50	2.0	1650	-
		0.41 x 50	3.5	940
		0.70 x 110	6.0	550
f/6 K	1.90 - 2.49 w. either slits	0.23 x 50	2.7	1300
		0.47 x 110	4.5	780
		0.75 x 110	6.7	520
f/6 L	2.99 - 4.15	0.23 x 50	2.8	1100
		0.47 x 110	4.5	690
		0.75 x 110	6.7	460
	2.85 - 4.01 (w. 'blue' slits)	0.23 x 50	2.8	1100
		0.41 x 50	4.0	770
		0.70 x 110	6.3	490
f/6 M	4.45 - 5.45	0.23 x 50	2.8	1100			-
		0.47 x 110	4.0	770
		0.75 x 110	6.7	460
f/32 J	1.05-1.35	0.09 x 22	4	1000	-	-	-
		0.14 x 22	6.6	620
		0.23 x 50	9.0	450
f/32 H	1.49-1.79	0.09 x 22	6.2	880	-	-	-
		0.14 x 22	8.6	630
		0.23 x 22	10.5	500
f/32 K	1.96-2.41	0.09 x 22	4	1280	-	-	-
		0.14 x 22	6.6	775
		0.23 x 22	9	570

The following table gives wavelength ranges of the order-sorting filters in NIRI that are used with the grisms and also lists the physical properties of each f/6 grism. The unique Gemini identification number for each filter is recorded in image headers.

Waveband	Central Wavelength (microns)	Order-sorting Filter bandpass (microns)	Gemini ID	Grism Properties
				lines/mm	Prism angle	Throughput
J	1.30	0.97-1.63	G0209	312.6	15	39%
H	1.69	1.30-2.09	G0210	223.9	15	43%
K	2.20	1.90-2.49	G0211	166.7	15	46%
L	3.50	2.90-4.25	G0212	104.1	15	-
M	5.00	4.40-6.00	G0213	72.8	15	-

Last update 2007 July 6; Andrew Stephens, Tom Geballe & Joe Jensen