FLAMINGOS-2 currently contains three grisms (located in the grism wheel; see the FLAMINGOS-2 optomechanical diagrams). When used with a 2-pixel slit (0.36"), the two moderate-resolution grisms, JH and HK, have resolving power R = λ/dλ ~1250. These cover "octave" wavelength ranges: the JH grism wavelength range is 0.9-1.8 microns, and the HK grism wavelength range is 1.2-2.4 microns (see the filters page for details on the matching JH and HK bandpass filters). With these grisms spectra in two near-IR bands are obtained simultaneously. The HK grism may be used with the HK filter to obtain H- and K-band spectra in first order, or it may be used with the JH filter to obtain H-band spectra in the first order and J-band spectra in second order; the JH grism covers both the J- and H-bands in first order. The peak transmission for both grisms is 80%.

The third grism has resolving power R~3000, and is called the "R3K" grism. The R3K grism can be used with any of the single-band broadband filters -- Y, J-low, J, H, K_s -- to obtain spectra in near-IR one band at a time.

Predicted dispersions and resolving powers for the three grisms, when used with a 2-pixel wide slit (or slitlet), are presented below. The plots of resolving power as a function of wavelength use the laboratory-measured wavelength solutions for the grisms.

Grism	Filter	Wavelength [µm]	Order	2-pix Slit Dispersion [Angstrom/pixel]	R = λ/dλ (2-pix slit)	Resolving Power as Function of λ
JH	JH	1.25	1	6.667	938	Plot
		1.65	1	6.667	1238
HK	JH	1.10	2	4.000	1375
	HK	1.65	1	7.826	1054	Plot
		2.20	1	7.826	1406
R3K	J-low	1.10	6	1.667	3300	Plot
	J	1.25	5	2.022	3090	Plot
	H	1.65	4	2.609	3162	Plot
	Ks	2.20	3	3.462	3178	Plot

Plots of the net system throughput with the grisms are linked from the table below. These plots use the grism efficiency data produced by the grism manufacturer and scales them by the throughput curves for the FLAMINGOS-2 optical train. (Laboratory measurements made of the net system transmission made in Florida are consistent with these throughput curves.) These transmission curves do not include the effects of the atmosphere, the telescope, or the detector. Values for the mean transmission over the full data range (T_mean), the mean over the wavelength range where the transmission is greater than or equal to 80% of the maximum transmission (T_80%), and the maximum transmission (T_max) are given.

Grism	Filter	Tmean	T80%	Tmax	Plot
JH	JH	43%	54%	59%	Plot
HK	HK	47%	55%	63%	Plot
R3K (order 6)	J-low	8%	18%	20%	Plot
R3K (order 5)	J	TBD	TBD	TBD
R3K (order 4)	H	20%	36%	38%
R3K (order 3)	Ks	13%	34%	35%

 

Throughput curves for the grisms only, based on lab measurements, are also available.