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Grisms

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, Ks -- to obtain spectra in near-IR one band at a time.

Due to the ongoing issues with the image quality, the performance on the telescope has been affected and the spectral resolution is not within specs. We present the following links that show the measured spectral resolution for all slits (as measured at the center of the field of view) for grisms JH and HK. As a reference, the following plots show the measured spectral resolution maps for the R3K grism and the Ks filter and the HK grism . The vertical axis shows the wavelength and the horizontal axis shows the position along the slit when the 2-pixel wide slit is used. We urge PIs to take these plots into consideration when planning their observations.

As a reference, the predicted dispersions, when used with a 2-pixel wide slit (or slitlet), are presented below. Note that the values and plots of resolving power as a function of wavelength show the current performance of the instrument.

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 6.667 800
Plot
1.65 1 6.667 400
HK JH
1.10 2 4.000 TBD  
HK 1.65 1 7.826 500 Plot
2.20 1 7.826 500
R3K J-low
1.10 6 1.667 1600  
J 1.25 5 2.022 1500  
H 1.65 4 2.609 1600  
Ks 2.20 3 3.462 3000 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 (Tmean), the mean over the wavelength range where the transmission is greater than or equal to 80% of the maximum transmission (T80%), and the maximum transmission (Tmax) 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%