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Components

, Updated

Detector Array

FLAMINGOS-2 uses a Teledyne HAWAII-2 array with 2048x2048 pixels. Unlike more recent versions such as the HAWAII-2RG, the detector in FLAMINGOS-2 does not offer on-detector arithmetics such as image coadds before readout.
 

Array HAWAII-2 (HgCdTe) FPA 109
Pixel format 2048x2048 18 μm pixels
Pixel scale 0.18 arcsec (f/16), 0.09 arcsec (F/33 AO)
Dark current 0.5 e-/s/pix
Read noise (single CDS pair) 11.7 ± 0.2 e-
Read noise (nreads ≥ 8) < 5 e-
Gain 4.44 ± 0.35 e-/ADU
Well depth 155,400 e-
Linearity range (>99.5%) 4000-22000 ADU (K-band laboratory)
Saturation limit 35000 ADU
Quantum efficiency 66% (J-band), 50% (K-band)

The read noise of FLAMINGOS-2 is a function of both the detector and the MCE-4 electronics. The mean RMS read noise for a single CDS pair ("single read") is between 11.6 e- to 14.3 e- depending on the VRESET value. For multiple reads, the read noise drops more slowly than the theoretically expected 1/√(nreads) (see here). This is consistent with other HAWAII-II arrays using Fowler sampling.

An effective read noise of <5 e- RMS is achieved for nreads of 8 or higher. In imaging applications, the FLAMINGOS-2 detector is always sky-noise limited using single reads.

Filters

A set of broad- and medium-band filters are available for imaging and spectroscopy. Some filters are installed temporarily, changing on a roughly semesterly schedule controlled by scientific demand and the limited space in the filter wheels (see the optomechanical diagrams). Please contact us in advance if you plan a proposal requiring a temporary filter.

Spectroscopy mode always requires a filter, and all filters make sense for spectroscopy. Unlike GMOS, the grisms in FLAMINGOS-2 cannot be tilted. Fixed wavelength ranges are chosen using certain filter / grism combinations, not all of which are meaningful.  For example, the medium resolution R3000 grism projects the 3rd, 4th and 5th order on the detector simultaneously, covering the K, H and J wavelength ranges, respectively. The J, H or K filters can be used to select one of the wavelength ranges.

Most commonly, the JH and HK bandpass filters are used for long-slit spectroscopy with the respective low resolution R1200 JH and HK grisms. In multi-object spectroscopy mode (to be commissioned), a narrower filter allows two spectra to be packed in dispersion direction without overlap, effectively doubling the slit density in the mask. 

Only a subset of filters is suitable for imaging, such as the permanently installed J, H and Ks filters, and the K-band splitting filters K-blue and K-red. 

The following table lists all available filters. The throughput data include the entrance window, filter, nine lenses, and two flat mirrors, i.e. all transmissive elements. Excluded are the detector, grisms, telescope and atmosphere. The "cut-on" and "cut-off" wavelengths bracket the wavelength ranges that have at least 50% (80%) of the peak transmission.

Filter
name
 Currently installed Central
λ
[µm]

Band Width

[nm]

80% cut-on
Wavelength
[µm]
80% cut-off
Wavelength
[µm]
50% cut-on
Wavelength
[µm]
50% cut-off
Wavelength
[µm]
Throughput
curves
 Imaging and spectroscopy:
Y_G0811  NO 1.020 89.4 0.985 1.066 0.969 1.068 plot / data
J_G0802  ALWAYS 1.255 151 1.178 1.328 1.175 1.333 plot / data
H_G0803 ALWAYS 1.631 274 1.490 1.767 1.486 1.775 plot / data
Ks_G0804  ALWAYS 2.157 318 1.997 2.313 1.991 2.320 plot / data
K-blue_G0814  YES 2.060 227 1.941 2.168 1.934 2.177 plotdata
K-red_G0815  YES 2.310 248 2.189 2.437 2.181 2.446 plotdata
Spectroscopy:
JH_G0809 ALWAYS 1.390 720 1.163 1.774 0.970 1.805 plot / data
HK_G0806  ALWAYS 1.871 1067 1.308 2.401 1.261 2.511 plot / data
Jlow_G0801  NO 1.122 132 1.056 1.189 1.048 1.192 plot / data
K-long_G0812 YES 2.200 600 1.906 2.474 1.900 2.483 plotdata

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 a 2-pixel slit maximum resolving power R = λ/dλ ~1250, at the center of each spectral range. 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 a 2-pixel slit maximum resolving power R~3200, at the center of each spectral range, 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.

After the remediation work in the camera in April-May 2014, the spectral resolution performance has improved. We present the following links that show the measured spectral resolution for the 2-pixel wide slit (as measured at the center of the field of view) for grisms HK and the R3K grism and the Ks filter. 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. Note that the image quality across the slit is uniform for 80% of its length, dithering offsets as large as 3 arcminutes can be performed for extended objects.

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, i.e. the average spectral resolution for 75% of each spectral range 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 6.667 900 Plot
1.65 1 6.667 900
HK JH 1.10 2 4.000 TBD
HK 1.65 1 7.826 900 Plot
2.20 1 7.826 900
R3K J-low 1.10 6 1.667 2700
J 1.25 5 2.022 2800
H 1.65 4 2.609 2800
Ks 2.20 3 3.462 2900 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%