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Detector Characteristics


The GSAOI detector is formed by four Rockwell HAWAII-2RG (H2RG) arrays with 2048 x 2048 18 μm pixels each. The arrays are mounted in a 2 x 2 mosaic that create a 4080 x 4080 pixel focal plane. There is a gap of ~2.5 mm between the arrays, corresponding to ~2.4" on sky. The arrays have an additional four rows and columns around the outer edges that read out as reference pixels and are not illuminated. The HAWAII-2RG arrays are sensitive to light out of 2.6 μm and use a HgCdTe array layer. Each array is readout through four amplifiers that simultaneously read out 512 x 2048 pixels in ~ 5.3 seconds. GSAOI arrays are normally read out using the Fowler sampling technique (see below for details) to reduce read noise. The table below summarizes the basic GSAOI array properties


Type Rockwell HAWAII-2RG HgCdTe 
Array sizes 2048 x 2048 pixels each (2040 x 2040 active)
 Detector area 4080 x 4080 pixels (~ 85" x 85")
Physical Pixel size 18 μm
Pixel scale 0.02"
Spectral Response 0.9  μm to 2.6 μm (data / plot)
Gains ~ 2.4 e-/ADU
 Dark current  ~ 0.01 e-/s/pix (~12 e- in the maximum integration time of 20 minutes)
 Saturation ~ 50,000 ADU
 Well Depth ~ 120,000 e-

Read-out  modes

GSAOI arrays are normally read out using a technique known as ''Fowler sampling'' (Fowler & Gatley 1990, ApJL, 353, L33) to reduce read noise. In this mode, a set of multiple non-destructive reads are made at the beginning of an exposure and at the end of the exposure. The two sets of non-destructive reads are averaged and the signal is formed from the difference between these two averages. Each array is continuously read out and reset in “idle” mode between science exposures. This prevents the arrays saturating, and maintains the readout electronics at a stable operating temperature. When a science exposure is initiated, the current idle mode read is interrupted and the science exposure is started.

The table below summarizes the three read-out modes implemented in GSAOI for Bright, Faint and Very Faint objects. Note that the readout noise can be reduced by optimizing the readout method. For example, tests performed in the laboratory and confirmed during the commissioning, show that the read noise can be reduced to ~10 e- if  we use 8-8 Fowler sampling (16 NDRs). However, the penalty in the read noise reduction is the increase in the read out time by a factor of two, from 26.2sec to 47.7sec.

 Mode Fowler Samples (FS) Non-Destructive Reads (NDR) Read Noise Read Out time Min. Exp. time
Bright Object 1 2 28 e- 10.0 sec 5.3 sec
 Faint Object 4 8 13 e- 26.2 sec 21.1 sec
 Very Faint Object 8 16 10 e- 47.7 sec 42.2 sec

The table below shows the gain and read noise values for each GSAOI array.

Bright Object Faint Object Very Faint Object
Array Gain (e-/ADU) Read Noise (e-) Read Noise (e-) Read Noise (e-)
1 2.434 26.53 13.63 10.22
2 2.010 19.10 9.85 7.44
3 2.411 27.24 14.22 10.61
4 2.644 32.26 16.78 12.79

Dark current and Bad Pixels

The dark current in the GSAOI arrays is very low (~0.01 e-/s/pix) and stable. This produce a dark charge of ~ 12 e- in the maximum integration time of 20 minutes (set by cosmic ray hits, see below). However, there are a large number of "hot" pixels. The "hot" pixels appear as a single high dark current pixels with adjacent moderately high dark current pixels ("cross" shape). An example is given below, where the "cross-shaped" hot pixels are clearly seen in the 300 sec dark current image. The "cross-shaped" hot pixels appears to be due to high leakage current in the central pixel, which affect its neighbors through capacitive coupling between adjacent pixels.

Dark current image showing

Dark current images show spatial structure within each array. The level of the structure can get up to 10 ADU above the mean of the full array, but it is of the order or smaller than the standard deviation of the pixel values around the mean. This means that in any dark subtracted image, the dark structures will be within the noise. The existing spatial structures show temporal variations in time scale of seconds to hours, however the variations are smaller than the standard deviation around the mean values. The dark current also shows temporal variation in amplitude within each array, in the same time scale, of the order of 2 to 6 ADUs. This amplitude variation is smaller than the standard deviation from the mean. All these effects are very small and dark subtraction in the form of dark frames are not required for the different types of science and calibration frames. The normal near-IR observing techniques will be enough to take care of any dark current counts/structure.

The GSAOI arrays have imperfect cosmetic quality. This is due to the existence of 'dead' pixels (debonded), light emitting-diode pixels, "killed" spots, and excessive dark current areas. Bad pixel masks are necessary to construct to correct by the imperfect cosmetic and hot pixels. The procedure to derive a bad pixel masks can be found here.

Cosmic Rays

The GSAOI arrays are sensitive to the cosmic rays. Cosmic rays interact with the CdZnTe substrate above the HgCdTe array layer and appear to forward scatter photons or electrons that strike the detector in a diffuse pattern extending over a diameter of ~ 20 pixels. Generally, these events add only a few hundred electrons to the pixel signal. However, their diffuse nature makes cosmic ray detection difficult in real time.

The cosmic rays removal appears to be more effective if multiple science exposure, at the same location on the sky, are recorder and then median of these frames are used to remove these event. However, this requires a reduction of the integration time to a point where cosmic ray hits contaminate only a small fraction of the detector area.The test results show that this integration time limit is around 15 minutes. This is supported by the results obtained during the GSAOI commissioning.

Linearity Correction

The GSAOI arrays are intrinsically non-linear due to their source-follower-per-detector architecture. The linearity correction per detector has been derived during the commissioning, and is implemented in the standard GSAOI IRAF reduction package. Once corrected by non-linearity, the data are found to have less than 2% departure from linearity at 96% of the saturation limit (see Saturation and Persistence). For count levels below 80% of saturation, the linearity correction is better than 1%. 

The implementation takes the form of a quadratic polynomial fit:

Linearity Correction Factor = a+b*ADU+c*ADU*ADU

The data used to derive the parameters a,b,c above was a set of well-illuminated H and Z-band flats,  and the resulting values are shown in the table below. Sigma(res) corresponds to the residual dispersion of all points in the fit.

  Array  a b c  sigma(res) 
1   0.9947   7.4E-7    2.5E-11  0.003
2 0.994 6E-7  3.7E-11 0.005
3 0.9947 7.1E-7  2.2E-11 0.003
4 0.9959 5.7E-7  2.7E-11


Saturation and Persistence

Saturation of the GSAOI arrays occurs at about 50,000 ADU, which also results in a strong persistence. The persistence causes saturated images to appear as faint artefacts in subsequent exposures. Read out methods using more non-destructive readouts result in lower noise that reveals residual images for several hours so care should be taken to avoid saturation. Note that while not exposing, the arrays are continually reset pixel-by-pixel at 5s per frame. This will normally prevent the arrays saturating on the exposed field between integrations.

The table below shows the GSAOI array saturation values and count levels (in percents of the saturation level) where the data diverges >5% from linear relation before the linearity correction is applied. The table also shows the level where the linearity corrected value diverges >2% from the linear relation.

Because the arrays become >5% non-linear at ~50,000, it is recommended to keep the counts below ∼40,000 ADU (~96,000 e-) per co-add. Saturation can be checked using the GSAOI ITC.

  Array  Saturation (ADU) 5% (without lin. correction) 2% (with lin. correction)
1   52,400   73%   97% 
2   50,250   64%   97% 
3   53,700   76%   98% 
4   52,300   75%   96% 


GSAOI contains two filter wheels and one utility wheel. The two filter wheels have room for 27 broad- and narrow-band (zero redshifted) emission- and absorption line- filters. Each filter wheels also contains one blocked position for recording bias and dark frames. The list of the installed filters, central wavelengths, 50% cut-on and cut-off wavelengths, the Gemini Filter Number, transmission curves (plot and ASCII files), and the filter ID (internal) are listed in the table below.

The utility wheel contains a pupil viewer and two defocus lenses (convex and concave). The pupil viewer is used to accurately align the cold stop with GeMS exit pupil viewer and so minimize the background reaching the imager detector. The convex and concave defocus lenses produce defocused images that are used to derive static wave front phase errors at the imager detector. These phase errors are nulled using the GeMS deformable mirrors.

Transmission curves for the filters currently installed in GSAOI are shown in the figure below. The atmospheric transmission between 0.9 and 2.5 μm is superimposed as a dark green line.

Transmission curves

The table lists the properties of the filters based on lab measurements. The measurements were taken at room temperature and then calculated to 70 K using correlation measurements from a cold test witness. The measured witnesses were obtained at 298 K and 77 K and the slopes were used to determine the 70 K shift. To access the graph or the ASCII data, click on the links in the table below (plot or data). Note that the filter name is the concatenation of filter and the Gemini filter Number e.g. Z_G1101 for Z broad-band filter and CH4short_G1109 for CH4(short) narrow-band filter.

Filter Name  Central Wavelength (μm)  Coverage (μm)  50% cut-on Wavelength (μm)  50% cut-off Wavelength (μm)  Gemini Filter Number  Transmission Curves  Filter ID
 Broad-band filters
Z 1.015  0.170  0.930  1.100 G1101 plot / data  ED205-1
 J 1.250  0.160  1.170  1.330 G1102 plot / data  ED191-1
 H  1.635  0.290  1.490  1.780 G1103 plot / data  ED169-1
 K(prime)  2.120  0.340  1.950  2.290 G1104 plot / data  ED278-2
 K(short)  2.150  0.320  1.990  2.310 G1105 plot / data ED196-1
 K  2.200  0.340  2.030  2.370 G1106 plot / data  ED192-1
 Narrow-band (zero redshifted) emission- and absorption-line filters
 J-continuum 1.207  0.018 1.198 1.216 G1107 plot / data ED190
H-continuum  1.570  0.024 1.558 1.582 G1108 plot / data  ED141
 CH4(short)  1.580  0.100 1.530 1.630 G1109 plot / data  ED144
 CH4(long)  1.690  0.100 1.640 1.740 G1110 plot / data  ED151
 K(short) continuum  2.093  0.031 2.078 2.108 G1111 plot / data  ED168
 K(long) continuum  2.270  0.034 2.253 2.287 G1112 plot / data  ED163

He I 1.083 μm

 1.083 0.016 1.075 1.091 G1115 plot / data ED185
 H I Pγ  1.094 0.011 1.089 1.100 G1116 plot / data ED186
 H I Pβ  1.282 0.019 1.272 1.292 G1117 plot / data ED231
 [Fe II] 1.644 μm  1.644 0.025 1.631 1.656 G1118 plot / data ED150
 H2O  2.000 0.080 1.960 2.040 G1119 plot / data ED188
 He I (2p2s)  2.058 0.031 2.042 2.073 G1120 plot / data ED165
 H2 1-0 S(1)  2.122 0.032 2.106 2.138 G1121 plot / data ED162
 H I Brγ  2.166 0.032 2.150 2.182 G1122 plot / data ED166
 H2 2-1 S(1)  2.248 0.034 2.231 2.265 G1123 plot / data ED164
 CO Δv=2  2.360 0.080 2.320 2.400 G1124 plot / data ED187