EMIR

 

EMIR (Especrografo Multiobjecto Infra-Rojo) is the first second-generation GTC instrument to enter regular operations. It is a near-infrared (0.9 - 2.5 µm) wide-field imager and medium-resolution multi-object spectrograph installed at the Naysmith-A focal station. The center piece of the instrument is the CSU (Configurable Slit Unit) allowing to configure and observe in real time up to 55 slits over the 6.64' x 4' spectroscopic field of view. Long slits with different dimensions could be configured as well. The disperser elements are formed by combining high-quality diffraction gratings, manufactured by photo-resistive procedures with large conventional prisms. In imaging mode the 6.64' x 6.64' field of view could be observed through 11 narrow and broad-band filters (including the standard 2MASS JHKs). The detector is a 2048 x 2048 Teledyne HAWAII-2 HgCdTe near-infrared optimized chip with a pixel scale in imaging mode 0.2"/pixel.

The EMIR project led by the IAC with the participation of the Laboratoire d'Astrophysique - Observatoire Midi-Pyrenees (France), Universidad Complutense de Madrid and the Laboratoire d'Astrophysique - Observatoire de Marselle (France).

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icon Instrument Features

The most important features and characeristics of EMIR are summarized below, followed by a representation of the instruments optics and a picture showing it attached to Naysmith A focal station of the GTC.

 

Focal Location Nasmyth A
Spectral Range (λ) 0.9 - 2.5 µm
Optimization All Spectral Range
Spectral Resolution 4000 - 5000 for bands JHK (one window at a time)
987 for YJ and HK (selectable range)
Spectral Coverage YJHK observational window in each exposure
Array Format Teledyne HAWAII-2 HgCdTe 2048x2048 pixels
Plate Scale 0.1945"/pixel
Limiting magnitude Y=24.9, J=24.7, H=23.9, K=23.4 for S/N=5;
Texp. = 1h.
OH suppression In software
Cryostat
Spectrograph temperature 77 K

 

emir_optics

Light path inside EMIR.

 

emir

EMIR attached to the Nasmyth A focal station of the GTC telescope.

 

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icon Instrument Detector

EMIR is equipped with a HAWAII-2 IR detector manufactured by Teledyne. It is a HgCdTe array of 2048 x 2048 pixels (18 μm square each), operating between 0.9 and 2.5 μm and optimized for the K band athmospheric transmission window (~ 2.1 μm) at cryogenic temperatures. The detector is divided into 4 quadrants (1024 x 1024 pixels each). Individual quadrants are read out through 8 channels, permitting a full frame rate of slightly over one frame per second. The read out of the 32 channels is performed simultaneously. The following table summarize some detector parameters of interest.

 

Detector Characteristics Value
Pixel Size (λ) 18 μm/pixel
Filling Factor (λ) 90
Dark Current < 0.15 e-/sec.
Read Noise 5.23 ADU - single read
3.5 ADU - 10 reads ramp
Gain 4.2 e-/ADU
Well Depth (< 1 % linearity) 42571 ± 727 ADU
Quantum Efficiency (77K) 85%@2.20μm
80%@1.60μm
65%@1.25μm
Cosmetics 0.05% bad pixels (~ 2.1 kpix.)
1.11% hot pixels (~ 46.6 kpix.)

 

Below you could see an example of the detector Bad Pixel Mask (BPM) as of 14.11.2016. The current BPM can be downloaded from here. Note that this is the BPM utilized by the instrument Data Reduction Pipeline (DRP).

bpm

Example of the BPM for the instrument. Dark areas (zeros) represent the good pixels, while the light regions (positive values) indicate the known bad pixels. Keep in mind that the features will be evolving with time as detector properties are changing.

 

Due to the calibration specifics of the detector, keep into account that only a pre-defined set of quantified exposure times is available. The exposure times are summarized in the table below, along with the corresponding readout modes.

 

Readout Mode Exposure times [sec.]
Correlated Double Sampling (CDS) 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15
Ramps (5 points/samples) 20, 30
Ramps (10 points/samples) 60, 120, 160, 200, 240, 280, 320, 360

 

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icon Observing Modes

 

FOV

Representation of the EMIR Field of View (FOV). The image in the background is a J band image of NGC6946 spiral galaxy, acquired during the instrument tests at GTC. The imaging mode FOV covers the entire extent of the detector (2048 x 2048 pixels / 6.67 x 6.67 arcmin). The Multi-Object Spectroscopy (MOS) FOV is the central (1234 x 2048 / 4.0 x 6.67 arcmin) or the grayscale represented part of the image with X coordinates roughly in the range from 410 to 1640. The three distinct pre-defined Long-Slit (LS) positions currently in use are outlined in red, green and cyan for the right, central and left slits respectively. The two LS positions at the borders of the Cold Slit Unit (CSU) range are represented in grey. Note that the color coding of the slits matches the one used in the wavelenth calibration section of this page. For more details - see the sections dedicated to each observing mode.

 

icon Broad Band Imaging

EMIR allows broad-band imaging over a 6.67' x 6.67' FOV, covering the atmospheric transmission windows in the 0.9 - 2.5 μm spectral range, utilizing the Y (1.03 μm), standard J (1.25 μm), H (1.63 μm), Ks (2.16 μm) 2MASS filters and Johnson K (2.23 μm) filter. Note that due to the tilt of the detector with respect of the focal plane, the Image Quality (IQ) varies across the imaging FOV. The best results, uniform PSF and IQ are achieved in the central 4' x 4' portion of the field. The plate scale in imaging mode is 0.1945 arcseconds/pixel.

 

filter_imaging

Transmission curves for the broad and narrow-band imaging filters currently installed in EMIR. The atmospheric transmission between 0.9 and 2.5 μm is superimposed as a grey line. To avoid clutter, the H2(1-0) filter is not shown on this figure since its transmission curve almost fully coincides with the transmission of the BrγC filter.

 

The broad-band photometric zero-points for EMIR are presented in the following table, as well as estimates of the limiting magnitudes, sky brightness and the times to saturate the sky in the most utilized near-IR broad- band filters. Note that the Zero Points (ZP) are given in the Vega system and the limiting magnitudes are for 5σ detection in 1 hour exposure time.

 

Filter λc

[μm]
Zeropoint

[ADU/sec.]
Zeropoint

[e-/sec.]
Limiting
Magnitude
[mag.]
Sky
Background
[mag./asec.2]
Time to
Saturate Sky
[sec.]
Y 1.03 25.02 ± 0.08 26.52 ± 0.08 24.9 18 230
J 1.25 25.27 ± 0.08 26.77 ± 0.08 24.7 16.6 140
H 1.63 25.60 ± 0.10 27.10 ± 0.10 23.9 14.4 24
KS 2.16 25.09 ± 0.13 26.59 ± 0.13 23.4 12.5 12

Technical details for the EMIR broad-band filters are summarized below:

Filter λCut-ON
[μm]
λCentral
[μm]
λCut-OFF
[μm]
FWHM
[μm]
Transmission
[%]
Y 0.965 1.032 1.098 0.133 Image / Text
J 1.174 1.253 1.333 0.160 Image / Text
H 1.489 1.629 1.770 0.281 Image / Text
KS 2.004 2.160 2.316 0.312 Image / Text
K 2.073 2.234 2.395 0.322 Image / Text

 

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icon Medium Band Imaging

Medium-band filters for EMIR (resolution 30-40) are available (see details in the table below). These were purchased by Dr. Pablo Pérez González from the Universidad Complutense de Madrid and Dr. José Miguel Rodríguez Espinosa from Instituto de Astrofísica de Canarias. These filters have been offered for general use to the whole GTC community. The FOV and pixel scale for the narrow-band imaging mode are the same as for the broad-band mode. The best results, uniform PSF and IQ as well are achieved in the central 4' x 4' portion of the field.

The filter currently installed in the instrument is highlighted in blue. Keep in mind that replacing a filter in EMIR requires a thermal cycling of the instrument.

Filter λCentral
[μm]
FWHM
[μm]
F0960HBP40 0.950 0.039
F1000HBP40 1.000 0.035
F1042HBP42 1.048 0.039
F1084HBP45 1.084 0.047
F1180HBP50 1.180 0.050
F1230HBP50 1.230 0.048

 

We kindly invite people interested in these filters to contact the owners expressing their commitment to include an acknowledgment in any publication arising from their use. This is the suggested acknowledgment text: "This work is (partly) based on observations carried out with EMIR and medium-band filters purchased with funds from Spanish Government grants AYA2015-63650-P and AYA2015-70498-C2-1-R".

 

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icon Narrow Band Imaging

The FOV and pixel scale for the narrow-band imaging mode are the same as for the broad-band mode. The best results, uniform PSF and IQ as well are achieved in the central 4' x 4' portion of the field. The properties of the narrow-band filters installed in EMIR are summarized in the table below:

    Filter λCut-ON
[μm]
λCentral
[μm]
λCut-OFF
[μm]
FWHM
[μm]
Transmission
[%]
    [FeII] 1.632 1.647 1.662 0.030 Image / Text
    [FeII]Cont. 1.701 1.714 1.728 0.027 Image / Text
    Brγ 2.158 2.176 2.193 0.035 Image / Text
    BrγCont. 2.112 2.127 2.193 0.030 Image / Text
    H2(1-0) 2.110 2.125 2.141 0.031 Image / Text
    H2(2-1) 2.235 2.249 2.264 0.029 Image / Text

 

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icon Long Slit Spectroscopy

In long slit observing mode the Configurable Slit Unit (CSU) is used to form slits with pre-defined dimensions at three distinct positions in the spectroscopic FOV: one at a central position and two at one arcmin to the left/right from the center (that can be used in order to better exploit the wavelength coverage along the FOV). Currently, slit widths of 0.6", 0.8", 1.2", 1.6" and 5.0" are available. Technically slits with arbitrary widths and lenghts could be defined within the entire extend of the 6.67' x 4' field. However these should be considered and treated like MOS configuration files. As well as in imaging mode, the tilt of the detector with respect of the focal plane also applies to the spectroscopy mode, producing that the FWHM of the spectral traces deteriorate towards the left edge of the detector (affecting to bluer wavelengths).

 

filter_spectroscopy

Transmission curves for the spectroscopic filters currently installed in EMIR. The atmospheric transmission between 0.9 and 2.5 μm is superimposed as a grey line.

 

The properties of the spectroscopic filters installed in EMIR are summarized in the table below.

Filter λCut-ON
[μm]
λCentral
[μm]
λCut-OFF
[μm]
FWHM
[μm]
Transmission
[%]
YJ 0.899 1.115 1.331 0.432 Image / Text
HK 1.454 1.929 2.405 0.952 Image / Text
Kspec 2.001 2.214 2.428 0.427 Image / Text

Next table summarizes the main characteristics of EMIR dispersive elements. There are three pseudo-grisms which offer high resolution (J, H and K bands) and a low resolution normal grism (LR). Last column gives the measured value of the central wavelength with the slit in the central position.

ID λc
[μm]
Range
[μm]
D
[A/pix]
Resolution
(0.6" slit)
λc
[μm]
J 1.25 1.17-1.33 0.76 5000 1.25
H 1.65 1.52-1.77 1.22 4500 1.65
K 2.20 2.03-2.37 1.71 4000 2.21
YJ 1.00 0.85-1.35 3.43 987 1.10
HK 2.00 1.45-2.42 6.86 987 2.01

The table below gives information about the central wavelength and the wavelength range covered (in micrometers [μm]) for the different pre-defined LS positions. Note that the color coding in the table concides with the one of the figure illustrating the EMIR FOV (above) and the one utilized in the wavelength calibration section below.

ID Slit
X position
[px.]
λc

[μm]
Range

[μm]
J 430
730
1030
1330
1630
1.30
1.27
1.25
1.23
1.20
1.22 - 1.38
1.20 - 1.35
1.17 - 1.33
1.15 - 1.30
1.13 - 1.28
H 430
730
1030
1330
1630
1.73
1.69
1.65
1.61
1.58
1.60 - 1.85
1.57 - 1.81
1.53 - 1.78
1.49 - 1.74
1.46 - 1.70
K 430
730
1030
1330
1630
2.30
2.26
2.21
2.15
2.10
2.14 - 2.47
2.08 - 2.44
2.03 - 2.38
1.98 - 2.33
1.93 - 2.28

 

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icon Zero Order Ghosts in Long Slit Spectroscopy Mode

The zero order ghost position is measured for a bright telluric standard star, observed at the nominal position for LS spectroscopy with coordinates [1030,1215]. Ghost A is situated at a distance 60 - 80 arc-seconds from Spectrum A and Ghost B is at 30 - 50 arc-seconds from Spectrum B.

!!! Note that if the position (Y coordinate) of the spectrum does change, the ghost appears at other position. Do not place the spectrum at the center of the detector to avoid overlap with the ghost !!!

In the table below is summarized the information about the zero order and the corresponding ghost for each filter/grism combination and the relative intensity with respect of the spectrum for both A and B positions and 15 arc-seconds offsets. Take into account that this information is valid only for the nominal position for LS spectroscopy.

 

Filter/Grism
combination
Y position
spectrum
Zero Order
position
Zero Order
ghost
Intensity
[%]
J/J A [X,1222.0]
B [X,1142.0]
[1039,1206]
[1039,1126]
[1108,880]
[1108,959]
2
H/H A [X,1227.0]
B [X,1147.0]
[1017,1211]
[1117,1131]
[1019,804]
[1019,883]
13
Kspec/K A [X,1224.0]
B [X,1145.0]
[1029,1218]
[1029,1141]
[1033,802]
[1033,880]
9
YJ/LR
HK/LR
A [X,1132.0]
B [X,1053.0]
[1030,1216]
[1030,1137]

4

 

Relative offsets of the spectrum and the zero order ghost for the standard position for LS spectroscopy. N gives the numerical relation between the positions of the spectrum and the ghost.

 

Filter/Grism
combination
Offset
Spect./Ghost
[pix.]
N

J/J [78,340] 1.6
H/H [11,423] 2.1
Kspec/K [3,422] 2.1

 

The equation that predicts where will appear the zero order ghost image on the detector for the J, H and K grisms. (Ghost images are not detected in YJ and HK.) The N value for each grism are tabulated above.

 

PosY0 = N x (PosYSPEC - 1024) + PosYSPEC

 

PosYSPEC is the position of the spectrum, PosY0 is the position of the ghost. Respectively the N parameter is calculated according to the following equation:

 

N = (PosY0 - PosYSPEC) / (PosYSPEC - 1024)

 

ghosts1

The relative spectrum-ghost positions for both nods (A-B) for the J, H and K grisms (respectively from left to righ and top to bottom) for an offset of 15 arc-second. It is notable that the distance spectrum-ghost is not the same for A and B positions.

 

ghosts2

On this image it is clearly visible that if the spectrum is acquired closer to the center of the detector, the ghost position approaches to the spectrum.

 


 

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icon Multi-Object Spectroscopy (MOS)

 

Multi-object Spectrocopy Mode
Field of View (FOV) 6' x 4'
Multi-slits Multi-slit Masks Exchanger (up to 55 individual slits)

 

EMIR allows Multi-Object Spectroscopy (MOS) over a 6.67' x 4' FOV, covering the 0.9 - 2.5 μm spectral range, utilizing three spectroscopic filters, which properties are summarized in the table presented in the long-slit spectroscopy section. The properties of the grisms mounted in the instrument are also summarized there. Up to 55 individual slits could be configured over the FOV, utilizing the instrument´s Configurable Slit Unit (CSU). The following images illustrate the capabilities of the CSU.

 

x_csu

X-shaped configuration of all the slits, outlining the extent of the MOS FOV of the Configurable Slit Unit.

 

gtc_csu

GTC logo formed by configuring the slit unit.

 

The configuration of the CSU for a particular field is calculated and prepared by a dedicated software tool.

 

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icon Wavelength Calibration

 

EMIR is used in conjunction with the Naysmith A ICM (Instrument Calibration Module), which contains three dedicated Argon (Ar), Neon (Ne) and Xenon (Xe) arc lamps. The properties of the calibration lamps are summarized below. Also, a complete arc linelist for all the lamps available at the EMIR Instrument Calibration Module (ICM) can be retrieved here.

The images cover the entire spectral range that could be observed with EMIR with different slit positions. The data below was acquired with the 0.6 arcseconds long-slit CSU configuration and is representable for the middle line of the detector. Also, for the low resolution grisms some second order lines are visible in the spectra. Users must be aware of this, in order to avoid these in the line identification. In the last column (outlined in red) are presented the line calibration identification images for the three distinct pre-defined Long-Slit (LS) positions currently in use. The corresponding spectral ranges are presented in red, green and cyan for the right, central and left slits respectively. For more detaisl see the Long Slit Spectroscopy section above and the figure illustrating the EMIR FOV.

 

Band/Grism Calibration Lamp
YJ HgAr --- Xe HgAr+Xe
HgAr+Xe (2nd order)
HK HgAr --- Xe HgAr+Xe
HgAr+Xe (2nd order)
J HgAr Ne Xe HgAr+Ne+Xe HgAr+Ne+Xe
H HgAr Ne Xe HgAr+Ne+Xe HgAr+Ne+Xe
K HgAr Ne Xe HgAr+Ne+Xe HgAr+Ne+Xe

 

Another possibility widely used in the Near-IR spectroscopy is to utilize the OH atmospheric airglow lines. The data below was acquired with the 0.8 arcseconds long-slit CSU configuration and is representable for the middle line of the detector.

 

Band/Grism Lines
YJ Image
HK Image
J Image
H Image
K Image

 

The text file with the OH airglow lines identifications is available here.

 

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icon Observing Strategy and Phase 2

 

All the relevant information to prepare observations with EMIR can be found in the EMIR User Manual. We strongly recommend to potentially EMIR users to read that document for a complete description of the different observing strategies and currently available observing modes.

EMIR observing overheads are really severe, mainly in imaging mode. For typical science exposures, open-shutter efficiency for EMIR is about 50% (including dithering, readout overheads, etc.). For this reason, in order to optimize the telescope time for a predefined on-source integration time, users should make use of the EMIR efficiency calculator available here.

The PIs should select the appropriate telluric standards and prepare the corresponding OBs. Note that the telluric standards will be charged to your program. The time needed for those should be taken into account when preparing the proposal. Please indicate in the Phase II readme file which telluric standard OB corresponds to which science observation. From a scheduling point of view, the standard practice will be that the telluric standard will be observed after the science target and will be acquired with the ASG without opening the CSU to alleviate operational overheads. Keep this into account when selecting your standard stars. In case you need a different observing strategy, please justify it in your proposal. Observing the telluric after the science target is done in order to extend the CSU life by decreasing the number of re-configurations.

Here you could find the IAC EMIR web page with some suggested telluric standards.

The Gemini Telluric Standards Selection Tool is another possibility to select the stars needed to correct your science data.

 

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icon EMIR contact persons at GTC

 

contact email @ gtc.iac.es
Nieves Castro - main contact nieves.castro
Peter Pessev peter.pessev
Gabriel Gomez gabriel.gomez

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icon More information:

 

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Last modified: 27 October 2017