EMIR+

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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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", 1.0", 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 4' x 6.67' field. However these should be considered and treated like MOS configuration files.

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

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.

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.

The following plots show the limiting magnitudes (Vega system) in the different EMIR pseudo-grisms for getting S/N=3 per resolution element as a function of the exposure time for a point-source in 0.6 arcsec seeing, at airmass = 1.2 (by using a 0.8" slit width).

Limiting magnitudes achievable in EMIR spectroscopy mode as a function of time.

Comparison with previously observed data clearly demonstrates the gain in sensitivity with the current detector in EMIR+ configuration (see Figure below) .

Important Note: In an operative sense, observations with Hawaii-2RG detector in EMIR+ produce the S/N predicted by EMIR ETC.

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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.

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.

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.

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

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Multi-Object Spectroscopy (MOS)

EMIR allows Multi-Object Spectroscopy (MOS) over a 4' x 6.67' 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.

Left: X-shaped configuration of all the slits, outlining the extent of the MOS FOV of the Configurable Slit Unit. Right: 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, EMIR Optimized Slits Positioner (OSP), detailed description on it use can be found here.

While conceptually equivalent to Long Slit Spectroscopy once the CSU is configured and the objects have been acquired, the initial step of the MOS mode has sufficient number of specificities so as this section is entirely devoted to it. Once the CSU configuration file is obtained using the OSP tool, the instrument setup already done and the field acquired, the observation can proceed as in the Long Slit Spectroscopy.

At the time of planning MOS observations with EMIR, the user must be cared of the following issues:

• Good astrometry is essential. Bear in mind that MOS can accommodate a large number of objects, in theory up to 53, distributed over the spectroscopic FOV.
• The width of the slit assigned to each object would be typically around 1 arcsec o even less. Hence, absolute astrometric accuracy at the level of 0.1 arcsec or better is highly advisable. With the release of the Gaia results, that should not be a problem. Anyhow, it is totally up to the user the configuration of the EMIR CSU around the target list.
• The length of slit assigned to each object must be sufficient to allow for beam switch along the slit (ABBA pattern), if this is selected. To this end, more than one slitlet, each one of ~7.2 arcsecs in height, can be allocated to a single object. Also, for the success of the ABBA pattern, the location of each object on its slit must be commensurate with the selected nodding throw and the direction of the initial displacement.
• The central wavelength of each spectrum is placed at the location of the optical image of the corresponding slit on the detector. Hence, the spectral range covered by each of them will vary accordingly.
• Due to the optical distortion, the slits at both ends, numbers 1 and 55, are not fully imaged onto the detector. Hence, they cannot be used as primary slits to allocate objects but are useful to complement adjacent slits.
• The OSP tool has been designed to cope with most of the chorus associated with the CSU configuration. The use of OSP tool is mandatory to produce the configuration files.

Once the user has produced CSU configuration files for each of the fields to be observed. The only file that is strictly needed to configure the CSU is the corresponding CSU output from the OSP. The XML file(s) must be kept with the user for future use and/or additional verifications.

It is worth to repeat that the success of the observation is extremely dependent on the quality of the CSU configuration file that, in turns, depends critically on the astrometric accuracy of the target list. The centring of the mask in the field is as important as it is the astrometric accuracy. To this end, a number of reference objects have to be allocated in different slits, better if spread over the FOV. The minimum number of reference objects is 3, but it is advisable to include some more. Good reference objects must comply with the following:

• They need to be conspicuous, easily detectable in short integration times. That means, among other things, no to be located in crowded areas with objects around with similar brightness.
• The location of the reference objects in the slits should be as centred as possible, or at least not close to the ends of the slit. This is to maximize the probability of the object to lie within the slit in the first GTC pointing, which might be a bit out of position.

It has to be noted that there is nothing that prevents a target object to double as a reference object, provided that it complies with the above list. The reference slit have to be wider than the target slits. A reference value of 5 arcsec for the reference slit widths must be used. At this point, the user has two options:

• Prepare just one CSU configuration file in which the reference slits are open to 5 arcsecs and the target slits opened to the selected width for scientific observations. This has the caveat that the reference (bright) objects might contaminate the areas of the target spectra in the detector.
• Have two different CSU configuration files, the first of which with the reference slits opened to 5 arcsecs, and the second one with the reference slits closed or opened to a selected width for the scientific observations. The target slits are opened to the selected width in both files.

The user has to provide the two configuration files (.CSU) and the .XML to the GTC staff when using GTC Phase 2 Tool. Additionally, the .CONF file produced by the OSP can be also uploaded to include extra information to be incorporated to the image keywords, although this file is not mandatory.

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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.

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.

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

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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 important, mainly in imaging mode. As a side effect to ensure this better perfomance, Hawaii2-RG needs some time to signal stabilization, which implies that EMIR+ operation has now higher overheads than before: observing efficiency is 50% in imaging mode and 75% in spectroscopic modes (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.

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Telluric Standards

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.

Some typical exposure times (for the individual ABBAs) are suggested in the table below for the different spectroscopic configurations as a function of the magnitude of the teluric star. (The reference filter in the 2MASS system is listed in the second line of the table.) Note that this is still work in progress and the table below will be updated as more data comes in. For the time being, the values from the table should be treated with caution.

Selecting a brighter telluric standard assures that the required high SNR for the standard is achieved with minimal investment of observing time.

In the case of Multi-Object Spectroscopy observations with EMIR, each given slit covers a wavelength range that depends on the slit position. Hence, to take one single spectra of the standard with the slit placed at the center (like for longslit spectroscopy) will not cover the full wavelength range covered by a generic MOS configuration. In EMIR MOS mode two spectra of the standard will be obtained: one with the slit placed at the extreme left, and a second one with the slit at the extreme right. All in one frame. In this way it is guarantee the full wavelength range a MOS configuration might cover, it is recovered.

This mode uses a predefined MOS configuration (see Figure below), so no mask design with the OSP is required. However, note that for this configuration nodthrow must be defined as -7.4 arcsec in GTC Phase 2 in order to execute ABBA pattern properly within the two slits. Obviously, the duration of the observation will be two times larger than the required in longslit observations.

EMIR MOS Telluric configuration by using 5 arcsecs wide slits placed at two decentered positions in the FOV, in order to cover the full wavelength range required.

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EMIR contact persons at GTC

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Useful Documents

• EMIR description paper
• EMIR+ detector perfomance paper

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More information

• URL: Proyecto EMIR (IAC)
• URL: Telluric Standard Stars with EMIR
• URL: Gemini Telluric Standards Selection Tool

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Last modified: 07 November 2025