MEGARA

 

 


icon Summary of Features

 

MEGARA (Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía) is an optical integral-field Unit (IFU) and multi-object spectrograph (MOS) designed for the GTC. The MEGARA IFU mode will offer a fibre bundle covering 12.5 arcsec x 11.3 arcsec with a spaxel size of 0.62 arcsec, which makes use of 100 μm-core optical fibres. The MEGARA MOS will allow observing up to 100 objects in a region of 3.5 arcmin x 3.5 arcmin around the IFU bundle. Eight of these bundles will be devoted to the determination of the sky during the observation with the IFU, so only 92 of these positioners will be available for MOS observations. Both the IFU and MOS capabilities of MEGARA will provide intermediate-to-high spectral resolutions (R~5,500, 12,000 and 20,000 respectively for the LR, MR and HR modes).

MegaraLayout

A infographic view of the PseudoSlit, Collimator, dispersion elements (VPHs) on the wheel and the set Cryostat+Camera.

icon Observing modes

 

MEGARA has two different bundles of fibres distribution: the Large Compact Bundle IFU and the Multi-Object Spectrograph (MOS) mode.

With MEGARA, spatially spread individual targets could be observed in MOS mode, as well as compact or extended targets with fibre-limited spatial resolution in the centre of the instrument (IFU mode).

In the table below we provide a summary of the main characteristics of the two MEGARA modes (IFU and MOS) and the corresponding spectral resolutions yield for each set of VPHs. Also, the current VPHs available in MEGARA can be found here.

IFU MOS
Field of View 12.5 x 11.3 arcsec2 3.5 x 3.5 arcmin2
Spaxel size 0.62 arcsec
Sampling (1D FWHM) 3.6 pix
LR VPHs R(λ/Δλ) ~ 5 500
MR VPHs R(λ/Δλ) ~ 12 000
HR VPHs R(λ/Δλ) ~ 20 000

icon Volume Phase Holographic

 

Figure below shows the distribution of the VPHs in the resolving power versus wavelength coverage plane for all MEGARA VPHs in the case of the 100 μm-core fibres (LCB IFU and MOS modes) when theoretically projected at designing stage (in different colours) and black/grey after empirical estimates at lab. Average resolutions at midpoint of range are above expectations.

MegaraVPHs

Coverage of the MEGARA VPHs in resolving power (RFWHM) and wavelength for the LCB IFU mode (for MOS mode is very similar) (100 μm-core fibres).

The scientific empirical performance (from commissioning measures) of the complete MEGARA gratings is given in the following table:

VPH ID Setup RFWHM λ12 λC Δλ(@λC) lin res
(Å) (Å) (Å) (Å/pix)
VPH405-LR LR-U 5750 3654.32-4391.88 4025.90 0.700 0.176
VPH480-LR LR-B 5000 4332.05-5199.96 4785.32 0.957 0.207
VPH570-LR LR-V 5850 5143.74-6168.19 5687.63 0.971 0.244
VPH675-LR LR-R 5900 6096.54-7303.21 6729.61 1.141 0.287
VPH799-LR LR-I 5750 7224.11-8640.37 7976.31 1.387 0.337
VPH890-LR LR-Z 5800 8042.74-9634.92 8873.16 1.530 0.379
VPH410-MR MR-U 13100 3919.81-4282.17 4102.87 0.313 0.086
VPH443-MR MR-UB 13050 4226.38-4625.79 4429.44 0.339 0.095
VPH481-MR MR-B 13200 4585.66-5025.07 4809.46 0.364 0.105
VPH521-MR MR-G 12000 4963.22-5445.00 5208.79 0.434 0.115
VPH567-MR MR-V 12600 5413.11-5923.90 5664.96 0.450 0.122
VPH617-MR MR-VR 12100 5894.23-6448.26 6165.79 0.510 0.132
VPH656-MR MR-R 12150 6243.10-6865.26 6560.33 0.540 0.148
VPH712-MR MR-RI 12200 6764.58-7440.85 7109.81 0.583 0.161
VPH777-MR MR-I 8600 7386.53-8127.95 7766.14 0.903 0.177
VPH926-MR MR-Z 11600 8810.52-9698.97 9274.84 0.800 0.212
VPH665-HR HR-R 20050 6405.61-6797.14 6602.59 0.329 0.093
VPH863-HR HR-I 20500 8380.20-8882.38 8626.01 0.421 0.120


icon Efficiency

 

Based on designs parameters and some features estimates at the lab, compared expected efficiency of MEGARA lead us to think that it will be a very insteresting instrument for resolving lines even in faint targets case. Despite its great power resolution, design of MEGARA is intended to improve spectroscopic capabilities/efficiency respect to the current optical instrument working at GTC, OSIRIS.


Compared efficiency

MEGARA detection efficiency for the different sets of VPHs.

Back to top


icon Details about CCD

 

The MEGARA spectrograph is equipped with a 4k x 4k CCD. Given the wide wavelength coverage of the instrument the detector chosen will be a deep-depleted E2V CCD231-84 (see left panel above). This detector provides very low fringing and relatively good efficiency in the blue end of the optical window, specially when combined with the astro-2 coating provided by E2V. The main characteristics of the MEGARA CCD are summarized in the table below.

SPECIFICATION VALUE
CCD provider E2V
CCD model CCD 231-84 (Deep Depleted)
Coating astro-2
Number of pixels 4k x 4k (15 micron per pixel)
N of amplifiers 2 used (of 4 available) in operation mode
Readout time ~ 50s
Readout noise 3.4 electrons
Efficiency 90% (between 400-800nm)


No binning will be offered so operations would only produce frames 1x1. Bias level is quite stable around 2 thousend ADU. There is a little difference (less than 50 ADU) between channels.

As it is a Deep-Depletion (DD) CCD this detector would be more sensible to cosmic rays (CRs) than those with non-depletion characteristics (i.e. OSIRIS' CCD). But after some studies in lab we found that even among DD devices, the MEGARA's CCD shows a greater sensitivity to CR events than others. It will be studied in depth after normal operation would start. Nevertheless it is strongly recomended to split every exposure into, at least, 3 different exposures no longer than 20 minutes (i.e. 1200s) to minimize the impact of CRs in observations.

Back to top


icon IFU Overview

 

The IFU, also called as Large Compact Bundle (LCB), is, in fact, a bundle containing 567 fibres (0.62 arcsec diameter), completed with the 8 minibundles (with 7 fibres every one) that sample the sky background emission. All those 623 fibres are organised in a total of 81 sub-units of 7 fibres distributed in 17 boxes on the pseudo-slit. These boxes are grouping and parceling out the fibres toward left and right of the center of the pseudo-slit in order to take advantage of the "covering".

MEGARA_IFU

MEGARA central bundle of fibres distribution: the Large Compact Bundle or IFU.

The IFU capability of MEGARA samples the 12.5 x 11.3 arcsec2 of the pointed sky area. The fibres distribution is the one that minimizes the cross-talk and/or the contamination between adjacent fibres. Two spaxels adjacent in the detector are physically separated (placed in different locations of the bundle). This is thought like this in order to prevent the concentration of brightly illuminated fibres at the detector.

Back to top


icon MOS Overview

 

The MOS is composed of 92 robotic arms holding 7 fibres each of them in a minibundle. Every Robotic positioner (RP) can patrol an hexagonal area which is the result of dividing the whole MOS 3.5 x 3.5 arcmin2 FOV (on sky) around the central IFU.

The positioning of the fiber minibundle is performed by combining the interpolation of two rotations, which allows covering a circle with a radius of 11.605mm from the center of the positioner (this circle reaches the corners of the hexagon).


MOS_3D_design

There are one hundred minibundles but 8 are devoted to sky/background emission for IFU mode and they are fixed. The rest, 92, are able to patrol their cells and invade slitghly contiguous cell (non-zero risk of colliding).

That sums up a total of 644 fibres devoted to science and sky (upon user decission)

Back to top


icon Use of Cover

 

There is available the capability of using a Cover which will interrupt the pass of light all over the half of field of view selected (it could be LEFT or RIGHT)

Effect_of_Cover

Note the gain in the paths separation by using the cover.

Since the MEGARA design is thought to separate every two neighbor fibres onto different part of the detector, if used you will need a specific set of calibrations (Traces, Arcs, Flats), but as compensation the recovered signal is almost free of pollution from adjacent fibres. Cover will not obstruct the fibres in the centre of the Field Of View (FOV), hence for a number of fibres of the IFU and depending on the MOS setup somes of the non-used robots might be vignetted by the cover edge.

Back to top


icon Preparing an observation with MEGARA

 

In order to prepare the observations with MEGARA, first of all an intensive use of Exposure Time Calculator should be considered. This link takes us to the UCM webpage to pre-compute our expected SNR or needed integration time. This is the only thing to take into account when observing with the IFU. After this, on Phase-II you will be asked for coordinates, orientation of the field, grating or gratings to be used, exposure times and number of exposures and other relevants aspect of a MEGARA observation.

A bit different is to observe in Multi-Object Spectroscopy (MOS) mode. When preparing observations for MOS mode we would have to use the MOS preparation tool. A stand-alone java tool (java 1.8 or above is required) to configure and select the sources. It is important to note that, this (java) tool should be (more than very convinient) executed with internet connection in order to allow update of the config file from GTC web pages. To feed the tool you need to follow a particular ascii format. Please, contact with us if you find any problem.

M1-92_2D_reconst

View of the Fiber MOS Positioning Tool for MEGARA.

The tool comes with an user manual very descriptive and complete.

Then, a file containing needed figures (Robots positions, Field centre, Position Angle and others), would be produced onto your system. By uploading that file to your Phase-II the Fiber MOS assigning file (extension .meg) needs to be checked against our system. For this reason an automatic process would start and you will have on the spot feedback about the upload validity. When passed that check most of details requested for this mode would be recovered from the .meg file. And the only thing that user would have to define is which is the grism (or are the grisms) and integration time(s) for the block.

Further questions, please contact Daniel Reverte.

Back to top


icon Data Reduction package

MEGARA instrument has its own data reduction package, designed and developed as a pipeline (Sergio Pascual, UCM) that allows a complete cycle of work on the science images to produce flux and wavelength calibrated spectra. This data reduction package is based on python and performs automatically the reduction, wavelength and flux-calibration (with just one input file of definitions -- image types and reduction desired)

The MEGARA Pipeline package requires the following packages installed in order to be able to be installed and work properly:

• python 2.7
• setuptools
• numpy >= 1.7
• scipy
• astropy >= 1.0
numina >= 0.14
• scikit-image

The MEGARA Data Reduction Package could be installed in a few simple ways. All of them well explained at the document installation instructions.

icon Dependence on temperature

During tests carried out in the Lab review stage (on 2017, at the UCM), some dependence of the trace map on temperature was found. It was stated that there was a small shift in spatial-direction of the track that defines the path to recover every single fibre spectrum. That shift was found to be proportional to the shift in temperature respect to a work standard.

In any case, the effect is not neglectable when changes in temperature are above 2℃, in which case a proper trace map should be provided.

Back to top


icon Scientific Cases

 

MEGARA Science Team is keen on a wide range of scientific interests. Some of them can be grouped in two categories:

  • 1. the study of Galactic and extragalactic nebulae.
  • 2. the study of (or close to) point-sources with intermediate-to-high surface densities.

Among the former their interests include the study of nearby galaxies, Planetary Nebulae and the emission of the resonant UV lines from the high-redshift IGM and among the latter the study of Galactic open stellar clusters, stellar populations in Local Group galaxies, GC systems in nearby galaxies, intermediate-redshift dwarf and starburst galaxies, and high-redshift galaxy clusters are the main subject of their research activities.

For further details, please visit the MEGARA team collection of web pages:

- Nearby Galaxies
- Intermediate Redshift
- High Redshift
- Planetary Nebulae
- Stellar Clusters


Back to top


icon MEGARA contact persons at GTC

 

contact email @ gtc.iac.es
Daniel Reverte - main contact daniel.reverte
Riccardo Scarpa riccardo.scarpa

Back to top


icon More Information:

Back to top

 


Last modified: 05 July 2018