Instrument: Spectrograph

The MEGARA spectrograph will be located on GTC Nasmyth A platform while the optical fiber bundles will be routed from the Folded Cassegrain F focus to the entrance of the spectrograph at the pseudo-slit position. MEGARA has a fully refractive optical system. The spectrograph is composed by a pseudo-slit, where the fibers are placed simulating a long slit 119 mm length and with a RoC of 1075 mm. Then the light passes through the collimator, which is composed by 5 lenses and has a focal length of 483 mm, and a slit shutter placed behind the first collimator lens. The geometry is fixed for all spectral resolution modes and it is 68 degrees between collimator and camera. The pupil has 160 mm diameter and it is the location for the VPH-gratings (see figure below).

MEGARA Spectrograph optical design

The spectrograph subsystems include the pseudo-slit, with a XY mechanism that allows (1) exchanging among the three positions (LCB, MOS, OPEN) and (2) moving towards and away the first lens of the collimator for focusing; the shutter; the cryostat, the pupil elements at their mechanism, and the camera, which is joined to the cryostat through the last lens, through the last lens, with the CCD detector inside. The cryostat is based on liquid nitrogen. The detector is a deep-depleted Silicon 4k x 4k E2V CCD231-84 15μm pixels and 4 outputs with astro-2 coating. The CCD is < 3e- read noise and has excellent QE across almost the whole visible spectrum. In order to minimize electronic cross-talk between amplifiers we will read using two diagonally-opposed amplifiers.

The MEGARA spectrograph provides three ranges of spectral resolution: low (LR), medium (MR) and high (HR), with excellent transmission thanks to the use of VPHs. The MEGARA VPHs characteristics are summarized in the Table below.

VPH Name Set-up R
(FWHM)
λ1 - λ2
(Å)
λc
(Å)
Δλ (@ λc)
(Å)
Δv
(km/s)
lin res
Å/pix
1 VPH405-LR LR-U 6028 3653 - 4386 4051 0.672 50 0.17
2 VPH480-LR LR-B 6059 4332 – 5196 4800 0.792 49 0.20
3 VPH570-LR LR-V 6080 5143 – 6164 5695 0.937 49 0.23
4 VPH675-LR LR-R 6099 6094 – 7300 6747 1.106 49 0.28
5 VPH799-LR LR-I 6110 7220 – 8646 7991 1.308 49 0.33
6 VPH890-LR LR-Z 6117 8043 - 9630 8900 1.455 49 0.36
7 VPH410-MR MR-U 12602 3917 - 4277 4104 0.326 24 0.08
8 VPH443-MR MR-UB 12370 4225 – 4621 4431 0.358 24 0.09
9 VPH481-MR MR-B 12178 4586 – 5024 4814 0.395 25 0.10
10 VPH521-MR MR-G 12035 4963 – 5443 5213 0.433 25 0.11
11 VPH567-MR MR-V 11916 5393 - 5919 5667 0.476 25 0.11
12 VPH617-MR MR-VR 11825 5869 - 6447 6170 0.522 25 0.13
13 VPH656-MR MR-R 11768 6241 - 6859 6563 0.558 25 0.14
14 VPH712-MR MR-RI 11707 6764 - 7437 7115 0.608 26 0.15
15 VPH777-MR MR-I 11654 7382 - 8120 7767 0.666 26 0.17
16 VPH926-MR MR-Z 11638 8800 - 9686 9262 0.796 26 0.20
17 VPH665-HR HR-R 18700 6445 - 6837 6646 0.355 16 0.09
18 VPH863-HR HR-I 18701 8372 - 8882 8634 0.462 16 0.12

The wavelength coverage and spectral resolutions achieved by each VPH in the LCB and MOS modes is summarized in the figure below.

Resolution and wavelength coverage of the MEGARA VPHs

In order to accommodate and exchange the gratings, the MEGARA spectrograph has a Pupil Mechanism composed by the VPH wheel and a insertion mechanism. The wheel can host 11 VPHs mounted simultaneously. The mechanical mount of each VPH is screwed to a platform.

The figure below shows the 3D of the MEGARA spectrograph.

MEGARA spectrograph


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