In this work we present the coatings of the spectrograph red camera of WEAVE -the new multiobject survey facility for the 4.2m William Herschel Telescope. The initial requirements of WEAVE red camera lenses, with reflectances as low as 0.4% through the wavelength interval from 590 nm to 959 nm at angles of incidence of 18° +/- 17° represented a challenge for both design and production. Based on initial requirements, several solutions to the same problem were achieved and tested. The customized designs have been continuously improved through theoretical and experimental approximations. From transmittance measurements at normal incidence we developed a method to determine the reflectance at different angles of incidence. We show the designs and coating transmittance obtained for the four glasses on test runs to guarantee that the designs were achievable experimentally. Additionally, we present the reflectance obtained on the lenses of the the first four lenses of WEAVE red camera.
MEGARA is the new integral field unit (IFU) and multi-object (MOS) spectrograph successfully commissioned at Gran Telescopio Canarias, in August 2017. MEGARA provides spectral resolutions R (fwhm) ~ 6000, 12000 and 20000, via volume phase holographic gratings, at very high efficiency in both IFU and MOS modes. In the case of MEGARA main optics and pupil elements optics, the surfaces in contact with air have an anti-reflective (AR) coatings to minimize the Fresnel losses at the interface glass-air. In this work we present the designs and calculation of the total throughput of the optical system based in the transmission measurements of the AR coated witness samples. The results reflect the benefits of having implemented customized AR coatings for the mean angle of incidence on each surface as the measured throughput was better than the requirements. We analyze the effects of the pupil elements AR coatings for each spectral configuration.
MEGARA is the new generation IFU and MOS optical spectrograph built for the 10.4m Gran Telescopio CANARIAS (GTC). The project was developed by a consortium led by UCM (Spain) that also includes INAOE (Mexico), IAA-CSIC (Spain) and UPM (Spain). The instrument arrived to GTC on March 28th 2017 and was successfully integrated and commissioned at the telescope from May to August 2017. During the on-sky commissioning we demonstrated that MEGARA is a powerful and robust instrument that provides on-sky intermediate-to-high spectral resolutions RFWHM ~ 6,000, 12,000 and 20,000 at an unprecedented efficiency for these resolving powers in both its IFU and MOS modes. The IFU covers 12.5 x 11.3 arcsec<sup>2</sup> while the MOS mode allows observing up to 92 objects in a region of 3.5 x 3.5 arcmin<sup>2</sup>. In this paper we describe the instrument main subsystems, including the Folded-Cassegrain unit, the fiber link, the spectrograph, the cryostat, the detector and the control subsystems, and its performance numbers obtained during commissioning where the fulfillment of the instrument requirements is demonstrated.
We present the antireflection coatings of the optical elements of MEGARA, the new integral field and multi-object spectrograph for the Gran Telescopio Canarias. We describe the methodology for optimizing the solutions. We also present the results of the final deposited coatings. The main optics require broadband coatings in the range from 370 nm to 980 nm for different materials with a mean R<1.3% at specific angles of incidence in each surface. For each material a specific arrangement of thicknesses of the same eight layers were produced and tested. For the spectrograph pupil elements four layer coatings were designed and produced R<0.3%. The design of main optics and pupil elements coatings have been shared between INAOE and CIO. The coating depositions have been performed at CIO in the Integrity 39 Denton Vacuum Deposition System. The main optics final coatings fulfill MEGARA requirements.
Optical tolerances are specified to achieve the desired performance of any optical system. Traditionally the diverse sets of
tolerances of a system are proposed by the designer of each of the subsystems. In this work we propose a method to
corroborate the design tolerances and simultaneously to provide extra data of each parameter to the manufacturer. It
consists of an inverse analysis in which we fix a modified merit function as a constant and evaluate distinct models of
perturbed lenses via Monte Carlo simulations, determining the best possible tolerance for each parameter, and indirectly
providing information of sensitivity of the parameters. The method was used to carry out an extensive tolerance analysis
of MEGARA, a multi-object spectrograph in development for the GTC. The key parameters of the optics are discussed,
the overall performance is tested and diverse recommendations and adjustments to the design tolerances are made towards
fabrication at INAOE and CIO in Mexico.