BTFI2: a simple, light and compact Fabry-Perot instrument for the SOAR telescope
ABSTRACT
We present the concept of a new Fabry-Perot instrument called BTFI-2, which is based on the design of another Brazilian instrument for the SOAR Telescope, the Brazilian Tunable Filter Imager (BTFI). BTFI-2 is designed to be mounted on the visitor port of the SOAR Adaptive Module (SAM) facility, on the SOAR telescope, at Cerro Pach´on, Chile. This optical Fabry-Perot instrument will have a field of view of 3 x 3 arcmin, with 0.12 arcsec per pixel and spectral resolutions of 4500 and 12000, at H-alpha, dictated by the two ICOS Fabry-Perot devices available. The instrument will be unique for the study of centers of normal, interacting and active galaxies and the intergalactic medium, whenever spatial resolution over a large area is required. BTFI-2 will combine the best features of two previous instruments, SAM-FP and BTFI: it will use an Electron Multiplication detector for low and fast scanning, it will be built with the possibility of using a new Fabry-Perot etalon which provides a range of resolutions and it will be light enough to work attached to SAM, and hence the output data cubes will be GLAO-corrected.
INTRODUCTION
Numerous integral field spectrographs (IFS) have been deployed on 4 to 8 meter-class telescopes during the last few decades. These instruments are built to deliver 3D spectroscopy by sampling different regions of the sky at the same time using different techniques, principally: (1) lenslet arrays, e.g. TIGER/CFHT,1 OASIS/CFHT and SAURON/WHT,2 OSIRIS/Keck.3 (2) mirror slicers, e.g. SPIFFI/VLT,4 NIFS/Gemini,5 GNIRS/Gemini,6 SWIFT,7 SINFONI/VLT,8 KMOS/VLT,9 MUSE/VLT.10 (3) fiber bundles, e.g. DensePak/KPNO,11 SILFID/CFHT,12 INTEGRAL/WHT,13 FLAME-GIRAFFE/VLT (Pasquini, 2003SPIE.4841.1682P) and more recently SAMI/AAO.14 These IFS have relatively wide spectral range and intermediate spectral resolution (∼ 1000 - 5000), but generally have small field-of-views (FoV). For larger FoV another class of instruments, the imaging spectrometers, are used. The most common types are the Fabry-Perot spectrometers (FPS) and Fourier Transform spectrometers (FTS). For a long time, large FoV scanning FPS have been developed in the optical, e.g. TAURUS,15 CIGALE,16 HIFI,17 MMTF,18 SAM-FP,19 and they have been used with great success, especially for projects which required small wavelength coverage and large FoV, of the order of a few arc-minutes squared. This paper describes one such instrument, the Brazilian Tunable Filter Imager 2 (BTFI-2): a Fabry-Perot imaging spectrometer with high image quality provided by ground-layer adaptive optics (GLAO). This opens a whole new window for scientific exploration in areas such as the study of the physics of nearby galaxies, their kinematics, and their star formation history from pc to kpc scales by taking advantage of the large FoV (3 x 3 arcmin) and high spatial resolution, thanks to the SAM facility.20 Galaxies consist of baryons distributed in multiple morphological and kinematic components (bulges, disks, bars, black holes) trapped into the deep potential well of dark matter halos. High resolution spatially resolved spectroscopy on large FoV allows gathering detailed spatial and spectral information to measure numerous physical parameters such as (1) the interaction and feedback between the gas, the stars, the dust and the black holes on galactic (10-100 pc) scales and, (2) mass loss mechanisms in stars, HII regions, planetary nebulae and supernova on smaller (pc) scales for galactic structures.
BTFI-2 is a new instrument concept based on the Brazilian Tunable Filter Imager (BTFI, Ref. 21), a FPS instrument designed for use with the SOAR Adaptive Module (SAM), at the 4.1 m SOAR Telescope, at Cerro Pach´on, in Chile. BTFI is now officially decommissioned due to a number of reasons described further on in this paper. Instead, a much simpler instrument called SAM-FP has been used since 2016 to meet the needs of the SOAR community in obtaining spectroscopy with the SAM instrument. SAM-FP yields 3 × 3 arcmin GLAO-corrected data cubes for a variety of studies, as mentioned in section 2. However, SAM-FP is equipped with a normal CCD, and not with a photon counting system like an EMCCD, given that it uses the regular SAM camera called SAMI, which limits the detector power at low flux of the instrument. Standard CCDs still exhibit readout noise of a few electrons per pixel which does not allow short exposures and furthermore fast scanning of elementary FP cycles for which sub-electron readout is requested (see section 6 for details). The most important gain expected with BTFI-2 will be to get deeper data, thanks to the capability of scanning the data cube with shorter exposure times to averaging sky transmission fluctuations.
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