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10 July 2018 The advanced image slicers of OCTOCAM
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Abstract
OCTOCAM is the new large Gemini instrument in building. It is an imaging spectrograph with 8 cameras covering the range 370 nm to 2350 nm at a typical resolution of 3000-4000. It will have 2 IFUs, one for normal operation over all wavelengths, the other for AO in the NIR only and with a smaller field but a higher spectral resolution. Currently, no IFU exists that covers the entire range of VIS and NIR in a single observation. Such an IFU would have a number of applications: It can be used for resolved studies of HII regions over a broad wavelength range and emission line galaxies over a broad redshift range using the same set of emission lines. Another application is to observe transients with only arcseconds localization very early without waiting for a sub-arcsecond position, hence allowing to obtain very valuable early data. For bright transients such as SNe and GRBs we can study the immediate environment in detail, and even use the actual transient as AO tip-tilt star to study the environment at high spectral and very high angular resolutions. The IFUs will be Advanced Image Slicers, a proven concept now in use in many instruments around the world including Gemini NIFS, VLT MUSE and KMOS, and JWST NIRSpec. The normal operation slicer will have a field of 9.7" x 6.8" with 17 slices 0.4" wide giving 0.18" x 0.4" spaxels. The slices are smaller than the standard slit size of 0.54" (3 pixels) so will deliver higher spectral resolution. This IFU will deliver much higher performances than the GMOS IFU and NIFS with a larger field of view and spectral range but also considerably fewer pixels per arsec2 then reducing the readout noise. With its wavelength range starting at 370 nm, diamond machining cannot be used. A glass slicer system will have to be used as in MUSE. The wavelength range will however be much larger covering the whole VIS and NIR range. Modern reflection coatings as UV enhanced silver can be used but a trade-off may be better by starting at a longer wavelength to get higher transmission. Special consideration is necessary for the fore-optics which cannot be diamond machined and for the overall design due to the limited space envelope. The AO slicer will have a field of 2.5" x 3.6" with 31 slices 0.08" wide imaged on 2 pixels in the spectral direction to get proper sampling. The fore-optics will magnify the beam in both directions but with different magnifications to get spaxels of 0.08" x 0.08". The smaller slice image width will give a spectral resolution of about 5000 including aberrations, about the same than NIFS but covering all 4 NIR bands at once. This slicer uses a slit 60% longer than OCTOCAM is designed for. It is possible because the magnification reduces the beam size so the aberrations and vignetting.
© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Robert Content, Antonio de Ugarte Postigo, Christina Thöne, and Andrew Sheinis "The advanced image slicers of OCTOCAM", Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 107066L (10 July 2018); https://doi.org/10.1117/12.2314424
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