Optical design for the TAIPAN and HECTOR transmissive spectrographs

TAIPAN is a multi-fibre project for the UK-Schmidt Telescope and Hector is a multi-IFU project for the Anglo- Australian Telescope (AAT) using fibres. Many different transparent designs were studied covering a large parameter space. An important trade-off study was between the use of microlenses on the slit or just bare fibres. Microlenses have disadvantages but permit considerable simplification of the collimator by making the beam very slow. The disadvantages are more important with the UK-Schmidt due to the faster beam from the telescope. With microlenses, the collimator can be a unique spherical plano-convex lens significantly smaller than the mirror that would be needed in a reflective design. For Hector, 26 different camera designs where done to cover the parameter space for 2k x 2k, 2k x 4k, or 4k x 4k detectors, and for 50, 75 or 100 micron fibre cores, with or without microlenses, with a triplet in the camera or a doublet plus singlet, and with a maximum wavelength of 1 or 1.05 microns. Not all combinations were designed but for each parameter there are at least two representative cameras with all other parameters identical. A preliminary cost estimate was made for the most promising designs which permitted to reduce them to 3 for more detailed designing. Also, a theoretical study was done of the PSF obtained with highly packed microlenses at the slit with no space between them and imaging to 2 pixels per microlenses. This maximizes the number of fibres per spectrograph, and thus the total field of view of all IFUs together, but it comes with some disadvantages.