New Developments in Integral Field Spectroscopy

Abstract

The development of wide field adaptive optics (AO) capabilities greatly increases the potential information content (spatial resolution elements × spectral resolution elements) per telescope pointing. This is illustrated in Table 1, which lists the wide field AO systems for the VLT. The multi-conjugate adaptive optics (MCAO) demonstrator MAD has already proven that a 1′ corrected field of view (FoV) with point spread function (PSF) full width half-maximum (FWHM) of 0.1′′ is possible [1]. This corresponds to a number of spatial resolution elements, Nsp = 4×104. The two ground-layer adaptive optics (GLAO) systems currently under development promise comparable numbers. Comparing these figures with the seeing limited case for the maximum VLT FoV and good seeing we see that in terms of the number of spatial resolution elements the wide field AO foci of the VLT will be just as ‘wide field’ as any seeing limited instrument. This simple comparison highlights the need for large spatial format AO instruments, including integral field spectrographs (IFS’s), to fully exploit the scientific potential of wide field AO. However, large format IFS’s are complex and technically challenging instruments. A good example of the current state of the art is MUSE, a 300× 300 format, R = 2000–4000 IFS for wavelengths of 0.465–0.93 μm, giving a total of 7×107 resolution elements. One of the most obvious difficulties in building such instruments is simply physical size, MUSE will essentially fill the Nasmyth platform at the VLT. The technical difficulties are compounded for near infrared (NIR) IFS. The need for more extensive cooling leads to even greater size and complexity. Furthermore NIR IFS is less capable than optical IFS because the NIR sky background is much higher, which limits NIR IFS to high surface brightness objects. However, AAO technology developments, in particular integrated photonic spectrographs and OH suppression fibres, offer possible solutions.