Gravitational eccentric correction optics (GECO): an optical-gravitational device to compensate for flexures in astronomical spectrographs.
Mechanical flexure is a source of major failures in astronomical spectrographs, for which the reimaging of a focal-plane pinhole has to be maintained in position within a fraction of a CCD pixel that has dimensions of the order of 15 microm. The d.o.lo.res. (an acronym for device optimized for low resolution) spectrograph for the Italian national telescope, Galileo, showed displacements of the image of the pinhole more than 10 times greater than expected. The mechanical failure was overcome by the insertion of a passive optical wedge that can add an out-of-phase circle to the flexure ellipse. The results encourage the use of the gravitational eccentric correction optics (GECO) optical-gravitational device in all astronomical observations made with the d.o.lo.res. spectrograph.
Design study of an adaptive optics visual echelle spectrograph and imager for the VLT
We present a preliminary design study for an adaptive optics visual echelle spectrograph and imager/coronograph for use as parallel instrument of the Nasmyth Adaptive Optics System (NAOS) on unit UT3 of the VLT. The spectrograph is intended for intermediate resolution spectroscopy of faint sources. It could be used for observations of late-type dwarfs in distant Galactic clusters and in galaxies of the local group as well as for spectroscopy of extra galactic objects like quasars and Lyman break galaxies down to a limiting magnitude of V equals 22.5. The implementation of an imaging gand coronograph mode increases the versatility of the instrument and its scientific objectives. The instrument takes advantage of Adaptive Optics at visible wavelengths both for imaging and spectroscopy. With NAOS at the VLT, the light concentration in these bands will be above approximately 60 percent of the flux in a 0.3 arcsec aperture for typical Paranal conditions. Simulations show that a gain of more than one magnitude with respect to compatible non-adaptive optical spectrography will be possible for sky- and/or detector limited observations. In addition, the smaller diffraction limit in the optical than in the IR will allow a significant gain in imaging and coronography as well. Finally, the instrument will allow gathering unprecedented experience on the performances of AO at visible wavelengths, which will be fundamental for further development of AO systems, in particular for very large telescopes.
Integral field spectroscopy with the GEMINI multi-object spectrographs
The GEMINI Multiobject Spectrograph (GMOS), due for delivery in late 2000, will include a powerful integral field spectroscopic capability. The instrument scan switch to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of multiobject masks. The initial implementation of the GMOS IFU will cover a field in excess of 50 square arcsec with a sampling of 0.2 arcsec via 1500 spatial elements with spectra covering up to 3000 pixels. The spectrum length may also be doubled by halving the field. A separate field is provided at fixed offset to facilitate accurate background subtraction. The system employs a fiber-lenslet technique that provides significant benefits over unlensed fiber reformatters and fiberless lenslet arrays. The specific advantages are unit filling factor, high throughput and long spectra. The IFU has been designed in the light of our experience with two other successful devices of this type. We summarize the design of the device and discuss how the IFU will be operated within the context of GMOS and the GEMINI telescopes. Finally, we present options for implementing IFUs with finer spatial resolution on GMOS.