Abstract
A multimode spectrograph for tridimensional spectroscopic observations of faint extended objects is under development. The spectrograph has four modes, i.e., the imager mode with narrow (or wide) band filters, the long-slit spectrograph mode for area spectroscopy by means of spacial scans, the Fabry-Perot interferometer imager mode, and the microlens-array (MLA) spectrograph mode. Selection of the mode is made by moving optical parts relevantly in the spectrograph. The principles of the optics of the first three modes are ordinary ones. In the imager mode, a field lens is inserted behind the telescope focus and filters are set between the collimator and the camera lenses, while a Fabry-Perot etalon is inserted instead of the filters in the Fabry-Perot imager mode. In the long-slit mode, a slit is set at the telescope focus, and a grism replaces the filters or the etalon. The principle of the MLA spectrograph mode is essentially same as that of TIGER for the Canada-France-Hawaii telescope. Our spectrograph, however, is equipped with a dual-channel preoptics system for the MLA, one channel acquiring a target object and the other the neibouring sky appreciabily apart from the target object. The image of the sky is projected on the microlenses along a side of the array of the microlenses. The spectra of the sky are used to eliminate of contamination by sky background from spectra of the target object. The design of this spectrograph is primarily optimized for the MLA spectrograph mode at the 1.88 m telescope at Okayama. With this telescope, about eighty spectra of an area about 9# x 15 of an object and fourteen spectra of the nearby sky are obtained with a spacial resolution of 1.3" which matches the average seeing size at the telescope site. Performances of this spectrograph when it is used for other telescopes, including the SUBARU, are also presented.
