First-Light Images from Low-Dispersion Spectrograph-Cum-Imager on 3.6m Devasthal Optical Telescope

A low-dispersion spectrograph-cum-imager has been developed and assembled in ARIES, Nainital. The optical design of the spectrograph consists of a collimator and a focal reducer converting the f/9 beam from the 3.6 m Devasthal Optical Telescope (DOT) to a nearly f/4.3 beam. The instrument is capable of carrying out broad-band imaging, narrow-band imaging and low-resolution (λ /Δλ < 2000) slit spectroscopy in the wavelength range 350–1050 nm. A closed-cycle cryogenically cooled charge-coupled device camera, also assembled in ARIES, is used as the main imaging device for the spectrograph. The first images from the spectrograph on the telescope assert seeinglimited performance free from any significant optical aberration. An i-band image of the galaxy cluster Abell 370 made using the spectrograph shows faint sources down to ~25 mag. The quality and sensitivity of the optical spectra of the celestial sources obtained from the spectrograph are according to the expectations from a 3.6 m telescope. Several new modes of observations such as polarimetry, fast imaging and monitoring of the atmospheric parameters are being included in the spectrograph. Using a test set-up, single optical pulses from the Crab pulsar were detected using the telescope. The spectrograph is one of the main back-end instruments on the 3.6 m DOT for high-sensitivity observations of celestial objects.