VPHG-based upgrade of the low-resolution spectrograph d.o.lo.res. at the Italian Galileo National Telescope

摘要

The upgrade of already existing instrumentation keeps always the flavor of a second-hand choice when an insufficient amount of financial and material resources can be allocated to a project. The advent of VPH gratings into the astronomical instrumentation scenario offers a new possibility to reach spectral resolutions and efficiencies that could not be dreamt of in existing low dispersion spectrograph. d.o.lo.res. is a spectrograph of the FOSC series, which couples imaging to spectroscopy by means of a grism that can be put in the parallel beam between collimator and camera. The recent upgrade with a 1435 gr/mm VPH of d.o.lo.res. is the final point of a study carried out in the gOlem labs at Brera Observatory. The lab is equipped with an modified spectrophotometer (modi) which is able to measure transmission of optical elements which will be equipped for cryogenic environment. The working study lead to the design of a high resolution VPHG in the Hα spectral region for the low resolution spectrograph. An increase of the hitherto limiting resolution of 2250 up to 5150 together with an increase of global efficiency of a factor of 1.4 give the added value of a 5000 USD investment for an 'old' instrument.