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The VPH Grism installed on Dolores@TNG

After a successful test phase in the past months, a new 1435 gr/mm VPH (Volume Phase Holographic) Grism is now available on Dolores@TNG, and will replace the existing Echelle Grism.
This VPH Grism, developed by the gOlem group of the Brera Observatory, in Merate, provides a substantial upgrade of the observing capabilities of Dolores, being the only instrument in the northern telescopes currently equipped with such a new-generation grism.
Up to date, the best spectroscopic resolution (namely, 2.8 Å FWHM, or ~140 km/sec in radial velocity, with a 1" slit) was reached by the 600 gr/mm HR grisms, with a wavelength dispersion of tipically ~0.8 Å/pixel.
Now, with the new VPH grism, Dolores resolution can be increased up to ~1.2 Å (i.e. ~60 Km/s in radial velocity with a 1" slit) in the 6200-6900 Å range. An intrinsic accuracy in the wavelength calibration as good as 15 km/sec can be reached for spectra with good S/N emission lines, taking advantage, in addition, of a 40% better throughput efficiency with respect to the HR-R grism.

As an example of the new VPH grism performance, Figure 1 shows the spectrum of the R=10.1 mag barred galaxy NGC157, taken at the TNG on December 6, 2001, under quite poor seeing conditions (~2 arcsec FWHM).



Figure. 1 - A 900s exposure spectrum of the barred galaxy NGC157. The 1" slit was used, centering on galaxy nucleus.


Lines identification, together with observed and restframe wavelenghts is given in Tab.1

Tab. 1 Lines identification and wavelenghts

Line #   Line ID  Rest Lambda   Obs. lambda

1        [NII]    6548.0        6585.69
2        H alfa   6563.0        6600.12
3        [NII]    6583.0        6620.81
4        [SII]    6717.0        6754.30
5        [SII]    6731.0        6769.57


Credit to these data should be properly acknowledged.