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.