Logo
INAF Logo
  

New Volume Phase Holographic Grisms installed on DOLORES

During the first week of October 2005, a set of 9 Volume Phase Holographic (VPH) Grisms were installed on Dolores and successfully commissioned. The grisms used in Dolores were assembled and laboratory-tested by the Golem group (Brera-INAF Observatory in Milan).
Since July 2001, one experimental VPH grism (centered on the H_Alpha) was regularly used in Dolores and its superior performances indicated that this would have been the natural evolution for a serious up-grade of Dolores.
Thanks to their higher-than-average efficiency, the spectroscopic capabilities of the TNG at optical wavelenghts are now improved by at least a factor of ~50%.
Moreover, the highest resolving power of Dolores increased now up to Rs~5000, to be compared with the former Rs~3000.

Three of the new VPHs do replace exactly (both in wavelegth coverage and resolution) the old grisms: HR-V, HR-R and HR-I.
These grisms were thus christened: VHR-V, VHR-R and VHR-I, the preceeding "V" indicating that they are VPHs.
The remaining 6 grisms were choosen to cover ~600-800 Angstrom intervals centered on emission/absorption lines of particular interest in astrophysics (see Table 1).


Table 1: The main characteristics of the new VPH grisms installed on Dolores @ TNG.
  V390 V486 V510 V589 V656 V860 VHRV VHRR VHRI
  O3727
4000
HeII 4670
H? 4861
OIII 5007
Mg5200
Na D
5890
Ha 6563 Ca 8600 / / /
gr/mm 1511 2040 1886 1600 1400 1060 566 660 685
Eff. Lmed 0.85 0.77 0.81 0.84 0.86 0.86 0.80 0.81 0.85
Lmin 3600 4550 4850 5590 6200 8100 4650 6200 7360
Lmax 4200 5000 5350 6200 6920 9100 6800 7800 8900
Rs 3100 5200 5100 4700 4550 4300 1480 2430 2950

The response of the new VPH grisms was measured by observing a spectrophotometric star during a stable and photometric night.
Since the grism wheel has only 11 usable positions,for the test it was decided to mount all the new VPHs and leave the old HR-I to do a one-to-one comparison with its equivalent VPH.

The global efficiency of the TNG+Dolores+Grism with both the old HR-I and the new VHR-I is shown in Figure 1.

Comparison HRI vs.
VHRI

Figure1: Plot of the global efficiency of the system TNG+Dolores+Grism as measured from a standard star.

At 7550 Angstroms, the TNG+Dolores global efficiency was 0.14 with the old HR-I and is 0.21 with the new VHR-I, a notable ~50% improvement.
Almost all the new VPHs, with the exception of V510 and V390, do have a little ghost above or below the spectrum , as shown in Figure 2.
It should be noted, however that the reddest VPHs (i.e. V860, VHR-R and VHR-I) suffer of significant second-order effect.
For observations it is recommended to couple them with the ~520 nm cut-on filter ( Rasmussen Filter #2 ) available on the filters wheel.

VPH Ghost

Figure 2: The typical ghost which affects almost all the VPH grisms. In this case the spectrum taken with V656 is shown.

The New Dolores Configuration

Starting from January 1st 2006, the Dolores grisms configuration will be modified according to the scheme shown in Table 2.
Note that, since the pyramid focuser is also mounted on the grism wheel, not all the VPHs will be available at the same time on Dolores.
Starting from AOT-14, observers can also request those VPHs that are normally not mounted on the instrument (e.g. V390, V589) but ONLY in visitor mode.


Table2: The "old" and the "new" grism configuration of Dolores. Note that the old VPH-Halpha will be decommissioned since its twin has a more efficient grating.
OLD CONFIGURATION NEW CONFIGURATION
LR-B LR-B
LR-R LR-R
MR-B MR-B
MR-I V486
HR-B V510
HR-V V656
HR-R V860
HR-I VHR-V
Rasmussen VHR-R
VPH-Halpha VHR-I