Grating and Prisms

Primary Instrument Configuration

Each channel has a grating turret containing an imaging flat mirror and up to 3 dispersers.  Currently 2 dispersers are installed.  The configuration is summarized below:

 

Channel	Grating ID	Description
Blue	Flat	Imaging Flat with enhanced Al
	G400L	400 line/mm Reflection Grating
	P450L	FuSi+Al Double-Pass Prism
Red	Flat	Imaging Flat with Protected Ag
	G670L	250 line/mm Reflection Grating
	P700L	TIH6+Ag Double-Pass Prism

Reflection Gratings

Channel	Grating ID	Lines/mm	Blaze Angle	Order	Nominal Range (Å)	Linear Dispersion	Spectral Pixels	Resolution (0.6” Slit)
Blue	G400L	400	4.4°	1	3200-5800	0.515 Å/pix	5200	1850
Red	G670L	250	4.3°	1	5800-10000	0.845 Å/pix	5700	2300

 
Grating resolutions quoted are measured at the reference wavelength (4000Å for Blue, 7600Å for Red) in a 0.6″ reference slit. The nominal linear dispersions quoted here are from the MODS1 Commissioning Report.

 

MODS currently has a set of red and blue gratings giving R≈2000 (λ/δλ) in first order with a 0.6″ slit. Space is reserved in the grating turret for a future set of higher resolution gratings (up to R≈8000 with a 0.6″ slit).  The available gratings are tilted to give the optimal wavelength coverage for the full spectral range of MODS.

When observing with the red configuration only, a GG495 filter must be used to block out the second order from contaminating the first order.  In dual mode the dichroic acts as an order blocking filter.

Prisms

MODS has two double-pass prisms with immersed reflection coatings to provide a very low resolution (R=100-500) mode. The blue prism is made of Fused Silica glass with an immersed aluminum coating, the red prism is made of TIH6 glass with an immersed silver coating.

 

Channel	Grating ID	Nominal Range (Å)	Linear Dispersion	Spectral Pixels	Resolution (0.6” Slit)
Blue	P450L	3200-6000	3-15 Å/pix	650	420-140
Red	P700L	5800-10000	4-20 Å/pix	650	500-200

The prisms are non-linear dispersers, with a resolving power that decreases with increasing wavelength. The larger spectral pixels (more Angstroms per pixel) at the red end can lead to saturation in continuum or lines, and leakage of blue light through the dichroic.

In prism mode, a GG495 Filter is used with the red-only and with dual mode, to block any blue light that might leak through the dichroic.

Because prism spectra map onto a small number of pixels in dispersion (~650 vs. ~5000 pixels for the gratings), it is possible to horizontally stack slits on the multi-slit masks to increase the number of objects observed.