Science Operations

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.


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.