Observing Modes

Nominal Wavelength range	320-1100nm
Red/Blue Dichroic Cross-Over	565nm
Segmented Long-Slits	0.3-arcsec Segmented long-slit 0.6-arcsec Segmented long-slit 0.8-arcsec Segmented long-slit 1.0-arcsec Segmented long-slit 1.2-arcsec Segmented long-slit 2.4-arcsec Segmented long-sli 60 x 5-arcsec long-slit
Diffraction Gratings	Red: 250 l/mm (R=2300) Blue: 400 l/mm (R=1850)
Double-Pass Prisms	Red: TIH6 + Ag (R=500-200) Blue: Fused Silica + Al (R=500-125)

All resolutions in the above table are quoted for the 0.6″ slit. Grating spectra are ~ 5000pixels long.  In the spectral modes, dispersion is mapped onto the long (x) axis of the 8K x 3K detector.  The blue channel spectra run blue-to-red from left-to-right, whereas red-channel spectra run red-to-blue from left-to-right.   Raw prism spectra are oriented the same but map into only ~650 pixels.

Multi-Slit Spectroscopy

Multi-object spectroscopy is accomplished with user-designed laser-machined slit masks loaded into a 24- position mask cassette.  In multi–slit mode, a customized slit mask will be used to precisely locate a series of small and precisely positioned slits or apertures centered on targets within the MODS field of view.  The full width of the grating spectrum is ~5000 pixels. Even with displaced slitlets the entire spectra will land on the detector (unlike with LUCI).

The performance is comparable to that of the long-slit, since the same detector, dichroic, gratings or prisms, filters, etc are used.  The difference lies in that the image quality is dependent on the slit placement in the field.  MODS is designed to deliver good images inside the central 4×4′ field of view and reduced image quality in a 6×6′ “extended” science field. In practical terms, the image quality decreases rapidly outside a 5.6-arcminute diameter circle, primarily due to a combination of astigmatism from the off-axis paraboloid collimator mirrors and off-axis aberrations in the LBT f/15 direct Gregorian focal plane. For purposes of locating MOS mask slits and alignment stars, it is recommended that primary science targets slits are kept within a 5.6′ circle, and alignment stars within a 5.0′ circle.  Details on Mask design and optimization are described in the Mask Preparation Section.

Pre-defined (default) configurations:

For example:  When making and running the MODS Target Acquisition script, the default is the Red Camera, 60s r_sdss filter. Long Slit acquisition images are the central 1K x 1K ROI (Central 2 arcmin) and MOS acquisition images are the full 3K x 3K.  When generating the acquisition script with the OT or with:

>>  mkMODSAcq [options] rootName

options can be used to change the standard default configuration.

The options and how to use them when preparing scripts are outlined in the MODS Observing Script Preparation Toolkit  page.