Science Operations

Observing Modes


Each MODS can be used for long-slit or multi-object spectroscopy or for direct imaging.

  1. Direct Imaging
  2. Long-slit Spectroscopy
  3. Multi-Slit Spectroscopy

MODS1 as seen from chamber floor – S Allanson, LBTO

Direct Imaging

A set of SDSS filters is available in each MODS for Direct imaging. The following summarizes the imaging performance.

Wavelength Range 320-1100nm (Atmospheric to CCD limits)
Field of View ~6.0×6.0 arcminutes (~2800×2800 pixels)
CCD Pixel Scale Blue: 0.120 arcsec/pixel
Red: 0.123 arcsec/pixel
Imaging Filters SDSS u,g (Blue Channel) and SDSS r,i,z (Red Channel)
Image Quality Best IQ within inner 2 arcmin, Distortions grow as r2
When imaging, there is an imaging field-stop mask in place that acts as a field stop, insuring proper baffling at the edges of the FoV.  Images are oriented North=Up, East=Left when the instrument is rotated to PA=0°.

Long-slit Spectroscopy

Each MODS has a set of segmented long slits (5 60-arcsec long segments with a 3″ gap in between). The slit can be maintained at any position angle. The 0.6 arcsec wide slit will take advantage of the typical median seeing at the LBT and projects to ~5 pixels at the detector.  Below summarizes the long-slit performance:

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:

There are four  basic operating modes: acquisition, imaging, grating spectroscopy, and prism spectroscopy. Within in each mode, MODS may be configured for dual-channel (red+blue), blue-only, and red-only operation. The instrument configuration also sets the basic CCD subframe readout for that mode.

Each of the standard operating modes for MODS has a default instrument configuration.  A summary of these default configurations or “Auto Configurations” is summarized below.


Mode Active Channel Grating Filter ROI
and Multi-Slit
Dual Grating Red G670L Clear 8K x 3K
Blue G400L Clear 8K x 3K
Red Grating Red G670L GG495 8K x 3K
Blue Grating
Blue G400L Clear 8K x 3K
Dual Prism Red P700L GG495 4K x 3K
Blue P450L Clear 4K x 3K
Red Prism Red P700L GG495 4K x 3K
Blue Prism
Blue P450L Clear 4K x 3K
Dual Imaging Red flat SDSSr,
3K x 3K
Blue flat SDSSg,
3K x 3K
Red Imaging Red flat SDSSr,
3K x 3K
Blue Imaging
Blue flat SDSS g,
3K x 3K
and Multi-Slit
Red Acquisition Red flat SDSSr,
1K x 1K
Blue Acquisition
Blue flat SDSSg,
1K x 1K


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.