MODS

MODS1 and MODS2 are UV to near-IR (0.32-1.1 micron) multi-object double spectrographs mounted at the left and right direct Gregorian focal stations of the LBT. They were designed and built by The Ohio State University Department of Astronomy in Columbus, Ohio. The full field of view of each MODS is 6’ x 6’, and at the focal plane, either a custom multi-object slit mask, one of the 0.3”, 0.6”, 0.8”, 1.0”, 1.2”, 2.4” or 5″ long-slits or an empty mask frame for imaging can be inserted. Light can go straight through to the blue channel, be reflected to the red channel, or be split by a dichroic at 565 nm and directed to both blue and red channels. Both blue and red channels contain medium resolution (R~2000) gratings and low resolution (R~500-150) double-pass prisms, as well as flats for imaging at the Sloan u and g filters (blue channel) and the Sloan r, i and z filters (red channel).

Both MODS underwent significant upgrades in August 2025. The original custom-built head electronics were replaced with Archon controllers for the detectors and ‘mini-HE’ boxes to assume the other functions, including reading the IR quad cell used by the IMCS. The 15-year old instrument control and data-handing computers were also replaced.

This upgrade has occasioned changes in the way investigators prepare their observations (see Preparing to Observe), in the images and data files produced (see Detector and Looking at your Data) and in the way observers run the instrument (see Observing with MODS). (The linked pages are being updated 14-Apr-2026.) Data produced by the upgraded MODS will be stored in multi-extension FITS files which have a new keyword, CCDCTRLR, with the value “Archon” to differentiate them from old data.

• Optical Design: Seeing-limited, dichroic-split double-beam grating spectrometer
• Wavelength Coverage: 3200 — 10000Å
• Field of View:~ 6×6-arcminutes
• Pixel Scales:
  • Blue: 0.120 arcsec/pixel
  • Red: 0.123 arcsec/pixel

Mode	Channel	Filter(s)	Wavelength Range (nm)	CCD Readout	Notes
Direct Imaging	Dual	B:SDSSug R:SDSSriz	330-1000	3088 x 3088	Simultaneous blue & red filter imaging with pairs of filters
	Blue	SDSS ug	330-600		extended red coverage
	Red	SDSS riz	500-1000		extended blue coverage
Grating Spectroscopy (R~2000 for 0.6″ slit)	Dual	B: Clear R: Clear	320-1000	8288 x 3088	Lower s/n at the dichroic crossover wavelength at 5650Å.
	Blue	B: Clear	320-600		Extended red coverage thru the dichroic cross-over
	Red	R: GG495	500-1000		Extended blue coverage thru the dichroic cross-over
Prism Spectroscopy (R~150-500 for 0.6″ slit)	Dual	B: Clear R: GG495	320-1000	4096 x 3088	Lower s/n at the dichroic crossover wavelength at 5650Å. GG495 suppresses leaks from very blue targets
	Blue	B: Clear	320-600		Extended red coverage thru the dichroic cross-over
	Red	R: GG495	500-1000		Extended blue coverage thru the dichroic cross-over