MODS Overview
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).
Image of MODS1 & MODS2 while the telescope is pointed at horizon, taken from Level 9. Photo credit: Rick Pogge.
- 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 |
