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Department of Astronomy

OSMOS Instrument Characteristics

Baseline Operating Modes
Physical Characteristics
Expected Imaging Performance
Expected Spectroscopic Performance
Multi-object Slit Masks

Baseline Operating Modes

Spectroscopic modes include long-slit and multi-slit modes. The details of these modes are still under development. Potential OSMOS users interested in particular spectroscopic capabilities are strongly encouraged to contact Paul Martini as soon as possible.

Mode Scale Resolution Total Range
Coverage Order Notes
Low-Res Grism 3.6 pixel/arcsec 1000-2000 TBD TBD 1st 1
Triple Prism var. 550-40
350-1000 all N/A ...
Imaging Filters 350-1000 Filter N/A ...
Medium-Res Grism TBD 3000-5000 TBD TBD 1st 2


  1. Currently being designed
  2. A grism with these parameters is planned for the future (post deployment)

Physical Characteristics

Design All refractive, spherical optics
Focal Stationf/7.8 RC at MDM 2.4m telescope
Focal Surface Scale11.528 arcsec/mm
Approx Dimensions1.5x0.5 meters
Approx Mass160kg
ConstructionOptical bench supported by welded steel frame
Flexure goal4um P-V (~1/4 pixel) over a 1hr integration
Slit Masks6 position mask weel, spherical surface to match focal-plane curvature, slits laser-machined in electroformed NiColloy masks approximately 105mm in diameter
Long SlitsFixed long slits (0.6, 0.9, and 1.2-arcsec width corresponding to 2, 3, and 4 pixels or 52, 78, 104 microns physical size) that are curved to match the focal surface
SlitletsArbitrary size, shape, and orientation
Dispersers6 position disperser wheel with one 100x85mm direct vision prism and up to 4 VPH grisms (TBD)
ImagingOpen aperture in slit and disperser wheel
FiltersTwo 6 position filter wheels tilted by 8 degrees, 100x100mm cells (compatible with MDM 'Buckeye' filter wheels).
Collimator Designf/7.8, 14 degree FOV Double Gauss optimized for telescope input
Collimator Pupil Relief170mm
Collimator Focal Length430mm
Collimated Beam Diameter55mm
Camera Designf/4.9, 18 degree FOV Petzval with pupil relief and focal plane relief
Camera Focal Length270mm
Camera Pupil Relief1100mm
Detector Focal Plane Relief150mm
Detector Focal Plane Scale18 arcsec/mm, 55microns/arcsec
Imaging Pixel Scale0.273 arcsec/pixel (15 micron pixels)
(De)magnification1.6x reimaging of telescope focal surface
CCD4096x4096 (15 micron pixels) STA0500A thinned Silicon
Guider/AcqusitionMDM guider, acquisition via imaging through slit
Calibration SystemExisting MIS (Multi Instrument System) calibration lamps
User AccessFilter wheels, Slit Mask cassette
  1. Relief is the distance from a given location to the nearest optical element

Expected Imaging Performance

Wavelength Range350-1000nm
Field of View20 arcminute diameter circle
CCD Pixel Scale0.273 arcsec/pixel (15um pixels)
Standard FiltersMDM filter set
Throughput152% at 350nm, 62% from 400-1000nm
Designed Image Quality (80%EE)
U band: 0.6 arcsec (zenith3), 0.9 arcsec (40 deg zenith angle3)
B band: 0.3 arcsec
V band: 0.3 arcsec
R band: 0.3 arcsec
I band: 0.4 arcsec
350-1000nm (polychromatic): 0.5 arcsec
Expected Count Rates2Blue: TBD e-/sec for B=21mag
Red: TBD e-/sec for R=21mag


  1. Transmission through OSMOS collimator and camera only, excluding atmospheric transmission, telescope coating, and CCD quantum efficiency. 2% AR coatings on all air-glass surfaces are assumed (although the goal is 1.5% coatings).
  2. Magnitudes are at the top of the telescope, rates are for standard Kron-Cousins B and R filters.
  3. Includes atmospheric dispersion

Expected Spectroscopic Performance

Max Slit Length20 arcminutes
Total Slitlets50-100 slits/mask
CCD Pixel Scale0.273 arcsec/pixel
Wavelength range350-1000nm
Fixed Long-Slits0.6, 0.9, 1.2-arcsec (2, 3, and 4-pixel width)
Gratingsup to 4
Planned Gratings R=1000 (params TBD)
R=5000 (params TBD)
Expected Counts/resolution element TBD
S/N on point source in 1 hour TBD

Multi-Object Slit Masks

Mask Surface ShapeSpherical, matching f/7.8 focal plane
Slitlet ShapeArbitrary shape & size
Slitlet OrientationTilted up to +/-30° from vertical
Slitlet Length>0.6arcsec (>52um), typically <20" (1.73mm)
Slit Widths>0.6-arcsec (>52um), typically 1.2-arcsec (104um)
Edge Smoothness~1.8um rms (2% of 1.2-arcsec wide slit)
Slit-to-Slit Position Tolerance5um rms perpendicular (5% of 1.2-arcsec width)
150um rms parallel (10% of 20-arcsec length)
End ShapesSquare or Round
Cost per maskEstimated to be $100-$200
Mask GenerationLaser machine at the University of Arizona (primarily used for LBT instruments)


  1. Masks will be laser-machined and mounted in metal frames for loading into the slit mask wheel. The substrate will be electroformed NiColloy metal (Nickel-Cobalt alloy) coated with copper black.
  2. During manufacturing, a barcode could be applied for tracking purposes. This barcode would be read with a hand scanner when the mask is stored off-instrument.

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Updated: 2008 May 23 [rz/pm]