Contents

Overview

Instrument Configurations

Filters

Gratings & Prisms

Standard Facility Slit Masks

Multi-object Slit Masks

Imaging Performance

Spectroscopic Performance

Physical Characteristics

Overview

A dichroic below the slit splits incoming beam into separate red- and blue-optimized channels at a wavelength of 565nm. Each channel has separately optimized collimators, dispersers, cameras and detectors, allowing simultaneous operation across the entire wavelength region. The beam selector can also direct light into the red or blue channels alone, providing blue-/red-only modes to extend wavelength coverage into the dichroic cross-over region in one other channel.

The MODS science detectors are e2v CCD231-68 3Kx8K 15-micron pixel monolithic CCDs; blue-coated standard silicon on the blue channel and extended-red coated 40-micron deep depletion silicon on the red channel.

Basic Parameters

Design: Seeing-Limited Optical Double Spectrograph

Wavelengths: 320-1100nm

Field of View: 6x6-arcminutes (~3000x3000 pixels)

CCD Detectors: e2v CCD231-68 3072x8192, 15um Pixels

Pixel Scale:

Blue: 0.120 arcsec/pixel

Red: 0.123 arcsec/pixel

Operating Modes:

Direct Imaging, SDSS ugriz filters

Grating Spectroscopy R~2000

Prism Spectroscopy R=500-150

Slits: Laser-cut spherical slit masks (up to 24)

Dichroic: Blue-transmit/Red-reflect, 565nm cross-over wavelength

Instrument Configurations

There are three basic operating modes: 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.

Mode1	Channel	Filter(s)2	Resolution3 (0.6" Slit)	Wavelength Range (nm)	CCD Readout4	Notes
Direct Imaging	Dual	B: SDSS ug R: SDSS riz	n/a	330-1000	3072x3072	Simultaneous blue & red filter imaging with pairs of filters
	Blue	SDSS ug	n/a	330-700		extended red coverage
	Red	SDSS riz	n/a	400-1050		extended blue coverage
Grating Spectroscopy5	Dual	B: Clear R: GG495	B: 1850 R: 2300	320-1050	3088x8288	No coverage ~560-570nm in the dichroic cross-over GG495 suppresses leaks from very blue targets
	Blue	Clear	1850	320-600		Extended red coverage thru the dichroic cross-over
	Red	GG495	2300	500-1050		Extended blue coverage thru the dichroic cross-over
Prism Spectroscopy	Dual	B: Clear R: GG495	B: 420-140 R: 500-200	320-1050	3072x4096	No coverage ~560-570nm in the dichroic cross-over GG495 suppresses leaks from very blue targets
	Blue	Clear	420-140	320-600		Extended red coverage thru the dichroic cross-over
	Red	Clear	500-200	450-1050		Extended blue coverage thru the dichroic cross-over

Notes

1. Instrument configurations are requested in MODS observing scripts by specifying the channel configuration and mode name. For example, "instconfig dual grating" or "instconfig red imaging".
2. See MODS Filters for details on the current MODS filter set. At present we do not support addition of user-provided filters.
3. See MODS Gratings & Prisms for the detailed properties of the current suite of MODS gratings and prisms.
4. The CCD readout region is the default size for the mode. For imaging it includes the full FoV plus imaging stop, and for the spectroscopy modes it is sized for the blue and red extremes when used with MOS masks with slitlets extending to the outside edges of the mask. See MODS CCD Detectors for details on readout times, etc.
5. For initial operations only the primary grating tilt is available. This "primary" configuration is the optimal tilt to center the full grating spectral coverage on each science CCD.

Filters

All filters are mounted in the camera f/3 beam in front of the dewar window field flattener lens.

See MODS Filters for detailed specifications and transmission curves. Not all filter slots are occupied at this time, they are currently reserved for future expansion (e.g., additional grating order separation and prism band-limiting filters).

At this time we have not plans to support installation of user-provided filters.

Channel	Position	Filter ID1	Description	Use Notes
Blue	1	Clear	Fused Silica Clear	General Spectroscopic Mode Filter2
	2	g_sdss	SDSS g	Imaging
	3
	4
	5	UG5	UG5+Fused Silica	Red Blocker for UV spectral flats
	6	u_sdss	SDSS u	Imaging
	7	ND1.5	Neutral Density 1.5	Spectral Flats and Bright Comparison Lamps
	8
Red	1	Clear	BK7 Clear	General Spectroscopic Mode Filter2
	2	r_sdss	SDSS r	Imaging
	3
	4	i_sdss	SDSS i	Imaging
	5	GG495	Schott GG495	Red Grating Order Blocker
	6	z_sdss	SDSS z	Imaging
	7	ND1.5	Neutral Density 1.5	Spectral Flats and Bright Comparison Lamps
	8

Notes

1. The Filter ID is the name used to select the filter in MODS observing scripts. For example, "red filter r_sdss" or "blue filter nd1.5". Note that all Filter IDs are case-insensitive and have no spaces. It also stored in the FILTNAME keyword in the image FITS headers.
2. The clear filters in each channel are designed to ensure proper camera focus balancing between unfiltered and filtered configurations. The cameras cannot be used with no filter in the beam without substantial refocussing.

Gratings & Prisms

Each channel mounts up to 3 dispersers and an imaging flat in a 4-position turret.

See MODS Gratings & Prisms for detailed specifications and resolution and blaze curves.

Channel	Position	Grating ID1	Description	Resolution2 (0.6" Slit)	Wavelengths (nm)
Blue	1	Flat	Imaging Flat with Enhanced Al	n/a	320-600
	2	G400L	400 l/mm Blue Grating	1850	320-600
	3	P450L	FuSi+Al Double-Pass Prism	420-1403	320-600
	4		Position Empty4
Red	1	Flat	Imaging Flat with Protected Ag	n/a	320-600
	2	G670L	250 l/mm Red Grating	2300	500-1000
	3	P700L	TIH6+Ag Double-Pass Prism	500-2003	500-1000
	4		Position Empty4

Notes

1. The Grating ID is stored in the GRATNAME in image FITS headers.
2. Grating resolutions quoted is as measured at the reference wavelength (400nm for Blue, 760nm for Red). Prism resolutions are quoted for the full range of wavelengths, blue to red.
3. Prisms have continuously variable dispersion that declines roughly quadratically from blue to red.
4. Position 4 is currently unoccupied, reserved for a future third disperser (e.g., higher-resolution grating).

Standard Facility Slit Masks

Each MODS reserves the first 9 mask slots for the standard facility slit masks. See MODS Slit Masks for technical details.

Position	Mask ID1	Description
1	DarkMask	Dark Mask (uncut mask shell)2
2	Imaging	6x6-arcminute Direct-Imaging Field Stop3
3	LS5x60x0.3	0.3-arcsec Segmented long-slit4
4	LS5x60x0.6	0.6-arcsec Segmented long-slit
5	LS5x60x0.8	0.8-arcsec Segmented long-slit
6	LS5x60x1.0	1.0-arcsec Segmented long-slit
7	LS5x60x1.2	1.2-arcsec Segmented long-slit
8	LS60x5	60x5-arcsec spectrophotometric "fat slit"
9	SieveMask	Pinhole Sieve Mask (focus and geometric calibration)

Notes

1. The Mask ID is the name used to select the mask in MODS observing scripts (e.g., "slitmask ls5x60x0.6"). Mask IDs are case-insensitive and have no spaces in them. The Mask ID also stored in the MASKNAME keyword in image FITS headers.
2. The dark mask is used to block the view out of the instrument/telescope focal plane to measure internal scattered light, and to provide protection for the post-slit field lens when the instrument is put to sleep.
3. The imaging field-stop mask ensures proper baffling at the edges of the full science field of view.
4. Long-slit masks are segmented long-slits composed of a line of five (5) 60-arcsec long slits separated by 3-arcsec wide struts (with slit centers at 0, ±63, and ±126-arcsec from the nominal science field center). This is required to ensure the structural integrity of the spherical mask shells over their 190mm length.

Multi-Object Slit Masks

Observers may have up to 15 user-designed multi-object masks laser cut in special spherical mask material mounted in each MODS spectrograph per night. Unlike LUCI, masks may be exchanged in the cassette each afternoon.

The MMS mask design software, a modified version of the LUCI LMS program, is provided to help design MODS MOS masks.

Mask Material	150um thick electroformed NiColoy®1
Mask Surface Shape	Spherical (R=1040mm)
Mask Coating	Copper (II) Oxide (CuO) black
Slit Shape	Rectangular
Slit Lengths3	0.6-arcsec (360um) to 60-arcsec (36mm)
Slit Widths3	0.3-arcsec (180um) up to 10-arcsec (6mm)
Edge Smoothness	~1um rms over 180mm
Parallelism	<1% of width in 6mm (10") for 0.3-arcsec wide slit
End Shapes	Square

Notes

1. NiColoy® is a proprietary electro-deposited Nickel-Cobalt alloy made by the mask blank vendor, NiCoForm, Inc.

2. The MMS mask design program enscribes a unique 6-digit ID number in the upper left-hand corner of each mask. The Mask ID assigned to user-designed masks will be like "ID503123", similar to the practice with LUCI. It is this code that must be used in MODS observing scripts (e.g., "slitmakk id503123"), and it will appear as the value of the MASKNAME keyword in image FITS headers.

3. Minimum sizes are set by being at least as wide as the mask material's vertical thickness (150-180um). Maximum sizes are set by the size beyond which local out-of-plane flexure begins deform the slits (with a generous margin). We have not rigorously measured this for all batches of mask material to date, so we are being conservative.

Imaging Performance

Wavelength Range	320-1100nm (Atmospheric to CCD limits)
Field of View	~6.0x6.0 arcminutes (~2800x2800 pixels)
CCD Pixel Scale	Blue: 0.120 arcsec/pixel Red: 0.123 arcsec/pixel
Standard Filters	SDSS ugriz [Details] UG5 and GG495 blocking filters ND1.5 Spectroscopic Calibration
Throughput w/Dichroic1	Blue: 48% at 400nm Red: 62% at 800nm
Throughput w/o Dichroic1	Blue: 52% at 400nm Red: 67% at 800nm
Measured Image Quality (80%EE)	Blue Camera (400nm): 0.25-arcsec inner 2-arcmin (best focus) 0.43-arcsec inner 4-arcmin 0.55-arcsec 6-arcmin Red Camera (700nm): 0.29-arcsec inner 2-arcmin (best focus) 0.41-arcsec inner 4-arcmin 0.51-arcsec 6-arcmin

Notes

1. Total instrumental throughput plus detector and reference CCD detector QE, excluding telescope and atmosphere, based on laboratory measurements. On-sky commissioning estimates are in MODS Performance

Spectroscopic Performance

All performance numbers include the LBT primary and secondary mirrors and the atmosphere normalized to 1.2 airmasses.

Nominal Wavelength range	320-1100nm
Red/Blue Dichroic Cross-Over	565nm
Design Slit Width	0.6-arcsec (projects to ~4 pixels)
Maximum Slit Length	6-arcmin
Segmented Long-Slits	5x60x0.6, 0.8, 1.0, 1.2-arcsec
Dispersers	Gratings and Prisms
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)
Throughput w/dichroic	Blue Grating: 36% @ 400nm Red Grating: 38% @ 750nm Red Prism: 51% @ 750nm Blue Prism: 43% @ 400nm
Throughput w/o dichroic	Blue Grating: 39% @ 400nm Red Grating: 40% @ 750nm Red Prism: 54% @ 750nm Blue Prism: 48% @ 400nm

Physical Characteristics

Design	Double imaging spectrograph with a dichroic beamsplitter, reflective collimators, decentered Maksutov-Schmidt cameras.
Focal Stations	Left and Right Direct f/15 Gregorian Foci MODS1: LDG Focus (as of September 2010) MODS2: RDG Focus (coming in 2012)
Field of View	6x6-arcminute Science Field 5x6-arcminute off-center Guide Patrol Field
Focal Plane Scale	1.667 arcsec/mm
Science CCDs	Blue: e2v CCD231-68 3072x8196 (15um pixels) Std Silicon + Astro-Broadband coating Red: e2v CCD231-68 3072x8196 (15um pixels) 40um Deep-Depletion Silicon + Astro-ER1 coating
Imaging Pixel Scale	Blue: 0.120 arcsec/pixel Red: 0.123 arcsec/pixel
Slit Masks	24 slot mask cassette Slit masks are laser-machined in spherical electroformed NiColloy blanks 9 mask slots reserved for standard facility slit masks 15 mask slots available for user-designed slit masks
Long Slit Masks	Fixed long slits (0.6, 0.8, 1.0, and 1.2-arcsec width), curved to match the focal surface
Multi-Slit Masks	Up to 15 user-designed masks mounted in the cassette
Dispersers	R=2000 Gratings R=500-100 double-pass prisms
Imaging	Flat mirror replaces grating, 6x6-arcmin FoV
Filters	8-position filter wheels in each channel SDSS imaging and spectroscopic blocking filters
Collimator Focal Length	3450mm
Pupil Beam Diameter	230mm
Camera Design	Decentered Maksutov-Schmidt
Camera Focal Length	700mm
Dichroic Type	Blue Transmit/Red Reflect 38-layer dielectric
Dichroic Cross-Over	565nm
AGw Guide Camera	Steward e2v CCD57-10 512x1024 frame transfer CCD 50x50-arcsec FoV 4-position filter wheel: Clear, F525LP, Bessel-B, and ND1.0 filters
AGw Wavefront Sensor	Shack-Hartmann WFS Steward e2v CCD57-10 512x1024 frame-transfer CCD Potsdam 15x15 GRIN array 5x5-arcsec pickoff at the top of the guide field
Calibration System	Internal integrating sphere with lamps and projection optics QTH and variable-intensity incandescent flat lamps Hg, Ne, Ar, Kr, and Xe Pen-Ray wavelength calibration lamps
Approx Dimensions	~4.0x2.5 meters
Approx Mass	2700kg
Construction	Welded steel tube upper structure, welded/bolted steel-tube collimator support
Nominal Flexure	500um horizon-to-zenith, uncompensated
Flexure Compensation	Collimator tip/tilt/focus w/closed-loop IR reference laser feedback system
Flexure Specification	0.5-pixel rms motion over typical intergration (closed-loop)