MODS Filters

Each channel of the MODS spectrograph has an 8-position filter wheel with removable filter cells. The filter wheel can accommodate two types of filters:

Imaging Filters: 86x86mm square filters up to 8mm thick, mounted in removable cells. The filters are specified for operation in a fast f/3 beam.
Spectroscopic Filters: Rectangular 182x86mm (long axis along the dispersion axis), up to 8mm thick, mounted in removable cells. Uses include order-blocking, band-limiting, focus balancing (e.g., clear), neutral density, etc.

At this time we have no provision for supporting user-provided filters in MODS1 or MODS2.

MODS1 Filters

Channel	Position	Filter ID¹	Description	Dimensions	Vendor	Use Notes
Blue	1	Clear	Fused Silica Clear	128x86x6mm	JML	General Spectroscopic Mode Filter²
	2	g_sdss	SDSS g	86x86x6mm	Barr	Imaging
	3	unused
	4	unused
	5	UG5	Schott UG5	128x86x6mm	JML	Red Blocker for UV spectral flats
	6	u_sdss	SDSS u	86x86x6mm	Barr	Imaging
	7	ND1.5	Neutral Density 1.5	128x86x6mm	Omega	Spectral Flats and Bright Comparison Lamps
	8	unused

Channel Position Filter ID¹ Description Dimensions Vendor Use Notes

Red 1 Clear BK7 Clear 128x86x6mm JML General Spectroscopic Mode Filter²

2 r_sdss SDSS r 86x86x6mm Barr Imaging

3 unused

4 i_sdss SDSS i 86x86x6mm Barr Imaging

5 GG495 Schott GG495 128x86x6mm JML Red Grating 2nd Order Blocker

6 z_sdss SDSS z 86x86x6mm Barr Imaging

7 ND1.5 Neutral Density 1.5 128x86x6mm Omega Spectral Flats and Bright Comparison Lamps

8 unused

Notes

The Filter ID is the name used to select the filter with the MODS data-taking system. For example, "red filter r_sdss" or "blue filter nd1.5". Note that all Filter IDs are case-insensitive and have no spaces.
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.

Imaging Filters

The SDSS u and g filters are located in the Blue Camera filter wheel, while the r, i, and z filters are located in the Red Camera filter wheel.

SDSS Set #1:

Format:

Size:	86x86mm
Thickness:	6mm
AR Coating:	Standard
Vendor:	Barr Associates

Filter Characteristics:

Filter	Effective Center (nm)	Effective Width (nm)	T(peak)	Data
u	359.4	40.88	0.60	[ Raw Tx]
g	473.2	149.6	0.92	[ Raw Tx]
r	622.0	129.2	0.97	[ Raw Tx]
i	764.8	147.8	0.97	[ Raw Tx]
z	905.3	135.9	0.96	[ Raw Tx]

Notes:

The filter effective wavelengths are derived from the measured delivered filter transmission curves, using the formalism described by Schneider, Gunn, & Hoessel 1983, ApJ, 264, 337. The effective widths are computed in the usual way (i.e., the width of an ideal square-profile filter with the same peak transmission and effective area of the detailed transmission curve). Note that the "effective" width is more representative of the filter bandwidth than the FWHM.
"Raw Tx" is the raw transmission curve of the filter measured in parallel light by the filter vendor. Scale is 0 (opaque) to 1 (perfect).

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Updated: 2011 May 4 [rwp]