Science Operations


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 and Spectroscopic Filters. Each are summarized below.

Imaging Filters

The imaging filters are 86x86mm square filters. These filters are up to 8mm thick, mounted in removable cells. The filters are specified for operation in a fast f/3 beam. They are located in filter wheels specific to the blue or red camera, with the SDSS u and g filters located in the Blue Camera filter wheel, and the r, i, and z filters located in the Red Camera filter wheel. The following table summarizes the specifications for each of the filters as well as contains links to their filter curves.

Although both MODS1 and MODS2 use SDSS filters, there are slight differences. MODS1 uses Gen1 SDSS filters made by Barr Associates whereas MODS2 uses Asahi Gen1 SDSS filters. The MODS2 z filter is a long-pass filter without the interferometric red cut off of the MODS1 Barr z filter. Also, the factory-measured throughput of the MODS2 Asahi SDSS u filter is significantly higher than that of the MODS1 Barr SDSS u filter.

The tables below list the pivot and mean wavelengths, the effective widths and the FWHM for each filter used in both direct and dichroic modes. These terms are defined in Appendix B of the MODS Instrument Manual and references therein. The links under filter name go to plots of the filter transmission curves – raw, as measured at the factory and the calculated throughputs, for direct and dichroic mode, which include the optical elements in the instrument and the detector quantum efficiencies but exclude the telescope and atmosphere. The linked *txt files are the ascii tables used to generate the plots, where the 4 columns are: wavelength, raw (factory) transmission, and the efficiencies of the filter and instrument combination in both direct and dichroic modes.

Table 1: MODS1 Imaging Filters

Channel FilterID Name Mode Pivot Wavelength
Effective Width
Mean Wavelength
λ (Å)
Blue Camera u_sdss SDSS u
Direct 3589.4 415.4 3580.8 480
Dichroic 3592.9 408.8 3584.5 470
g_sdss SDSS_g
Direct 4767.4 1010.3 4728.7 1480
Dichroic 4773.8 1017.3 4735.3 1460
Red Camera r_sdss SDSS_r
Direct 6283.3 940.9 6257.8 1350
Dichroic 6323.4 878.7 6301.3 1270
i_sdss SDSS_i
Direct 7646.5 1018.5 7522.0 1500
Dichroic 7651.0 1014.4 7626.6 1500
z_sdss SDSS_z
Direct 8954.9 892.1 8938.7 1100
Dichroic 8952.9 890.8 8936.7 1100

Table 2: MODS2 Imaging Filters

Channel FilterID Name Mode Pivot Wavelength
Effective Width
Mean Wavelength
λ (Å)
Blue Camera u_sdss SDSS_u
Direct 3546.9 491.9 3534.7 480
Dichroic 3558.6 468.6 3547.6 470
g_sdss SDSS g
Direct 4731.9 1001 4693.4 1480
Dichroic 4729.6 995.8 4691.5 1460
Red Camera r_sdss SDSS r
Direct 6241.1 956.3 6214.6 1350
Dichroic 6294.8 874.6 6272.9 1270
i_sdss SDSS i
Direct 7636.8 967.7 7614.6 1500
Dichroic 7639.4 964.0 7617.4 1500
z_sdss SDSS z
Direct 8969.5 1008 8948.8 1120
Dichroic 8968.4 1009 8947.7 1100

Spectroscopic Filters

The spectroscopic filters in MODS are rectangular filters, 182x86mm, with the long axis aligned with the dispersion axis. These filters include order-blocking, band-limiting, focus balancing (e.g., clear), neutral density, etc.


Channel FilterID Description Use Pivot Wavelength
Effective Width
Mean Wavelength λ(Å)
Blue Camera Clear AR coated Fused Silica Dual- and single-channel Spectroscopy 3589.4 415.4 3580.8
UG5 UG5 + Fused Silica Red blocker for UV spectral flats 4767.4 1010.3 4728.7
ND1.5 Neutral Density 1.5 Spectral flats and comparison lamps
Red Camera Clear AR coated BK7 Dual-channel Spectroscopy 6283.3 940.9 6257.8
GG495 Schott GG495 2nd order blocking filter 7646.5 1018.5 7522.0
ND1.5 Neutral Density 1.5 Spectral flats and comparison lamps 8954.9 892.1 8938.7