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 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 λP(Å) |
Effective Width δλ(Å) |
Mean Wavelength λ (Å) |
FWHM (Å) |
|---|---|---|---|---|---|---|---|
| Blue Camera | u_sdss | SDSS u sdss_u_mods1.txt |
Direct | 3589.4 | 415.4 | 3580.8 | 480 |
| Dichroic | 3592.9 | 408.8 | 3584.5 | 470 | |||
| g_sdss | SDSS_g sdss_g_mods1.txt |
Direct | 4767.4 | 1010.3 | 4728.7 | 1480 | |
| Dichroic | 4773.8 | 1017.3 | 4735.3 | 1460 | |||
| Red Camera | r_sdss | SDSS_r sdss_r_mods1.txt |
Direct | 6283.3 | 940.9 | 6257.8 | 1350 |
| Dichroic | 6323.4 | 878.7 | 6301.3 | 1270 | |||
| i_sdss | SDSS_i sdss_i_mods1.txt |
Direct | 7646.5 | 1018.5 | 7522.0 | 1500 | |
| Dichroic | 7651.0 | 1014.4 | 7626.6 | 1500 | |||
| z_sdss | SDSS_z sdss_z_mods1.txt |
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 λP(Å) |
Effective Width δλ(Å) |
Mean Wavelength λ (Å) |
FWHM (Å) |
|---|---|---|---|---|---|---|---|
| Blue Camera | u_sdss | SDSS_u sdss_u_mods2.txt |
Direct | 3546.9 | 491.9 | 3534.7 | 480 |
| Dichroic | 3558.6 | 468.6 | 3547.6 | 470 | |||
| g_sdss | SDSS g sdss_g_mods2.txt |
Direct | 4731.9 | 1001 | 4693.4 | 1480 | |
| Dichroic | 4729.6 | 995.8 | 4691.5 | 1460 | |||
| Red Camera | r_sdss | SDSS r sdss_r_mods2.txt |
Direct | 6241.1 | 956.3 | 6214.6 | 1350 |
| Dichroic | 6294.8 | 874.6 | 6272.9 | 1270 | |||
| i_sdss | SDSS i sdss_i_mods2.txt |
Direct | 7636.8 | 967.7 | 7614.6 | 1500 | |
| Dichroic | 7639.4 | 964.0 | 7617.4 | 1500 | |||
| z_sdss | SDSS z sdss_z_mods2.txt |
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 λP(Å) |
Effective Width δλ(Å) |
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 |
