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Filters

The filter sets installed in GMOS-N and GMOS-S are largely redundant and therefore the previously separate webpages have been combined. It is also possible to install User-supplied filters in GMOS.

The GMOS filters u', g', r', i' and z' are similar to the filters used by the Sloan Digital Sky Survey (SDSS). A description of the SDSS photometric system is presented in Fukugita et al. (1996, AJ, 111, 1748). See the description of optical photometric standard stars for details on the photometric standard calibration. Please note that due to filter degradation discovered in semester 2004A the u' filter is no longer available for science use on GMOS-N. We are considering replacing this filter in the near future.

The filter CaT is a 150nm broad-band filter approximately centered on the Ca Triplet.

The Z and Y filters are similar to those installed in the United Kingdom Infra-Red Telescope (UKIRT) Wide Field Camera (WFCAM). A description of these filters is available from the UKIRT WFCAM user guide, see Table 1.1. The Z filter is designed to mimic the SDSS z' bandpass with red sensitive CCDs, while the Y filter occupies the clean wavelength range between the atmospheric absorption bands at 0.95µm and 1.14µm. Currently these filters are only offered with GMOS-N, though a set has been purchased for GMOS-S and will be installed in the near future.

The filters Ha, HaC, OIII, OIIIC, SII, HeII, and HeIIC are narrow-band filters for H-alpha, H-alpha continuum, ionized oxygen, ionized oxygen continuum, ionized sulphur, ionized helium, and ionized helium continuum, respectively.

The filter DS920 is a narrow-band filter centered on a wavelength region of very low sky background. This filter is only available with GMOS-N, and may be removed to make room for a u' filter in the future.

The filters GG455, OG515, RG610 and RG780 are 5mm thick Schott glass filters. These are long-pass filters which can be used to block 2nd-order contamination of the spectra or limit the wavelength range to allow multiple slit-banks in the multi-aperture mode. The RG780 filter is only available with GMOS-S.

Transmission curves for each filter set based on lab measurements are available: GMOS-N filters and GMOS-S filters. The new Z and Y filters will be added once they have been characterized.

The table lists the properties of the filters based on laboratory measurements. The wavelength interval given in the tables is the interval within which the transmission is larger than 50 per cent. For the long-pass filters and the z' filters the wavelength at the 50 per cent transmission at short wavelength edge is given.

Some of the filters can be combined, the allowed combinations are listed last in the table. The combinations are useful for using the IFU in 2 slit mode with the higher dispersion gratings, e.g. R831 with the filter combination r' + RG610.

Imaging with GMOS requires use of either a color filter (griz, CaT, Z, Y) or a narrow band filter. The long-pass filters GG455, OG515, RG610, and RG780 are not suitable for imaging. Also imaging without a filter is not supported since neither GMOS is equipped with an Atmospheric Dispersion Corrector (ADC) at this time.

Users may supply their own filters for queue observations; see filter policy and specifications for details.

 

GMOS Filters
Filter Filter number*
[North / South]
Effective
wavelength
[nm]
Wavelength
interval
[nm]
Transmission data
[North / South]
Broad Band Imaging Filters
u G0308
(no longer available)
350 336-385 GMOS-N data
G0332 GMOS-S data/plot
g G0301 475 398-552 GMOS-N data
G0325 GMOS-S data/plot
r G0303 630 562-698 GMOS-N data
G0326 GMOS-S data/plot
i G0302 780 706-850 GMOS-N data
G0327 GMOS-S data/plot
CaT G0309 860 780-933 GMOS-N data
G0333 GMOS-S data/plot
z G0304 >=925 >=848 GMOS-N data
G0328 GMOS-S data/plot
Z G0322 876 830-925 WFCAM Z
not available not available
Y G0323 1010 970-1070 WFCAM Y
not available not available
Narrow Band Imaging Filters
HeII G0320 468 464-472 GMOS-N data
G0340 GMOS-S data/plot
HeIIC G0321 478 474-482 GMOS-N data
G0341 GMOS-S data/plot
OIII G0318 499 496.5-501.5 GMOS-N data
G0338 GMOS-S data/plot
OIIIC G0319 514 509.0-519.0 GMOS-N data
G0339 GMOS-S data/plot
Ha G0310 656 654-661 GMOS-N data
G0336 GMOS-S data/plot
HaC G0311 662 659-665 GMOS-N data
G0337 GMOS-S data/plot
SII G0317 672 669.4-673.7 GMOS-N data
G0335 GMOS-S data/plot
DS920 G0312 920 912.8-931.4 GMOS-N data
not available not available
Spectroscopy Blocking Filters
GG455 G0305 >=555 >=460 GMOS-N data
G0329 GMOS-S data/plot
OG515 G0306 >=615 >=520 GMOS-N data
G0330 GMOS-S data/plot
RG610 G0307 >=710 >=615 GMOS-N data
G0331 GMOS-S data/plot
RG780 not available >=880 >=780 not available
G0334 GMOS-S data/plot
Allowed Filter Combinations
  g + GG455 506 460-552  
  g + OG515 536 520-552  
  r + RG610 657 615-698  
  i + CaT 815 780-850  
  z + CaT 890 848-933  

*The filter name is the concatenation of filter and filter number e.g. HeII_G0340 for the GMOS South HeII narrow-band filter.