Gratings: 2nd Order

Any of the GMOS gratings can be used in 2nd order; however only a few configurations are potentially interesting and none have been fully characterized. Prompted by a recent inquiry from the user community, we have recently made some daytime testing of the R831 grating in 2nd order. This mode offers higher resolution than delivered by the B1200 grating although the wavelength range is limited by the necessity to use broad-band color filters to block 1st order. Neither GMOS is currently equipped with red-blocking (short-pass) filters.

The configuration tested was R831 in 2nd order, central wavelength = 440 nm, using the g'-filter for order blocking. A Nod&Shuffle 1.0arcsec longslit data was used, which allowed for a characterization of scattered light beyond the edge of the slit. The detector was binned 2x2. The test data are available for download from the Gemini Science Archive. These test data were taken with GMOS-N:

These daytime tests were encouraging, and users whose science would benefit from higher resolution and do not need full wavelength coverage may wish to consider this combination. The R831 grating efficiency in 2nd order is comparable to that of the B1200 grating in 1st order. The g-filter unfortunately introduces a drop in throughput of about 15% because its peak transmission is 85%. The resolution compared to the B1200 grating increases by about 40% as expected, and the amount of scattered light contribution per pixel is comparable.

Please note: we have yet to confirm the behavior of the Gemini GMOS IRAF data reduction scripts with 2nd order spectral data.

   

GCALflat spectra for B1200 1st order (left) and R831 2nd order (right) at a central wavelength of 440nm. Both configurations use the g'-filter, the short wavelength filter cut-off is evident on the right edge of the spectra. The dispersion of the R831 2nd order data is 40% higher than that of the B1200 data.

   

CuAr spectra for B1200 1st order (left) and R831 2nd order (right), same configuration as the images above. The decreased spectral coverage due to the higher dispersion is evident in the R831 2nd order spectrum compared to the B1200 1st order spectrum, as is the comparable grating throughput.

   

2D spectral images of the B1200 1st order (left) and R831 2nd order (right) CuAr data zoomed to cover approximately the same small wavelength range. The higher resolution in the R831 data (R = 3250) compared to the B1200 data (R = 2125) is evident from the smaller width of individual lines and the clear separation of certain lines which in the B1200 1st order data are blended. The resolution values quoted are derived from FWHM values measured using IRAF gaussian fits to the CuAr lines. These data were taken using a 1.0 arcsec wide slit; the delivered resolution would increase by a factor of 2 with a 0.5 arcsec wide slit.