A new 1200 l/mm blue-sensitive grating (1200B) is availble for the observing community starting on Semester 18A. The grating, manufactured by Richardson Grating Lab, arrived at Keck on August 17, 2017. It was installed in DEIMOS and integrated in the DEIMOS support software through Fall 2017.

On October 23, 2017, we observed the standard star BD+28 4211 with the 1200B and the 1200G gratings, respecitvely, under photometric conditions. We took slitless spectra at three different central wavelengths in order to compare the efficiency of both gratings. The throughput curves between 400nm and 935nm are shown in the DEIMOS spectroscopic throughput page.

The following figure shows a comparison between on-sky spectroscopic troughput of the 1200B and 1200G gratings at a central wavelength of 500nm with the GG400 blocking filter. The blue curve is the throughput for the 1200B grating, while the green curve represents the throughput for the 1200G grating. The 1200B grating is clearly more sensitive than the 1200G grating in the range between 400nm and 590nm. Note that most of the drop in efficiency between 450nm and 400nm is due to the drop in the detector quantum efficiency at blue wavelengths.

The following figure represents the ratio between the efficiency of the 1200B and the 1200G grating in the same wavelength range as in the previous figure. Note that the 1200B grating a factor 3.3 more sensitive at 450nm than the 1200G grating.