The primary science drivers for NIFS require a two-pixel spectral resolving power of about 5000 to detect black holes in galactic nuclei, study the excitation and dynamics of the inner narrow-line regions of Seyfert galaxies, and study the dynamics of low mass disk galaxies at high redshift. A two-pixel resolving power of about 5000 is also sufficiently high to resolve much of the terrestrial OH airglow emission which plagues low resolution near-infrared spectroscopy. Greater sensitivity can be achieved by masking the OH emission-lines recorded in high resolution spectra in software. With a 2048 pixel detector, complete spectra in the H photometric band can be recorded with a two-pixel spectral resolving power of about 5340. Two 2048 pixel regions are required to record all of the J photometric band, and two regions are required to record all of the K band accessible from Mauna Kea with large overlap. Thus all regions of the 0.94-2.50 micron wavelength range accessible from Mauna Kea can be recorded using five fixed angle gratings.
An Ebert angle (i.e., camera-collimator angle) of 30° is required by the NIFS mechanical configuration. The off-the-shelf gratings listed below then produce the required central wavelengths in first order with grating angles of about 20°. A camera focal length is 288 mm. The collimator focal length is 421 mm.
|Grating||Central Wavelength||n||Blaze Angle||Grating Angle||R||Velocity Resolution||Wavelength Range|
The grating wheel will also contain a mirror to aid object acquisition.
Efficiencies for the NIFS gratings are shown above.
The NIFS spectrograph uses a Bouwers collimator and a refractive camera. The collimator has a focal length of 421 mm and the camera has a focal length of 288 mm. The Ebert angle is 30°. Anamorphic magnification at the grating causes the image scale to be different in the spatial and spectral directions; the scale is 0.04"/pixel along each slitlet, and each slitlet maps to 0.1" in the dispersion direction for the gratings used.
The NIFS system efficiency has been estimated assuming telescope mirror reflectivities of 98.6%, gold mirror reflectivities internal to the cryostat of 99%, and anti-reflection coatings on all lenses. A grating efficiency of 75% has been adopted, and the detector quantum efficiency varies from 51% to 62% over the 0.94-2.50 micron wavelength range. With these assumptions, the NIFS system efficiency including the Gemini telescope optics but excluding the transmission of the adaptive optics system is about 0.17 e-/photon at J, and 0.22 e-/photon at H and K. The detailed efficiency as a function of wavelength is shown in above figure with and without anti-reflection coatings on the lenses.