General Description of NIRSPEC on Keck


General: NIRSPEC is a near-infrared cross-dispersed echelle and grating spectrometer operating at the Nasmyth focus on Keck II. NIRSPEC will be available to the Gemini community on a limited basis during semesters 2000A - 2001B. The instrument features a 1024-squared InSb detector array (given by Gemini) and provides spectroscopy at resolving powers of R~1,500-3,000 or R~15,000-75,000 over the 1-5µm wavelength range. The observer uses an instrument configuration screen to specify the observing mode and monitor the instrument status and a data acquisition screen to choose the spectral region to be observed and monitor the progress of the observations. Array images in both grating and echelle modes are available from first-light observations made in late spring 1999.
Echelle Spectra: When the echelle is used orders are approximately horizontal on the array, although somewhat slanted. The total bandwidth of the spectrum is about 18 percent. However, at wavelengths longer than 1.04µm, each order of the echelle extends beyond the edges of the array, so more than one echelle setting is required to obtain a continous spectrum. Echelle programs seeking one spectral line only require one echelle setting (such that the desired line falls on the array). Programs seeking full high resolution spectra in a wavelength region or several lines will require multiple settings of the echelle at a fixed setting of the cross-disperser. The number of settings, available in the Sensitivity Table, must be stated explicitly in the proposal form (in the "Observing Details" section) and factored into the time estimate.
Grating spectra: When the echelle is removed from the optical path, the cross dispersing grating forms a low resolution spectrum running roughly vertically on the array. The bandwidth of this spectrum is approximately 18 percent. Blocking filters prevent the transmission of unwanted orders.
Slit Width: NIRSPEC contains a number of slits of different widths. Only one slit may be used at a time. As in most grating spectrometers there is a trade-off between throughput and spectral resolution. The highest resolving powers for both the grating and the echelle require the narrowest slits, but use of narrower slits results in loss of light and sensitivity. Note also that when using the narrowest slit, the 1-pixel wide slit (available with the echelle only), the spectral sampling is one point per resolution element.
Slit Orientation: NIRSPEC contains an image rotator which allows any position angle to be aligned with the slits. The image rotator tracks to compensate for field rotation.
Acquisition and Guiding: In addition to the InSb array used with the spectrograph, NIRSPEC has a standard CCD camera for acquisition and offset guiding; its FOV is an annulus of inner diameter 1' and outer diameter 8'. NIRSPEC also contains a slit-viewing 1-2.5µm camera (SCAM) with FOV ~ 1' in diameter. The field of the CCD camera is large enough that finding a star for offset guiding is not an issue. If the source is sufficiently bright at near IR wavelengths (J<18, H<17, K<16) , it is possible to guide on the light reflected from the slit jaws using the SCAM. If not it is helpful and possibly essential that the investigators supply an accurate offset from a nearby star brighter than the above limits. It is especially helpful, but not essential if such a star is located in the FOV of the SCAM. Non sidereal offset guiding is available at Keck.
NIRSPEC + Adaptive Optics:

We are not offering NIRSPEC+AO to the Gemini community.


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Last update September 1, 2001; Tom Geballe