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Description of Optical Path

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Description of Optical Path

The GNIRS design is fairly conventional for an infrared spectrograph. There are two main sections: a fore-optics section, which re-images the telescope focal plane onto the slit (or IFU) and provides field and pupil stops to limit excess background, and a spectrograph section, which collimates, disperses and re-focuses the light from the object of interest onto the detector.


The fore-optics comprise the following elements:

  • Entrance window
  • Pick-off mirror
  • Entrance fold mirror
  • Offner relay (primary and secondary mirrors)
  • Exit fold mirror
  • Filter wheels

The entrance window also acts as a weak lens, in order to ensure that the telescope secondary is imaged on the Offner secondary mirror, which is where the cold stop is placed.

The pick-off mirror acts as a crude field stop, defining the field accessible to the spectrograph. Light from the rest of the instrument field (roughly 3 arcminutes diameter) is available in principle to the OIWFS. The unvignetted field defined by the pick-off mirror is basically a 10 x 100 arcsecond strip with a superposed half circular field 15 arcseconds in radius centered on the optical axis, as shown schematically below:.

GNIRS Bench Image GNIRS Bench Image

The purpose of the semi-circle is to provide a somewhat larger field for target acquisition and identification, while at the same time allowing use of guide stars close to targets of interest. The pick-off mirror is tilted at 45 degrees, and is therefore exactly at the telescope focus only in the center of the slit. The mirror widens away from its center to ensure that the spectrograph field is unvignetted. The field available for guiding is discussed in section 2.1.3.

The Offner relay serves two functions: it produces an image of the telescope secondary on the Offner secondary, where a cold stop is located, and it re-images the telescope focal plane onto the spectrograph slit. The scale at the slit is the same as at the telescope focal plane.

The spectrograph contains two filter wheels. Each wheel can accommodate 9 filters, in addition to an "open" position. The first wheel uses several of the positions for focus masks, a dark position, and a lens used to view the telescope pupil during alignment (see below). These can be used in series with filters in the second wheel. The remaining positions in the first wheel can be used for back-up filters.

The current filter complement for the instrument is listed on the GNIRS filters page. The filters are slightly tilted (2.7 degrees) to reduce ghost images; this is also why the filters precede the slit.

Because the filters are located in a converging beam, they all have the same optical thickness in order to avoid refocusing the telescope each time a filter is changed. This also ensures that filter changes keep the object centered on the slit. Any user-supplied filters must have the same thickness (equivalent to 3 mm of BK7) in order to operate properly. 

Spectrograph optics

The spectrograph entrance slit is defined by two mechanisms. The width of the slit is defined by the one of several slits in a photo-etched mask located in the slit slide, while the length of the slit is defined by one of several openings in the decker slide. The integral field unit (IFU) is also mounted in the slit slide; there is a location for a second IFU unit, currently occupied by a dummy module of similar mass.

The slit mask is located at the re-imaged focal plane, while the decker apertures are slightly ahead of it, and therefore somewhat out of focus (by a few pixels). The decker sizes are matched to the full width of the array in long slit mode, or to the minimum spacing between adjacent spectra when the prisms are used. The slit mask in the instrument can be changed, although it should not be considered a routine operation. The slit widths and lengths currently available are listed on the GNIRS slit properties page.

In addition, the slit slide can be positioned to use the integral field unit or to use the pupil viewer; for the latter a second lens is placed in the beam (used in series with the lens in filter wheel 1).

The next element after the slit and decker is the collimator, an off-axis paraboloid of 1500 mm focal length. The collimator mount includes a system of adjustable weights, which provide partial compensation for internal flexure in the instrument. This is a passive system, where gravity acts on a set of weights and levers to tilt the mirror slightly with varying orientation of the instrument. The largest corrective tilt of the mirror is less than 7 arcseconds.

After the collimator, a mirror can be inserted in the beam to direct the light into the spectrograph cameras, without being dispersed. This acquisition mirror allows the observer to identify, acquire, or re-center objects via broadband imaging, without the need to alter grating and prism tilts. This facilitates prolonged observing sequences on faint objects, since the dispersive settings remain stable even while target positions are checked.

If the acquisition mirror is out of the beam, light goes from the collimator to the prism turret. The prism turret has four possible positions.

  • The mirror is used for work beyond 3um, or when one wants to work long-slit at shorter wavelengths. 
  • The two cross-dispersion prisms provide a cross-dispersed low resolution spectrum over the approximate range 0.9-2.4um, where the two prisms are matched to the two pixel scales produced by the cameras. A complete spectrum is produced at a resolution of ~1700 (2 pixels); use of higher spectral resolution results in more or less parallel portions of multiple orders but not a complete spectrum. 
  • The Wollaston prism separates the two linear polarization components of the light, and is usable through the L band. Because there is substantial internal polarization in the spectrograph itself, the Wollaston prism configurations must be used with GPOL on the telescope's up-looking port. Note that currently there are no plans to commission the polarization mode.

From the prism turret, light goes to the grating turret. The grating turret contains three gratings. All three gratings are blazed for 6.8um (first order Littrow), which provides an effective first order blaze wavelength of 6.6um in the configuration actually used (scattering angle of 27 degrees).

The different orders of the gratings then correspond fairly well to the atmospheric windows at 5, 3.5, 2.2, 1.6 and 1.2 um for orders 1 through 5 respectively; the order-sorting filters cover the free spectral range of the individual orders, with some allowance for filter roll-off. A filter for order 6 is also supplied; the orders above 5 don't match the atmosphere particularly well.

The resolutions provided by the gratings are tabulated on the GNIRS gratings page. The values given are with the gratings operated at the blaze peak. Tilts to longer wavelength provide somewhat higher resolution, while tilts to shorter wavelengths provide lower resolution. (The resolution in wavelength units is nearly constant for a given order, regardless of tilt.)

The quoted resolutions are all for 2 pixels at the detector, specified as l/Dl. The detector is 1024 x 1024 pixels, so there are roughly 512 resolution elements in the dispersion direction. For the R=1700 mode, this corresponds to coverage Dl/l of roughly 30%.

From the grating turret, light then passes to the camera turret. The camera turret contains four cameras.

The blue cameras will not work at longer wavelengths; the red cameras can be used at shorter wavelengths, but with somewhat degraded image quality and transmission. The main short wavelength use of the red cameras below 3um is for acquisition of targets in the K band (order-sorter 3).

All four cameras are close to parfocal; the longer focal lengths are achieved by folding the beam with a combination of mirrors in the camera barrel and external to the turret. The light path shown in the schematic diagram if for one of the long cameras, so one can see the folded light path.

The detector is mounted at the output of the cameras, on a focus stage. The focus stage provides correction for the small focus differences between the different cameras (and potentially other small focus changes produced by other changes in configuration). The detector is a 1K x 1K ALADDIN III InSb array, which is operated at a temperature of approximately 31K.

Last update December 10, 2005; Bernadette Rodgers and Greg Doppmann

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