INSTRUMENT LAYOUT

An All-reflective Integral Field Spectrograph for Far Ultraviolet

AstrophysicsStephen E. Kendrick1, Dennis Ebbets1, Chuck Hardesty1, Ken Sembach2, Matt Beasley3, Bruce Woodgate4

1 Ball Aerospace & Technologies Corp., 2Space Telescope Science Institute, 3University of Colorado, 4NASA Goddard Space Flight CenterABSTRACT

• This paper overviews the supporting optical technologies for an ultraviolet integral field spectrograph (IFS) that will be used for future space astrophysics missions. The new technology is an all-reflective image slicer that directs light to an array of imaging diffraction gratings. Previous UV instruments recorded the spectra of point sources or spatially resolved elements along a long slit. Our IFS has nominally only one reflection more than the Cosmic Origins Spectrograph for Hubble Space Telescope, which is the most sensitive UV spectrograph yet built, but is limited to point sources. An efficient UV IFS enables simultaneous spectroscopy of many spatially resolved elements within a contiguous two dimensional field of view in diagnostically important ultraviolet lines. The output is thus a data cube having one spectral and two spatial coordinates. This is the astrophysical analog to hyperspectral imaging in Earth sciences.

• The scientific benefits of such an instrument were developed during Vision Missions, Origins Probes, and Astrophysics Strategic Mission Concept Studies between 2004 and 2009. Implementation can be scaled for a small payload such as a sounding rocket or Explorer-class mission, leading to a flight experiment within the next few years. Of particular interest would be the application of this technology for an instrument on a version of the Advanced Technology Large-Aperture Space Telescope (ATLAST) which will have an 8+-m aperture. A 4-m version would be applicable to missions such as the Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy (THEIA).

• We will focus on the spectral region near Lyman alpha, but the all-reflective approach is applicable to other spectral regions when matched with wavelength appropriate gratings and detectors. Our project is a collaboration between Ball Aerospace & Technologies Corp., the University of Colorado, NASA Goddard Space Flight Center, and the Space Telescope Science Institute, all of which have extensive experience with the science and instrumentation for UV astrophysics.

BENEFITS OF REFLECTIVE IFS APPROACH WITH MULTIPLE GRATINGS• Multiple gratings and detectors provide field coverage without the use of multiple

beamsplitter optics – reduced optics in path means increased throughput to the detector.• Increased packaging flexibility.• Separated paths allow increased straylight suppression and control.• Instrument approach is scalable for different systems from small breadboard to

suborbital flight to 4 to 8 meter telescopes.

SCIENCE MISSION BENEFITS• The UV is a spectral region rich in diagnostic emission and absorption lines that provide

information about temperature, metallicity, ionization and excitation processes, and interaction of matter with radiation fields, shocks, winds and other processes. Species such as H I, C IV, N V, O VI, Si III and S IV are prominent in the spectra of a myriad of objects and environments. Planets exhibit UV aurorae when the solar wind interacts with their magnetic fields and atmospheres. Figure 1 is a beautiful picture of Saturn taken with the UV imaging mode of STIS, showing both the north and south pole auroral ovals. Stars produce strong UV features in their photospheres, chromospheres, coronae and winds. Diffuse gas in the interstellar and intergalactic medium is traced by UV lines. Star formation, starbursts, jets, fountains and galaxy mergers can be studied. Figure 2 illustrates how visible long-slit spectroscopy with STIS revealed the kinematics of matter in the powerful gravity field of the nucleus of M848. With future UV-optical telescopes and next-generation instrumentation, UV integral field spectroscopy will be a powerful diagnostic technique.

FUTURE DESIGN AND FABRICATION• Additional trades of F#, field of view, and physical size (and number) of the slicers.• Continued development of the microslicer element assembly.

– Control of edges and characterization of scattered light.• Fabrication of a brassboard assembly that can be tested and perhaps flown on a

suborbital flight to raise the TRL to an acceptable value for spaceflight programs.

SYNOPSIS OF CONCEPTIntegral Field Unit (IFU)• The secondary mirror acts as the relay mirror to achieve the appropriate image scale.

Depending on how slow or fast the system F# is chosen, a toroidal relay mirror may be needed. If packaging allows a slow F#, that additional mirror can be deleted. The image slicer is extremely small; only a handful of mm in the spatial direction and a mm or so in the dispersion direction (depending on the number of facets and spatial FOV). To envision how narrow the individual facets are in the dispersion direction, one could picture the thin edge of a microscope cover slide. Those small slicer faces must be polished without chips or artifacts that cause scatter and crosstalk. The facets are offset from each other by a few degrees, each directing its light to a separate grating. The reflective surfaces must be polished and coated to operate efficiently at Ly a. Figure 5 shows one approach to fabricating the slicer using a stack of thin rectangular plates.

Diffraction Gratings• Our UV IFS concept requires that the diffraction gratings provide both dispersion and

imaging of the spectrum. With the use of toroidal substrates we can produce excellent imaging over a broad bandpass. Beasley et al., 2004, have investigated and demonstrated UV holographic gratings with excellent performance over both a wide FOV and broad spectral coverage. These designs are particularly well suited to changing focal length and plate scale of a re-imaged beam.

Detectors• For the desired spectroscopic resolution, individual spectra would cover a few hundred Å,

with a resolution element of a few Å. Since the spectral formats are small, only modest detectors are required. Each spectrum will be flat, but the five spectra may not fall in the same plane. Separate detectors will be employed for each spectrum.

For additional information, contact Steve Kendrick (303-666-1108; skendric@ball.com) orDennis Ebbets (303-939-5964; debbets@ball.com) at Ball Aerospace & Technologies Corp.

113 mm

800 mm

Telescope Axis (Incoming Light)

Toroidal Relay Mirror

4x Image on Segmenter

(Image Slicers)

~80 mm

5 Toroidal Gratings

5 FPAs

330 mm

Figure 3. Integral Field Spectrometer - Top and Side Views

Figure 4. IFS Instrument Perspective View Figure 5. Individual Slicers Direct Sectors of the FOV to Individual Gratings and Detectors

Spatial direction = long dimension of facets

Dispersion direction = short dimension of facets.Each facet reflects light to a separate grating and detector.

Relay Mirror

Toroidal Grating (5x)

FPA (5x)

Integral Field Unit (5 Image Slicers)

Figure 1. Planets, satellites and comets all interact with the solar wind, producing conspicuous emission in the ultraviolet. A UV IFS could record the spectrum of every spatially resolved point in the region of interaction.

Figure 2. STIS observations revealed velocities of 400 km s-1 for matter orbiting the nucleus of M84, and inferred a mass of 300 million solar masses for the central object. A UV IFS will map the kinematics of hot stars and ionized gas in great detail.

A11685