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Astronomy

Payload design for the Wide-Field Infrared Survey Telescope

A new optical payload concept is suitable for a space telescope that probes dark energy, takes an exoplanet census, and surveys the near infrared sky.
26 July 2011, SPIE Newsroom. DOI: 10.1117/2.1201107.003798

Our view of the Universe has changed dramatically in recent years. We now know that most of the matter in the cosmos is invisible and that the expansion of the Universe is accelerating in an unexpected and unexplained way (due to a mysterious component dubbed dark energy). We also know that there are many exoplanets, but we have not fully determined their frequency or orbital distribution. Further, we have data on the near-IR (NIR) sky, but our picture of objects that emit radiation in this part of the electromagnetic spectrum is incomplete. Ground-based experiments to address these issues are ongoing and more are planned, but the atmosphere limits them. Atmospheric seeing and the limits of adaptive optics technology mean that wide field images are often blurred. In addition, NIR radiation is, aside from a few wavelength ranges, largely absorbed by water in the atmosphere. Another problem is that observing transient phenomena (such as supernovae or exoplanet detections) requires calibration among multiple observations, which are hampered by weather and the day/night cycle. A space telescope with a wide field-of-view in the NIR spectrum is ideal to probe dark energy, characterize exoplanets, and survey the near infrared sky.

The Wide Field Infrared Survey Telescope (WFIRST) was ranked first in new space missions by the US National Research Council Decadal Survey of Astronomy and Astrophysics.1 The mission results from a combination of the science and hardware concept from the Joint Dark Energy Mission (JDEM)2 with the science agendas of the proposed Microlensing Planet Finder3 and Near Infrared Sky Surveyor.4 WFIRST will take precision images and spectra over thousands of square degrees, with good image quality, and stability greater than that of the Hubble Space Telescope (HST). A field-of-view 50 times wider than Hubble's two-mirror telescope is required. Our design uses a 1 square degree field three mirror anastigmat (TMA) design. While TMAs have been used in Earth-mapping and ground telescopes, they have not been flown on an astrophysical space telescope because NIR imaging array detectors that ‘tile’ a large focal plane assembly were unavailable until recently.

Figure 1. Field-of-view layout for the WFIRST Interim Design Reference Mission-1. The fields-of-view of the imaging channel (ImC), spectroscopy channels (SpC A&B), and guiding modes are shown to scale with the Moon, Hubble Space Telescope (HST), and James Webb Space Telescope (JWST). Each square is a 4-megapixel visible and near-infrared sensor chip assembly.

Our mission concept deploys a space telescope to the second Sun-Earth Lagrange point where the full sky can be viewed every six months. Its primary mirror will be 1.3m, smaller than either HST or the James Webb Space Telescope (JWST), so it is lower in risk and cost. However, by using a mosaic of 28 NIR 4-megapixel sensors, we will still be able to achieve an imaging sky survey speed (aperture area times field-of-view area) over ten times larger than that of JWST or the widest field HST camera. Additionally, two spectrometer channels will map the distribution of galaxies 8–11 billion light years away. Each field-of-view is observed for 1–20 minutes, depending on the science measurement. During the galaxy survey, this will build up a map of a large region on the sky. During exoplanet measurements, repeated observations of the center of the Milky Way will catch brightening events when exoplanets gravitationally lens background stars.

For the wide field-of-view required, the on-axis aperture form of the TMA5 has a large central obscuration, blocking significant portions of the available signal and degrading the imaging resolution. We solve this problem by adopting an unobscured version of the TMA. Unobscured aperture telescopes have been used for Earth mapping and Earth science measurements, but no large (≥1m diameter) aperture of this kind has yet been used in a space-based observatory. Mirror polishing has advanced dramatically in the last ten years, driven by the needs of extreme ultraviolet lithography, the JWST, and other factors. The light weighting and polishing requirements of the WFIRST mirrors are safely within current capabilities.

Our WFIRST reference design consists of three channels, one for imaging and two for slitless spectroscopy. Figure 1 shows the overall field-of-view projected onto the sky. The imaging channel is in the middle and each small square is a 4-megapixel sensor chip assembly, devices that have good sensitivity over the 0.6–2.0μm visible to NIR spectral band pass of WFIRST. For sky scaling, the full Moon as seen from Earth and the full instrument suites of the HST and JWST are shown. The spectrometer designs are mirror images. The dispersions of the two instruments are 180 degrees opposed on the sky in order to resolve overlapping spectra of different objects. Prism assemblies are used rather than diffraction gratings to increase sensitivity, minimize confusion, and improve throughput. Our reference design uses prisms in a converging beam6 to allow a compact, simple instrument.

Our payload concept can accomplish the WFIRST mission as conceived in the US Decadal Survey of Astronomy and Astrophysics. This initial concept requires significant further engineering and science simulations to ensure that we can accomplish all the measurement goals. These aspects represent the focus of our future work. We hope to launch this observatory by the end of this decade.

We thank the JDEM science teams and the WFIRST science definition team for scientific support. Contributions to the JDEM optical design effort from M. Dittman, M. Lampton, M. Sholl, and R. Woodruff are acknowledged. This work is funded by NASA.


David A. Content
NASA Goddard Space Flight Center (GSFC)
Greenbelt, MD

David Content has worked at NASA GSFC since receiving a PhD in physics from Johns Hopkins University in 1988. He has worked on diffraction gratings for the Hubble Space Telescope and other missions and developed new lightweight mirror technologies. He is the optics lead engineer for the Wide-Field Infrared Survey Telescope.

John Lehan
Optics Branch, NASA GSFC SGT
Greenbelt, MD

John Lehan received his Bachelor's degree in physics from Virginia Tech and his Master's and PhD degrees in optical sciences from the University of Arizona. He worked as an industrial researcher prior to coming to NASA GSFC in 2002, working on the Suzaku mission. He joined the WFIRST team in 2009. His research interests include precision optical metrological methods, precision optical measurements, optical testing, optical design, precision optical manufacturing, and scatter from coated and uncoated surfaces.


References:
1. Committee for a Decadal Survey of Astronomy and Astrophysics, National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, 2010. http://www.nap.edu/catalog.php?record_id=12951
2. N. Gehrels, The Joint Dark Energy Mission (JDEM)/Omega, 2010. http://wfirst.gsfc.nasa.gov/science/astro2010_rfi/Astro2010_JDEM-Omega_RFI.pdf
3. D. P. Bennett, J. Anderson, J.-P. Beaulieu, I. Bond, E. Cheng, K. Cook, S. Friedman, et al., A census of exoplanets in orbits beyond 0.5 AU via space-based microlensing, 2009. http://arxiv.org/abs/0902.3000
4. Daniel Stern, The Near-Infrared Sky Surveyor 2009.  http://wfirst.gsfc.nasa.gov/science/astro2010_rfi/Astro2010_NIRSS_RFI.pdf
5. D. A. Content, M. G. Dittman, B. Firth, J. M. Howard, C. E. Jackson, J. P. Lehan, J. E. Mentzell, B. A. Pasquale, M. J. Sholl, Joint Dark Energy Mission optical design studies, Proc. SPIE 7731, p. 7731D, 2010. doi:10.1117/12.859144
6. M. J. Sholl, D. A. Content, M. L. Lampton, J. P. Lehan, M. E. Levi, Wide-field spectroscopy and imaging at two plate scales with a focal three mirror anastigmat, Proc. SPIE 7731, p. 7731F, 2010. doi:10.1117/12.857762