Steve B. Howell, NASA Ames Research Center, Senior Research Scientist
The NN-EXPLORE program provides about 50 percent of the observing time on the Kitt Peak WIYN (Wisconsin-Indiana-Yale-NOAO) 3.5-meter telescope to the exoplanet community. WIYN’s suite of instruments include HYDRA, a multi-fiber medium to high-resolution bench spectrograph, WHIRC, a near-IR imager, ODI, an optical wide-field optical imager, and for over a decade, a visiting speckle camera called DSSI (Horch et al. 2009). The telescope’s instruments also include several integral field units, bundles of optical fiber that feed light from the telescope to an instrument, in this case a spectrograph, that lives in an environmentally controlled room in the WIYN telescope basement. Beginning in 2017, thanks to funding support from the NASA Exoplanet program, DSSI has been replaced by a modern, more functional, community available observatory instrument named NESSI.
NESSI, the NN-EXPLORE Exoplanet & Stellar Speckle Imager, was commissioned during the fall of 2016 and is now available for community use. Speckle imaging allows telescopes to achieve diffraction limited imaging performance—that is, collecting images with resolutions equal to that which would be possible if the atmosphere were removed. The technique employs digital cameras capable of reading out frames at a very fast rate, effectively “freezing out” atmospheric seeing. The resulting speckles are correlated and combined in Fourier space to produce reconstructed images with resolutions at the diffraction limit of the telescope (see Howell et al., 2011). Achievable spatial resolutions at WIYN are 39 milliarcseconds (550 nanometers) and 64 milliarcseconds (880 nanometers).
Left image shows a set of fringes obtained by NESSI. On the right the resultant image showing a K2 exoplanet host revealed by NESSI as a 0.15 arcsecond-separated binary star.
NESSI provides simultaneous red and blue imaging, each beam passing through one of two filter wheels. Each wheel contains six filter slots and both standard Sloan Digital Sky Survey (SDSS) and narrow band filters are available. NESSI can also switch between a “wide-field” (80 by 80 arcsecond) mode and a “speckle” mode (19 by 19 arcsecond) field of view. The NESSI detectors are the Andor Technology company’s 1024X1024 Electron Multiplier Charge Coupled Device (EMCCD) capable of 24 fps full-frame readout with no read noise. The instrument is highly versatile and it is anticipated that many science programs, in addition to speckle imaging, will be accomplished.
We envision NESSI’s primary use to be validation and characterization of exoplanet targets starting with NASA's K2 and Transiting Exoplanet Survey Satellite (TESS) missions as well as exoplanets discovered by radial velocity and other means (e.g., Howell et al., 2016). The limiting magnitude for speckle observations is R=13-14 at WIYN, while wide-field, normal CCD imaging operation will be able to go much fainter, providing usual CCD imaging and photometric capabilities.
References: Horch, E., et al., 2009, AJ, 137, 5057; Howell S., et al., 2011, AJ, 142, 19; Howell et al., 2016, ApJ, 829, L1.
NESSI (black box, cameras sticking out) mounted next to WHIRC on the 3.5-meter WIYN telescope at Kitt Peak.