High-Resolution Optical Speckle Imaging at Gemini North

Steve B. Howell, Elliott Horch, Mark Everett, and David Ciardi

High-resolution Optical Speckle Imaging at Gemini North A visitor instrument at Gemini North takes the highest-resolution, ground-based optical image ever made of the Pluto-Charon system. Results have ramifications on both the study of the not-quite-a-planet Pluto system as well as the search for exoplanets.

“Clearly the best!” It’s the oft heard phrase Figure 1. about the atmospheric clarity and seeing on The reconstructed Mauna Kea, the purported first-born son of image of Pluto and Wākea and Papa, according to Hawaiian lore. Charon obtained at 692 nm. In the image, Well, we are here to tell you, we agree. north is up, east is to Mauna Kea is a premiere observing site, the left, and the image section shown here is especially for speckle interferometry, a 1.39 x 1.39 arcseconds technique that employs a sequence of short- across. No pixel exposure “snapshots” to obtain images at smoothing has been a telescope’s diffraction limit. In July 2012, applied to the image. a generous allocation of Director’s time allowed us to bring our Differential Speckle Survey Instrument (DSSI), a speckle camera, to Gemini North as a guest-instrument. When coupled with the superb 8-meter optics of Gemini, that dual-channel instrument, which employs an electron multiplying CCD sensor (EMCCD) with no readout noise, allowed our team to produce diffraction-limited shots of Pluto and its largest moon Charon. At an angular resolution of 20 milliarcseconds (+/-3-4 mas), these are the highest-resolution, ground-based optical images yet achieved for the Pluto-Charon system (see Figure 1).

December2012 GeminiFocus 5 positive sources masquerading as an exoplanetary transit event are background eclipsing bina- ries or a variety of other variable stars. By background, we mean a star that’s nearly co-aligned with the assumed exoplanet host star. Its light mixes in with the target star, mimicing a transit-like event. High-resolution imaging allows us to examine the area of space very near a potential host star and de- tect (or not) these confounding troublemakers.

Kepler, for example, has broad point-spread functions covering 1-2 pixels, with each one span- ning 4 arcseconds. Thus, the pa- Figure 2. The Scientific Purpose rameter space of close neighbors th th DSSI mounted on the to a typical 12 - to14 -magnitude target side port at Gemini You can see the DSSI mounted on a side- star is large, both in area and in magnitude. North. The instrument looking port of Gemini North in Figure 2. Therefore, one must try to examine the back- (top silver box with For scale, it’s about the size of a carry-on ground sky as close as possible to the target two EMCCD cameras suitcase. When we arrived at the telescope, star. It also requires looking as deep as pos- attached –– one the mounting, focus, and computer con- sible in magnitude. Since Kepler and CoRoT on the left and one nections and controls worked essentially both image in visible light, we performed towards the viewer) without a hitch. The Gemini day-crew mem- is surrounded by our speckle work at optical wavelengths as the larger standard bers were super at their jobs and made the well, to best match their bandpasses. Doing Gemini instrument setup process painless. But finding enough so also helps in confirming or denying a tar- cage enclosure. The small straps and bolts to lift the 35-kilo- get as a good exoplanet candidate. small box attached gram instrument with the dome crane, and As a demonstration of the power of com- underneath is the then balancing it with the three other side- instrument control bining DSSI with Gemini, our team suc- port monster instruments, proved to be a computer. ceeded in obtaining the sharpest ground- fun challenge. based snapshots ever obtained of Pluto and The scientific purpose of the observing run Charon in visible light (see Figures 1 and 4). was to use our high-resolution imaging abil- While this result only hints at the exoplanet ity to help the NASA Kepler spacecraft mis- verification power of a large state-of-the-art sion and the European Convection, Rotation telescope when combined with speckle im- and Planetary Transits (CoRoT) satellite mis- aging techniques, the data also verified and sion to validate small planets orbiting other refined previous orbital characteristics for stars. Both of these missions provide time- Pluto and Charon while revealing the pair’s series light curves of stars, which can reveal precise diameters. transit-like signals in them. They both also The Pluto-Charon result is of timely interest rely on ground-based follow-ups for exo- for scientists wanting to understand the or- planet confirmations. One of the largest false

6 GeminiFocus December2012 trometry to at least the 0.9 mas- Figure 3. level in a 3-minute observation and Summary of the Gemini- provide a photometric measure- limiting magnitude at ment of the stars observed to +/- 0.2 arcsecond and the 0.1 magnitude at Gemini North. target brightness we can reach with our speckle We have used our speckle camera camera. Filled circles are in a broad survey of exoplanet the results at 692 nm, and open circles are the candidate host stars employing results at 880 nm. The the WIYN telescope in Arizona boxed points are cases and now Gemini North. At WIYN where results from two we examined the bright star nights were combined. sample and used Gemini North The shaded area marks for a more selective program the region of most points aimed at the smallest, Earth-like in the same plot for WIYN telescope data. All bital dynamics of this pair for the 2015 en- exoplanet candidates orbiting within their points are marked at the counter by NASA’s New Horizons spacecraft. habitable zones. estimated 5-sigma limit Figure 3 shows a summary of our typical ob- at a separation of 0.2 arcsecond. Doubling the DSSI: The Instrument servations at the two telescopes. Gemini observation time would The DSSI consists of an optics box that con- North provides higher resolution (i.e., clos- move points upward by tains the field lens, beam splitter, and filters. er-in views near the target star) and deeper 0.25 magnitude, and It simultaneously sends the light to two iXon limiting magnitudes; Gemini’s superior if 3-sigma limits are desired, all points would EMCCD cameras. The filters and beam splitter telescope size, optics, and high-altitude location also allows us to observe fainter move upward by 0.55 are changeable, and due to their small size, magnitude. relatively inexpensive. A PC based software target stars than at WIYN. While our engi- program runs both the instrument and the two cameras. Typically, we observe in the visual through infrared bands, although any optical band is possible, being constrained only by the CCD quantum efficiency profile.

Speckle data are obtained Figure 4. through very narrow band The reconstructed image (~40 nanometers) filters of the Pluto-Charon centered on the wave- system with their length of interest. EMCCD angular sizes drawn readout generally consists (dotted circles inside of a windowed sub-region, “glare”) along with the angular sizes of four covering 2-3 arcseconds known stars. Note that on a side at a plate scale of our imaging system on ~0.01 arcsecond per pixel. Gemini can also resolve DSSI can also perform as- the largest stars.

December2012 GeminiFocus 7 neering run at Gemini was short, we tested References: our speckle imager on a wide sample of po- Howell, Steve B., et al., The Astronomical Journal, tential exoplanet host stars from the CoRoT 42: 1257, 2011 and Kepler missions. Howell, Steve B., et al., Publications of the Astro- When mounted on Gemini North, DSSI can nomical Society of the Pacific, 124: 1124, 2012 reach to deep limiting magnitudes and observe very faint target stars. Both of these Horch, Elliott, et al., The Astronomical Journal, factors represent new limits for any speckle 141: 45, 2011 imaging observations and promises to pro- Horch, Elliott, et al., The Astronomical Journal, vide a powerful tool for the validation of 144: 165, 2012 Earth analogues. Steve B. Howell is the Project Scientist for the To show the variety of possible programs NASA Kepler Mission, at NASA Ames Research that would benefit from the high-resolu- Center, near San Francisco, California. He can be tion images available with DSSI and Gemini reached at: [email protected] North, our fully-resolved observations of Elliott Horch is an associate professor of physics Pluto are shown in Figure 4 compared to the at Southern Connecticut State University. His e- sizes of some well-known stars. Pluto’s moon mail address is: [email protected] Charon, has a smaller angular size than the Mark E. Everett is a research associate at the Na- star Betelgeuse, highlighting the possibility tional Optical Astronomy Observatory in Tucson, of using our camera system to resolve and Arizona. His email address is: study surface details for large stars and other [email protected] resolved objects, as well. David Ciardi is an Associate Research Scientist at Given the great potential for a number of the NASA Exoplanet Science Institute at Caltech. types of high-resolution images, DSSI is like- His email address is: [email protected] ly to return to Gemini North for observations in mid-2013 for general user programs from across the international Gemini partnership. Any such arrangement will be announced along with the call for proposals for Semes- ter 13B, in February 2013.

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