Vidhya Vaitheeswaran

The Giant Magellan Telescope adaptive optics program

The Giant Magellan Telescope (GMT) adaptive optics (AO) system will be an integral part of the telescope, providing laser guidestar generation, wavefront sensing, and wavefront correction to every instrument currently planned on the 25.4 m diameter GMT. There will be three first generation AO observing modes: Natural Guidestar, Laser Tomography, and Ground Layer AO. All three will use a segmented adaptive secondary mirror to deliver a corrected beam directly to the instruments. The Natural Guidestar mode will provide extreme AO performance, with a total wavefront error less than 185 nm RMS using bright guidestars. The Laser Tomography mode uses 6 lasers and a single off-axis natural guidestar to deliver better than 290 nm RMS wavefront error at the science target, over 50% of the sky at the galactic pole. The Ground Layer mode uses 4 natural guidestars on the periphery of the science field to tomographically reconstruct and correct the ground layer AO turbulence, improving the image quality for wide-field instruments. A phasing system maintains the relative alignment of the primary and secondary segments using edge sensors and continuous feedback from an off-axis guidestar. We describe the AO system preliminary design, predicted performance, and the remaining technical challenges as we move towards the start of construction.

The Large Binocular Telescope mid-infrared camera (LMIRcam): final design and status

We report on the final design and the fabrication status of LMIRcam - a mid-infrared imager/spectrograph that will operate behind the Large Binocular Telescope Interferometer (LBTI) primarily at wavelengths between 3 and 5um (the astronomical L- and M-bands). Within LMIRcam a pair of diamond-turned biconic mirrors re-images a ten arcsecond square field onto a 1024x1024 HAWAII-1RG 5.1um cutoff array. The re-imaging optics provide two pupil planes for the placement of filters and grisms as well as an intermediate image plane. Flexible readout electronics enable operating modes ranging from high frame rate broadband imaging at the longest wavelengths to low background R=400 spectroscopy at shorter wavelengths. The LBTI will provide LMIRcam with a diffraction limited two-mirror PSF with first null dictated by the 14.4 meter separation of the two LBT mirror centers (22.8 meter baseline from edge to edge).

On-sky operations and performance of LMIRcam at the Large Binocular Telescope

The L/M-band (3−5 μm) InfraRed Camera (LMIRcam) sits at the combined focal plane of the Large Binocular Telescope Interferometer (LBTI), ultimately imaging the coherently combined focus of the LBT’s two 8.4-meter mirrors. LMIRcam achieved first light at the LBT in May 2011 using a single AO-enabled 8.4-meter aperture. With the delivery of LBT’s final adaptive secondary mirror in Fall of 2011, dual-aperture AO-corrected interferometric fringes were realized in April 2012. We report on the performance of these configurations and characterize the noise performance of LMIRcam’s HAWAII-2RG 5.3-μm cutoff array paired with Cornell FORCAST readout electronics. In addition, we describe recent science highlights and discuss future improvements to the LMIRcam hardware.

Commissioning the LBTI for use as a nulling interferometer and coherent imager

The Large Binocular Telescope Interferometer (LBTI) is a strategically important instrument for exploiting the use of the LBT as a 22.7 m telescope. The LBTI has two science cameras (covering the 1.5-5 μm and 8-13 μm atmospheric windows), and a number of observing modes that allow it to carry out a wide range of high-spatial resolution observations. Some simple modes, such as AO imaging, are in routine use. We report here on testing and commissioning of the system for its more ambitious goals as a nulling interferometer and coherent imager. The LBTI will carry out key surveys to Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS) and an LBTI Exozodi-Exoplanet Common Hunt (LEECH). The current nulling and coherent imaging performance is described.

First AO-corrected interferometry with LBTI: Steps towards routine coherent imaging observations

We report the first phased images using adaptive optics correction from the Large Binocular Telescope Interferometer. LBTI achieved first fringes in late 2010, with seeing-limited operation. Initial tests verified the feasibility of the setup and allowed us to characterize the phase variations from both the atmosphere and mechanical vibrations. Integration of the secondary-base AO systems was carried out in spring 2011 and spring 2012 for the right and left side respectively. Single aperture, diffraction-limited, operation has been commissioned and is used as a productive mode of the LBTI with the LMIRCam subsystem. We describe the initial observation for dual aperture observations and coherent imaging results.

Co-phasing the Large Binocular Telescope: status and performance of LBTI/PHASECam

The Large Binocular Telescope Interferometer is a NASA-funded nulling and imaging instrument designed to coherently combine the two 8.4-m primary mirrors of the LBT for high-sensitivity, high-contrast, and highresolution infrared imaging (1.5-13 μm). PHASECam is LBTIs near-infrared camera used to measure tip-tilt and phase variations between the two AO-corrected apertures and provide high-angular resolution observations. We report on the status of the system and describe its on-sky performance measured during the first semester of 2014. With a spatial resolution equivalent to that of a 22.8-meter telescope and the light-gathering power of single 11.8-meter mirror, the co-phased LBT can be considered to be a forerunner of the next-generation extremely large telescopes (ELT).

The HOSTS Survey—Exozodiacal Dust Measurements for 30 Stars

The HOSTS (Hunt for Observable Signatures of Terrestrial Systems) survey searches for dust near the habitable zones (HZs) around nearby, bright main sequence stars. We use nulling interferometry in N band to suppress the bright stellar light and to probe for low levels of HZ dust around the 30 stars observed so far. Our overall detection rate is 18%, including four new detections, among which are the first three around Sun-like stars and the first two around stars without any previously known circumstellar dust. The inferred occurrence rates are comparable for early type and Sun-like stars, but decrease from 60 (+16/-21)% for stars with previously detected cold dust to 8 (+10/-3)% for stars without such excess, confirming earlier results at higher sensitivity. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal excess luminosity function, we put upper limits on the median HZ dust level of 13 zodis (95% confidence) for a sample of stars without cold dust and of 26 zodis when focussing on Sun-like stars without cold dust. However, our data suggest that a more complex luminosity function may be more appropriate. For stars without detectable LBTI excess, our upper limits are almost reduced by a factor of two, demonstrating the strength of LBTI target vetting for future exo-Earth imaging missions. Our statistics are so far limited and extending the survey is critical to inform the design of future exo-Earth imaging surveys.

Astronomical imaging using ground-layer adaptive optics

Over the past several years, experiments in adaptive optics involving multiple natural and Rayleigh laser guide stars have been carried out by our group at the 1.5 m Kuiper telescope and the 6.5 m MMT telescope. From open-loop data we have calculated the performance gains anticipated from ground-layer adaptive optics (GLAO) and laser tomography adaptive optics corrections. In July 2007, the GLAO control loop was closed around the focus signal from all five laser guide stars at the MMT, leading to a reduction in the measured focus mode on the laser wavefront sensor by 60%. For the first time, we expect to close the full high order GLAO control loop around the five laser beacons and a tilt star at the MMT in October 2007, where we predict image quality of < 0.2 arc seconds FWHM in K band (λ = 2.2 μm) over a 2 arc minute field. We intend to explore the image quality, stability and sensitivity of GLAO correction as a function of waveband with the science instrument PISCES. PISCES is a 1-2.5 µm imager with a field of view of 110 arc seconds, at a scale of 0.11 arc seconds per pixel. This is well matched to the expected FWHM performance of the GLAO corrected field and will be able to examine PSF non-uniformity and temporal stability across a wide field. FGD.

TIGER: A High Contrast infrared imager for the Giant Magellan Telescope

The Thermal Infrared imager for the GMT which provides Extreme contrast and Resolution (TIGER) is intended as a small-scale, targeted instrument capable of detecting and characterizing exoplanets and circumstellar disks, around both young systems in formation, and more mature systems in the solar neighborhood. TIGER can also provide general purpose infrared imaging at wavelengths from 1.5-14 μm. The instrument will utilize the facility adaptive optics (AO) system. With its operation at NIR to MIR wavelengths (where good image quality is easier to achieve), and much of the high-impact science using modestly bright guide stars, the instrument can be used early in the operation of the GMT. The TIGER concept is a dual channel imager and low resolution spectrometer, with high contrast modes of observations to fulfill the above science goals. A long wavelength channel (LWC) will cover 7-14 μm wavelength, while a short wavelength channel (SWC) will cover the 1.5-5 μm wavelength region. Both channels will have a 30° FOV. In addition to imaging, low-resolution spectroscopy (R=300) is possible with TIGER for both the SWC and LWC, using insertable grisms.

Design and predicted performance of the GMT ground-layer adaptive optics mode

The Giant Magellan Telescope is planning to provide adaptive wavefront correction of the low layers (<1 km) of atmospheric turbulence in support of wide-field instrumentation. This ground-layer adaptive optics (GLAO) mode will use the adaptive secondary mirrors to provide improved image quality over approximately 7 arcminutes FOV. We present a comparison between the use of a sodium laser guide star asterism plus three tip-tilt natural guide stars versus natural guide stars only on the average seeing width improvement. The layout and components of both (laser beacon based and natural star only based) GLAO concepts are described and the impact and interaction with other GMT subsystems is analyzed.