Gudmundur Stefansson

The Habitable-zone Planet Finder: A status update on the development of a stabilized fiber-fed near-infrared spectrograph for the for the Hobby-Eberly telescope

The Habitable-Zone Planet Finder is a stabilized, fiber-fed, NIR spectrograph being built for the 10m Hobby- Eberly telescope (HET) that will be capable of discovering low mass planets around M dwarfs. The optical design of the HPF is a white pupil spectrograph layout in a vacuum cryostat cooled to 180 K. The spectrograph uses gold-coated mirrors, a mosaic echelle grating, and a single Teledyne Hawaii-2RG (H2RG) NIR detector with a 1.7-micron cutoff covering parts of the information rich z, Y and J NIR bands at a spectral resolution of R∼50,000. The unique design of the HET requires attention to both near and far-field fiber scrambling, which we accomplish with double scramblers and octagonal fibers. In this paper we discuss and summarize the main requirements and challenges of precision RV measurements in the NIR with HPF and how we are overcoming these issues with technology, hardware and algorithm developments to achieve high RV precision and address stellar activity.

A comprehensive radial velocity error budget for next generation Doppler spectrometers

We describe a detailed radial velocity error budget for the NASA-NSF Extreme Precision Doppler Spectrometer instrument concept NEID (NN-explore Exoplanet Investigations with Doppler spectroscopy). Such an instrument performance budget is a necessity for both identifying the variety of noise sources currently limiting Doppler measurements, and estimating the achievable performance of next generation exoplanet hunting Doppler spectrometers. For these instruments, no single source of instrumental error is expected to set the overall measurement floor. Rather, the overall instrumental measurement precision is set by the contribution of many individual error sources. We use a combination of numerical simulations, educated estimates based on published materials, extrapolations of physical models, results from laboratory measurements of spectroscopic subsystems, and informed upper limits for a variety of error sources to identify likely sources of systematic error and construct our global instrument performance error budget. While natively focused on the performance of the NEID instrument, this modular performance budget is immediately adaptable to a number of current and future instruments. Such an approach is an important step in charting a path towards improving Doppler measurement precisions to the levels necessary for discovering Earth-like planets.

Environmental control system for Habitable-zone Planet Finder (HPF)

HPF is an ultra-stable, precision radial velocity near infrared spectrograph with a unique environmental control scheme. The spectrograph will operate at a mid-range temperature of 180K, approximately half way between room temperature and liquid nitrogen temperature; it will be stable to sub -milli-Kelvin(mK) levels over a calibration cycle and a few mK over months to years. HPF‟s sensor is a 1.7 micron H2RG device by Teledyne. The environmental control boundary is a 9 m2 thermal enclosure that completely surrounds the optical train and produces a near blackbody cavity for all components. A large, pressure - stabilized liquid nitrogen tank provides the heat sink for the system via thermal straps while a multichannel resistive heater control system provides the stabilizing heat source. High efficiency multi-layer insulation blanketing provides the outermost boundary of the thermal enclosure to largely isolate the environmental system from ambient conditions. The cryostat, a stainless steel shell derived from the APOGEE design, surrounds the thermal enclosure and provides a stable, high quality vacuum environment. The full instrument will be housed in a passive „meat -locker‟ enclosure to add a degree of additional thermal stability and as well as protect the instrument. Effectiveness of this approach is being empirically demonstrated via long duration scale model testing. The full scale cryostat and environmental control system are being constructed for a 2016 delivery of the instrument to the Hobby-Eberly Telescope. This report describes the configuration of the hardware and the scale-model test results as well as projections for performance of the full system.

Toward Space-like Photometric Precision from the Ground with Beam-shaping Diffusers

We demonstrate a path to hitherto unachievable differential photometric precisions from the ground, both in the optical and near-infrared (NIR), using custom-fabricated beam-shaping diffusers produced using specialized nanofabrication techniques. Such diffusers mold the focal plane image of a star into a broad and stable top-hat shape, minimizing photometric errors due to non-uniform pixel response, atmospheric seeing effects, imperfect guiding, and telescope-induced variable aberrations seen in defocusing. This PSF reshaping significantly increases the achievable dynamic range of our observations, increasing our observing efficiency and thus better averages over scintillation. Diffusers work in both collimated and converging beams. We present diffuser-assisted optical observations demonstrating 62_(-16)^(+26) ppm precision in 30 minute bins on a nearby bright star 16 Cygni A (V = 5.95) using the ARC 3.5 m telescope—within a factor of ~2 of Keplers photometric precision on the same star. We also show a transit of WASP-85-Ab (V = 11.2) and TRES-3b (V = 12.4), where the residuals bin down to 180_(-41)^(+66) ppm in 30 minute bins for WASP-85-Ab—a factor of ~4 of the precision achieved by the K2 mission on this target—and to 101 ppm for TRES-3b. In the NIR, where diffusers may provide even more significant improvements over the current state of the art, our preliminary tests demonstrated 137_(-36)^(+64) ppm precision for a K_S = 10.8 star on the 200 inch Hale Telescope. These photometric precisions match or surpass the expected photometric precisions of TESS for the same magnitude range. This technology is inexpensive, scalable, easily adaptable, and can have an important and immediate impact on the observations of transits and secondary eclipses of exoplanets.

A VERSATILE TECHNIQUE to ENABLE SUB-MILLI-KELVIN INSTRUMENT STABILITY for PRECISE RADIAL VELOCITY MEASUREMENTS: TESTS with the HABITABLE-ZONE PLANET FINDER

Insufficient instrument thermo-mechanical stability is one of the many roadblocks for achieving 10cm/s Doppler radial velocity (RV) precision, the precision needed to detect Earth-twins orbiting Solar-type stars. Highly temperature and pressure stabilized spectrographs allow us to better calibrate out instrumental drifts, thereby helping in distinguishing instrumental noise from astrophysical stellar signals. We present the design and performance of the Environmental Control System (ECS) for the Habitable-zone Planet Finder (HPF), a high-resolution (R=50,000) fiber-fed near infrared (NIR) spectrograph for the 10m Hobby Eberly Telescope at McDonald Observatory. HPF will operate at 180K, driven by the choice of an H2RG NIR detector array with a 1.7micron cutoff. This ECS has demonstrated 0.6mK RMS stability over 15 days at both 180K and 300K, and maintained high quality vacuum (<

Ultra-stable temperature and pressure control for the Habitable-zone Planet Finder spectrograph

10^{-7}

A system to provide sub-milliKelvin temperature control at T~300K for extreme precision optical radial velocimetry

Torr) over months, during long-term stability tests conducted without a planned passive thermal enclosure surrounding the vacuum chamber. This control scheme is versatile and can be applied as a blueprint to stabilize future NIR and optical high precision Doppler instruments over a wide temperature range from ~77K to elevated room temperatures. A similar ECS is being implemented to stabilize NEID, the NASA/NSF NN-EXPLORE spectrograph for the 3.5m WIYN telescope at Kitt Peak, operating at 300K. A full SolidWorks 3D-CAD model and a comprehensive parts list of the HPF ECS are included with this manuscript to facilitate the adaptation of this versatile environmental control scheme in the broader astronomical community.

A Candidate Transit Event around Proxima Centauri

We present recent long-term stability test results of the cryogenic Environmental Control System (ECS) for the Habitable zone Planet Finder (HPF), a near infrared ultra-stable spectrograph operating at 180 Kelvin. Exquisite temperature and pressure stability is required for high precision radial velocity (< 1m=s) instruments, as temperature and pressure variations can easily induce instrumental drifts of several tens-to-hundreds of meters per second. Here we present the results from long-term stability tests performed at the 180K operating temperature of HPF, demonstrating that the HPF ECS is stable at the 0:6mK level over 15-days, and <10-7 Torr over months.

Adaptive optics fed single-mode spectrograph for high-precision Doppler measurements in the near-infrared

We present preliminary results for the environmental control system from NEID, our instrument concept for NASAs Extreme Precision Doppler Spectrograph, which is now in development. Exquisite temperature control is a requirement for Doppler spectrographs, as small temperature shifts induce systematic Doppler shifts far exceeding the instrumental specifications. Our system is adapted from that of the Habitable Zone Planet Finder instrument, which operates at a temperature of 180K.We discuss system modifications for operation at T ~ 300K, and show data demonstrating sub-mK stability over two weeks from a full-scale system test.

Extreme precision photometry from the ground with beam-shaping diffusers for K2, TESS, and beyond

We present a single candidate transit event around Proxima Centauri, found during a blind transit search using a robotic 30\,cm telescope at Las Campanas Observatory. The event lasted 1 hour, with an estimated depth of 5\,mmag, and is inconsistent with the transit window predicted for the recently discovered planet b. We modeled the lightcurve under the assumption that the event was caused by a transiting exoplanet, and our model predicts the planet has a radius