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

Abstract

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 Keplerʼs 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 targetand 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 -+ 3664 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 Sub-Neptune-sized Planet Transiting the M2.5 Dwarf G 9-40: Validation with the Habitable-zone Planet Finder

“…We observe that the RMS values largely follow the expected white-noise behavior, although we note that slight excursions below the Gaussian expected values are observed at the largest bin sizes, which still are largely consistent with the Gaussian expected precision within the reported error bars. Similar and larger departures below the Gaussian expected precision have been reported by a number of groups in the literature (e.g., Blecic et al 2013;Stefansson et al 2017), and Cubillos et al (2017) show that these excursions are not statistically significant after taking into account the increasingly skewed inverse gamma distribution of the RMS values at the largest bin sizes. Following our previous work (Stefansson et al 2017(Stefansson et al , 2018a, we argue that excursions much below the Gaussian expected precision is likely an overestimate of the actual precision achieved, and we conservatively say that we achieve 138ppm precision in 30 minute bins for these transit observations.…”

“…The target rose from airmass 1.55 to airmass 1.05 during the observations. We observed the transit using the Engineered Diffuser on the ARCTIC imager, which has been described in detail in Stefansson et al (2017Stefansson et al ( , 2018a. In short, the Engineered Diffuser molds the focal-plane image of the star into a broad and stable top-hat shape, allowing us to increase our exposure times to gather more photons per exposure while minimizing correlated errors due to point spread function (PSF) variations and guiding errors.…”

“…In short, the Engineered Diffuser molds the focal-plane image of the star into a broad and stable top-hat shape, allowing us to increase our exposure times to gather more photons per exposure while minimizing correlated errors due to point spread function (PSF) variations and guiding errors. The increased exposure time allows us to further average over scintillation errors by increasing the duty cycle of the observations (Stefansson et al 2017). We used a newly-commissioned, custom-fabricated narrow-band filter from AVR Optics 1 and Semrock Optics 2 , centered on 857nm with a 37nm width in a region with minimal telluric absorption.…”

“…After removing the best-fit transit model and additional 6 significant (> 3σ) outliers present in the data, we obtained an unbinned precision of 1225ppm which is further discussed in Section 4. The transit is shown in Figure 7 where the data is shown with error bars estimated following the methodology in Stefansson et al (2017) and accounting for errors from photon, dark, read, background, digitization and scintillation noise.…”

The Orbit of WASP-12b Is Decaying

“…An occultation of WASP-12b was also observed with the Wide-Field Infrared Camera (WIRC, Wilson et al 2003) on the Hale 200-inch telescope at Palomar Observatory on 18 March 2017. This observation was made in the K s band using a new Hawaii-II detector installed on WIRC in January 2017 (Tinyanont et al 2019) and a near-infrared Engineered Diffuser (Stefansson et al 2017). We obtained 1,828 images with an exposure time of 2 seconds, spanning 5 hours.…”