Nulling at short wavelengths: theoretical performance constraints and a demonstration of faint companion detection inside the diffraction limit with a rotating-baseline interferometer

Abstract

The Palomar Fiber Nuller (PFN) is a rotating-baseline nulling interferometer that enables high-accuracy near-infrared (NIR) nulling observations with full azimuth coverage. To achieve NIR null-depth accuracies of several x 10 -4 , the PFN uses a common-mode optical system to provide a high degree of symmetry, single-mode-fiber beam combination to reduce sensitivity to pointing and wavefront errors, extreme adaptive optics to stabilize the fiber coupling and the cross-aperture fringe phase, rapid signal calibration and camera readout to minimize temporal effects, and a statistical null-depth fluctuation analysis to relax the phase stabilization requirement. Here we describe the PFN's final design and performance, and provide a demonstration of faint-companion detection by means of nulling-baseline rotation, as originally envisioned for space-based nulling interferometry. Specifically, the Ks-band null-depth rotation curve measured on the spectroscopic binary  Peg reflects both a secondary star 1.08 ± 0.06 x 10 -2 as bright as the primary, and a null-depth contribution of 4.8 ± 1.6 x 10 -4 due to the size of the primary star. With a 30 mas separation at the time,  Peg B was well inside both the telescope's diffraction-limited beam diameter (88 mas) and typical coronagraphic inner working angles. Finally, we discuss potential improvements that can enable a number of small-angle nulling observations on larger telescopes.

Kernel nullers for an arbitrary number of apertures

“…This observing mode takes advantage of the spatial filtering provided by the resolving power of each of the 8 m telescopes of the Very Large Telescope Interferometer (VLTI), coupled into single mode fibers to reach the required contrast. Interferometric nullers (Bracewell 1978;Colavita et al 2009;Serabyn et al 2019;Hoffmann et al 2014;Defrère et al 2015;Norris et al 2020) offer the possibility to explore smaller angular separations through the use of fragmented apertures and long-baseline interferometry. Some combining solutions have been found that optimize the rejection of resolved stars (Angel & Woolf 1997;Guyon et al 2013).…”

Exoplanet detection yield of a space-based Bracewell interferometer from small to medium satellites

“…By operating at the null phase, the noise is minimal in comparison to standard interferometry. 40 This null depth can be decomposed as a timeaveraged null depth N in the presence of active OPD matching and intensity balancing, and an RMS fluctuation of the null level σ N . 39 The time-averaged null depth of a Bracewell interferometer observing a single star is given by E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 6 ; 4 8 6…”

Detecting and characterizing close-in exoplanets with vortex fiber nulling

“…These two improvements on traditional nulling interferometry have allowed PFN to demonstrate on-sky null depths of ∼ 10 −3 in the past 9,10 and more recently of ∼ 10 −4 with the detection of a faint companion well within the diffraction limit of the Palomar Telescope. 11 Building on this heritage, Vortex Fiber Nulling (VFN) is a new fiber nulling technique introduced by our team in 2018. 12 It is very similar to the PFN in that it relies on a pupil mask and SMF but instead of creating two Figure 1.…”