<p>This is an introduction to a US government program that conducted high-contrast imaging experiments with an electron multiplying charge coupled device (EMCCD) in an interferometric coronagraph. This report will introduce the concepts of “charge blooming” and “starlight saturation” in the context of high-contrast astronomical imaging. These phenomena adversely effect the performance of high-contrast photon-counting instruments that do not use a mask to physically block starlight in the science channel of the coronagraph. The problems will be presented with the help of images taken with a commercial EMCCD camera in the visible nulling coronagraph at the Goddard Space Flight Center. A new clocking scheme for EMCCDs—variable multiplication gain clocking—will be proposed as a means for suppressing horizontal blooming and starlight saturation in an astronomical camera. This opening report will conclude with a discussion of design considerations for a new controller for high-contrast photon-counting with an EMCCD in a coronagraphic instrument. This controller will allow a single frame from an EMCCD to be scanned in multiple modes—photon-counting and digitization—with a variable multiplication gain clock to enable direct imaging of an exoplanet and wavefront control of a coronagraph, simultaneously.</p>
This paper discusses charge blooming and starlight saturation – two potential technical problems – when using an Electron Multiplying Charge Coupled Device (EMCCD) type detector in a high-contrast instrument for imaging exoplanets. These problems especially affect an interferometric type coronagraph – coronagraphs that do not use a mask to physically block starlight in the science channel of the instrument. These problems are presented using images taken with a commercial Princeton Instrument EMCCD camera in the Goddard Space Flight Center’s (GSFC), Interferometric Coronagraph facility. In addition, this paper discusses techniques to overcome such problems. This paper also discusses the development and architecture of a Field Programmable Gate Array and Digital-to-Analog Converter based shaped clock controller for a photon-counting EMCCD camera. The discussion contained here will inform high-contrast imaging groups in their work with EMCCD detectors.
The key to broadband operation of the Visible Nulling Coronagraph (VNC) is achieving a condition of quasi-achromatic destructive interference between combined beams. Here we present efforts towards meeting this goal using Fresnel rhombs in each interferometric arm as orthogonally aligned half wave phase retarders. The milestone goal of the demonstration is to achieve 1 × 10−9 contrast at 2λ/D over a 40 nm bandpass centered at 633 nm. Rhombs have been designed and fabricated, and a multi-step approach to alignment using coarse positioners for each rhomb and pair has been developed to get within range of piezo stages used for fine positioning. The previously demonstrated narrowband VNC sensing and control approach that uses a segmented deformable mirror is being adapted to broadband to include fine positioning of the piezo-mounted rhombs, all demonstrated in a low-pressure environment.
Herein we report on our Visible Nulling Coronagraph high-contrast result of 10<sup>9</sup> contrast averaged over a focal plane
region extending from 1 – 4 λ/D with the Vacuum Nuller Testbed (VNT) in a vibration isolated vacuum chamber. The
VNC is a hybrid interferometric/coronagraphic approach for exoplanet science. It operates with high Lyot stop
efficiency for filled, segmented and sparse or diluted-aperture telescopes, thereby spanning the range of potential future
NASA flight telescopes. NASA/Goddard Space Flight Center (GSFC) has a well-established effort to develop the VNC
and its technologies, and has developed an incremental sequence of VNC testbeds to advance this approach and its
enabling technologies. These testbeds have enabled advancement of high-contrast, visible light, nulling interferometry to
unprecedented levels. The VNC is based on a modified Mach-Zehnder nulling interferometer, with a “W” configuration
to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic phase shifters.
We give an overview of the VNT and discuss the high-contrast laboratory results, the optical configuration, critical
technologies and null sensing and control.
Herein we report on the development, sensing and control and our first results with the Vacuum Nuller Testbed to realize
a Visible Nulling Coronagraph (VNC) for exoplanet coronagraphy. The VNC is one of the few approaches that works
with filled, segmented and sparse or diluted-aperture telescope systems. It thus spans a range of potential future NASA
telescopes and could be flown as a separate instrument on such a future mission. NASA/Goddard Space Flight Center
(GSFC) has a well-established effort to develop VNC technologies, and has developed an incremental sequence of VNC
testbeds to advance this approach and the enabling technologies associated with it. We discuss the continued
development of the vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated
testbed that operates under closed-loop control within a vacuum chamber. It will be used to achieve an incremental
sequence of three visible-light nulling milestones with sequentially higher contrasts of 10<sup>8</sup>, 10<sup>9</sup>, and ideally 10<sup>10</sup> at an
inner working angle of 2*λ/D. The VNT is based on a modified Mach-Zehnder nulling interferometer, with a "W"
configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic
phase shifters. We discuss the initial laboratory results, the optical configuration, critical technologies and the null
sensing and control approach.