Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published June 2020 | Published
Journal Article Open

Precision Photometric and Astrometric Calibration Using Alternating Satellite Speckles


Photometric and astrometric calibration of high-contrast images is essential for the characterization of companions at small angular separation from their stellar host. The main challenge to performing accurate relative photometry and astrometry of high-contrast companions with respect to the host star is that the central starlight cannot be directly used as a reference, as it is either blocked by a coronagraphic mask or saturating the detector. Our approach is to add fiducial incoherent faint copies of the host star in the image plane and alternate the pattern of these copies between exposures. Subtracting two frames with different calibration patterns removes measurement bias due to static and slowly varying incoherent speckle halo components, while ensuring that calibration references are inserted on each frame. Each calibration pattern is achieved by high-speed modulation of a pupil-plane deformable mirror to ensure incoherence. We implemented the technique on-sky on the Subaru Coronagraphic Extreme Adaptive Optics instrument with speckles which were of the order of 10³ times fainter than the central host. The achieved relative photometric and astrometric measurement precisions for 10 s exposure were respectively 5% and 20 milliarcsecond. We also demonstrate, over a 540 s measurement span, that residual photometric and astrometric errors are uncorrelated in time, indicating that residual noise averages as the inverse square root of the number of exposures in longer time-series data sets.

Additional Information

© 2020 The American Astronomical Society. Received 2020 January 8; revised 2020 April 8; accepted 2020 April 10; published 2020 May 6. The authors acknowledge the support from Subaru Telescope, NAOJ for lending out their facility. The development of SCExAO was supported by the Japan Society for the Promotion of Science (grant-in-aid for research No. 23340051, 26220704, 23103002, 19H00703, and 19H00695), the Astrobiology Center of the National Institutes of Natural Sciences, Japan, the Mt. Cuba Foundation, and the director's contingency fund at Subaru Telescope. The work of F.M. is supported by the ERC award CoG-683029. The authors wish to recognize the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Attached Files

Published - Sahoo_2020_AJ_159_250.pdf


Files (1.1 MB)
Name Size Download all
1.1 MB Preview Download

Additional details

August 22, 2023
October 20, 2023