TRIDENT: An Infrared Differential Imaging Camera Optimized for the Detection of Methanated Substellar Companions

Abstract

A near-infrared camera in use at the Canada-France-Hawaii Telescope (CFHT) and at the 1.6-m telescope of the Observatoire du Mont-Mégantic is described. The camera is based on a Hawaii-1 1024×1024 HgCdTe array detector. Its main feature is to acquire three simultaneous images at three wavelengths across the methane absorption bandhead at 1.6 µm, enabling, in theory, an accurate subtraction of the stellar point spread function (PSF) and the detection of faint close methanated companions. The instrument has no coronagraph and features fast data acquisition, yielding high observing efficiency on bright stars.The performance of the instrument is described, and it is illustrated by laboratory tests and CFHT observations of the nearby stars GL526, υ And and χ And.TRIDENT can detect (6σ) a methanated companion with ∆H = 9.5 at 0.5 ′′ separation from the star in one hour of observing time. Non-common path aberrations and amplitude modulation differences between the three optical paths are likely to be the limiting factors preventing further PSF attenuation. Instrument rotation and reference star subtraction improve the detection limit by a factor of 2 and 4 respectively. A PSF noise attenuation model is presented to estimate the non-common path wavefront difference effect on PSF subtraction performance.

Angular Differential Imaging: A Powerful High‐Contrast Imaging Technique

“…To our knowledge, this is the first time that such behavior is clearly demonstrated for an acquisition and reduction technique designed for speckle attenuation. The wall raised by quasistatic speckles that prevents a gain with longer integration time for standard observing techniques (Marois et al 2003(Marois et al , 2005Masciadri et al 2005) can thus be removed by ADI. Comparison with a classical imaging technique shows that ADI achieves 30 times better speckle attenuation in 30 minutes integration time.…”

“…The use of a multiwavelength instrument (Marois et al 2000(Marois et al , 2005Lafrenière et al 2004;Biller et al 2004) or an Integral Field Unit (IFU; Sparks & Ford 2002) to acquire simultaneous images at multiple wavelengths across the methane absorption band head at 1.6 m through the simultaneous spectral differential imaging (SSDI) technique (Racine et al 1999;Marois et al 2000Marois et al , 2005Biller et al 2004) could provide good short-lived and common-path speckle attenuations and increase detection limits. It has been shown that SSDI instruments are ultimately limited by non-common-path aberrations, which are expected to be stable over long periods of time, as they come almost entirely from the instrument itself (Marois et al 2005). Hence, the ADI technique nicely complements SSDI since it can be used to subtract the residuals caused by the non-commonpath aberrations.…”

“…If these two limitations were the only ones, a simple solution would be to integrate longer to average these random noises and gain as the square root of the integration time. However, observations have shown that for integrations longer than a few minutes, the PSF noise converges to a quasistatic noise pattern, thus preventing a gain with increasing integration time (Marois et al 2003(Marois et al , 2005Masciadri et al 2005). To achieve better detection limits, it is thus necessary to subtract the quasistatic noise using a reference PSF.…”

“…Both ground-and space-based imaging are plagued with this stellar PSF calibration problem caused by imperfect optics and slowly evolving optical alignments. For ground-based imaging, subtraction of a reference PSF obtained from a star close to the target achieves a factor $4 of PSF noise attenuation, leaving residuals that are also quasistatic and thus severely limiting detection of fainter companions (Marois et al 2005). For spacebased telescopes that have a better PSF stability, like the Hubble Space Telescope (HST ), a partial solution was found by subtracting two stellar images acquired during the same orbit with a different roll angle.…”

A wide planetary-mass companion to a young M3 star of the AB Dor moving group

“…This survey comprises two observational programs, both carried at Gemini-South, in Chile. The first uses NICI [2], a high-contrast imager, and takes advantage of Angular Differential Imaging (ADI [3]) and Spectral Differential Imaging (SDI [4]) to detect companions down to 2 M J up between 15 and 300 AU around the stars. Second epochs are currently being taken for this program, which surveyed over 40 stars.…”