Near-Infrared Coronagraphic Imager (NICI)

Introduction

The Gemini-South Near-Infrared Coronagraphic Imager (NICI) is a specialized dual-channel camera with a dedicated Lyot coronagraph and 85-element curvature adaptive optics system designed to search for and image large Jupiter planets around nearby stars. NICI employs spectral image differencing by comparing two images taken in or next to strong near-infrared methane features found in the atmosphere of large Jovian-type planets.

While other instruments exist with similar science objectives, NICI is the first to integrate all systems necessary for planet finding into a single facility class instrument. The design philosophy dictates that NICI be limited only by the residual atmospheric wavefront and scattering. Significant effort was put into minimizing optical light scattering within the instrument and minimizing non-common path aberrations.

NICI was developed for and currently being used to carry out the NICI planet-finding campaign. The campaign, organized by UH's Mike Liu, Beth Biller, and more, seeks to observe 300 stars that may harbor companions. As of the middle of 2010, over half of the targets have been observed and the campaign has produced some exciting results. Read more about this campaign by clicking on the "The NICI Planet-Finding Campaign" menu option to the left.

(Image gratis www.gemini.edu public images.)

NICI's Primary Subsystems

Light Entrance

The entrance focus of the NICI is the f/16 Cassegrain focus of the Gemini-South 8-meter telescope. The AO relay is a standard single pupil-conjugated DM design with two exceptions.

  • The AO system uses a curvature sensor/mirror with 85 correcting elements
  • The dichroic that splits the light for the wavefront sensor and the science camera reflects near-infrared light to the science channel.

This approach with the dichroic minimizes ghosting and chromatic aberrations with an all-reflective science optical train up to the transmissive focal plane occulting mask of the coronagraph.

Wavefront Sensor Channel

Inside the wavefront sensor channel, guide stars may be selected by a field-steering mirror that has an 18" field of view. A membrane mirror adjusts the optical gain of the 85-element wavefront curvature sensor.

Science Channel

In the science channel, after encountering the deformable mirror, the AO-corrected wavefront passes through:

  • One of several focal plane masks held in a rotating mechanism.
  • The dewar window.
  • The pupil plane mask of the coronagraph, which is implemented in two stages:
    • A combined spider-mask and inner Lyot stop (having an equivalent central obscuration of 27%)
    • Outer Lyot stops, which are placed in a wheel. A variety of fixed pupil stops can be selected from 80%-120% pupil diameter stops.
  • A beam splitter which sends the beam to two science channels. A wheel allows a choice between muliple beamsplitters.
  • Each science channel has its own:
    • Filter wheel
    • Focusing mirror
    • Science array - 1024 x 1024 InSb
    • Array controller

Science Channel Notes

The first focal plane masks the AO-corrected beam encounters are transmissive and have flat-top Gaussian-shaped transmission functions. Notably, the focal plane masks are not fully opaque at the center but were designed to provide partial transmission for astrometry and registration of images in the data reductions. The masks effectively boost the dynamic range of the detector, allowing accurate determination of the position and flux of the central star relative to any faint companion candidates. In principle photometry could also be reconstructed from the final images though this must take into account the focal plane transmission function, the point spread function, and the stars location on the focal plane mask.

In the coronagraph, the spider masks (30 times over sized) and the inner Lyot stop are combined into a single rotating mechanism that can never be fully retracted from the optical path. The outer Lyot stop can be removed but the inner Lyot stop is always in place.


Image Source: Chun et al. 2008 SPIE paper

AO System

The adaptive optics system in NICI consists of an 85-element curvature wavefront sensor and deformable mirror. The
AO control electronics, software, deformable mirror, and wavefront sensor were designed and built by the Institute for
Astronomy at the University of Hawaii. Integration and initial commissioning of the NICI AO system was done with a
deformable mirror from CILAS, but this was later replaced by deformable mirror by the University of Hawaii with a
smaller minimum radius of curvature and higher resonance frequency.

NICI Performance

An important aspect of assessing NICI's performance is the data reduction pipeline. Teams at UH developed separate data reduction pipelines that differ slightly in the precise order of data reduction steps and the algorithms employed, however all produce similarl results. Beth Biller adapted the University of Arizona's SDI pipeline while Zahed Wahaj and Etienne Artigau developed new pipelines.

Upon commissioning in 2008, NICI performed close to expectations. Contrast versus angular separation at the time of commissioning is shown below. The curves are the result of Wahaj's data reduction pipeline.


The dashed red line labeled NICI-RfP is NICI's expected performance from simulations. GDPC refers to the results from the Gemini Deep Planet Survey for reference.

 

 

 

Adaptive Optics Laboratory
University of Hawaii Institute for Astronomy
2680 Woodlawn Dr. Honolulu, HI 96813
(808) 956-7434

Site design: Katie Whitman; Header graphic design: Banana Grafeeks