Contrast and Sensitivity

NICI is optimized for detection of faint, sub-stellar companions of stars by utilizing the simultaneous Spectral Differential Imaging (SSDI) technique. The performance of system is intimately tied to the data reduction and analysis pipelines.

In summary, a typical reduction pipeline would follow these basic steps:  (1) normal image reductions (flat fielded, dark subtraction, and bad pixel removal), (2) high-pass filter the image (for example removing azimuthal average/median profile), (3) generating a 'quasi-static' PSF for removal taking advantage of the ADI observing, (4) registration and subtracting the two filters to produce the SDI images, and (5) finally rotating all of the ASDI images to a common field position angle and combining to create a final science image frame.

To date the commissioning of NICI has focused on bright star performance in the full AO + coronagraph + ASDI.  As such, the adaptive optics system performance characterized so far is limited to guide stars V~11 magnitude and brighter.

NICI's adaptive optics performance delivered for brighter stars compares well with simulations. The Strehl ratios are measured on the detector focal plane of long-exposure images and include all non-common path errors. The simulation curves account for the measured static Strehl of 0.8 at 1.6 microns as measured on NICI's internal calibration source. The seeing is estimated from the AO telemetry data. The WFS counts are per channel per wavefront sample. NICI nominally samples the wavefront curvature at 1.3kHz. The seeing during these runs was better than the median seeing for the site. For bright guide stars (Vgs = 11 or brighter) the measured Strehl ratios agree well with the predicted performance curve. The performance appears to saturate at SR1.6um ~ 50% under very good seeing conditions but note that the curvature wavefront sensor has been used at only one optical gain (extra-focal distance = 0.4m) during on-sky commissioning. At these seeing values the optimal extra-focal distance determined from the simulations is smaller and from the simulations this difference would account for most of this difference.

Fig. 2 shows contrast curves in ADI mode (Cassegrain rotator off) under median (0.7”) to better than median seeing conditions. The integration times are 30 minutes on stars with brightnesses between v=8 and v=11. The curves illustrate the contrasts that be achieved from the occulting mask edge (0.32” radius) to 1” from the star, in the region where contrast is limited by speckles. Note that TWA-7 is considerably fainter than the other two targets and, in addition, the number of non-destructive reads used was less than that for the other two fields.

The contrast outside 1 arcsecond should be dominated by photon noise for long exposure with a high number of NDR. This highlights ADI's strong requirement on minimizing detector noise. The right panel of Fig. 2 shows the contrast curves scaled to a 2-hour integration. Commissioning data  confirm that the contrast increases with the square root of the exposure time. However, this has only been examined with a small number of datasets, none longer than 45 minutes of integration.

As reference the contrast curves specified in the Gemini Science Campaign Request for Proposals (RfP) and contrast curves obtained during the Gemini Deep Planet Survey (Lafrenière et al. 2007) are shown.

Exposure times can be estimated using the contrast plot of Fig. 4, and the table of zero points.

NICI is not yet explored enogh to provide a integration time calculator (ITC). Rough estimates can be done using the shown contrast and sensitivity curves, and also the zeropoint values.

Figure 1: Bright star performance measured for median seeing conditions and better. Figure from Chun et al., 2008, SPIE.

Figure 2: Preliminary contrast curve derived from commissioning data, from Z. Wahhaj, M. Liu, and the NICI Campaign Team. The contrast curves on the left show the achieved contrasts for the actual integration times (approximately 30 minutes) as well as the simulation/RfP contrast curve scaled to 30 minutes. The V-band brightnesses of the stars are listed in the legend. Note that TWA-7 is considerably fainter than the other two targets and the number of non-destructive reads was less than that for the other two targets. The figure on the right shows the contrast curve obtained on the two brighter targets scaled by the square root of the integration time to 2 hours. (Scaling to such long exposures has not yet been fully demonstrated during the commissioning phase.) For reference, the NICI-RfP curve and two contrast curves from the GDPS are also shown scaled to the same total integration time. Figure from Chun et al., 2008, SPIE proceedings.

Overheads

Acquisition overheads associated with setting up on each new science target include time for slewing  the telescope, acquiring guide star on the peripheral WFS (P2) and AO guide star on NICI OI WFS.

Long imaging observations (> 1h) should be split to gain flexibility for changing conditions and ease queue scheduling.

In imaging mode automated dither patterns have an estimated on-source efficiency of ~75%, where 25% of the elapsed time is used for telescope offsetting, NICI OI re-acquisition, detector readout etc; the exact overheads vary with the size of the offset and the exposure time.

Created: 1 Sep 2008, M. Hartung