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NICI Imaging Modes
NICI offers several imaging configurations. The science camera is always fed by the AO system, and the basic imaging optics of the AO system and science camera are fixed, providing a pixel scale of 18 mas per pixel and a field size of 18x18 arcsec in all modes. In addition, both science detectors are read out in all modes. However, the science camera can be reconfigured by inserting different masks, dichroics or beamsplitters, and filters into the beam.
For planning and execution purposes, NICI observations are divided between two principal modes:
- Coronagraphic AO Imaging: Configured by selecting one of the Coronagraphic focal-plane masks and a hard-edged pupil-plane mask. The science target is placed under the coronagraph spot and is also used for Adaptive Optics guiding. For best AO performance, the guide star must be a point source with no close, bright companions. The mask and target are nominally placed near the center of the field, but can be at any position along an arc traced by the coronagraph spot as it moves with its wheel.
- Standard (non-Coronagraphic) AO Imaging: Configured by selecting the Clear focal-plane mask. The target must still be sufficiently bright for AO guiding, but can be anywhere within the 8" patrol radius of the AO steering mirror (i.e. there is no coronagraph spot limitation). The Strehl ratio degrades with the separation between the science target and the AO guide star, although this degradation should be small since the maximum offset radius is within the isoplanatic patch of about 20"
In standard imaging mode, NICI's performance is affected by several factors.
- The warm AO relay optics, the optical/IR beamsplitter, and the Focal Plane Mask all raise the instrumental emissivity. The sensitiivity to point sources is very good because of the high Strehl ratio of the AO correction, but sensitivity to extended emission, especially in the K and L bands, is relatively low compared to standard IR imagers.
- Even in standard imaging mode, the Focal Plane Mask is set to a Clear substrate to maintain proper focus. Imperfections on this substrate appear in focus in the science images, and at K and L these warm features glow. Standard sky subtractions and flat fielding remove most of these artifacts, but these calibration data must be taken carefully without moving the mask in between.
- The Spider Mask, with its oversized central obscuration and spider vanes, cannot be removed from the beam and therefore causes stronger diffraction effects in standard images. (These effects are suppressed by the occulting spot in coronagraphy.)
We consider these to be principal modes because there are significant operational differences between them. For example, in coronagraph mode, precise acquisitions are needed to center a target on the coronagraph mask, and careful monitoring of the centering and AO correction are required during the observation. Standard imaging observations, on the other hand, have more relaxed acquisition requirements and are generally simpler to execute.
Each of the two principal modes offers the following options:
- Dual- or Single-channel imaging: Dual-channel imaging is configured by selecting the Dichroic Wheel (DW) H50/50 Beamsplitter, which sends half the light to each channel in the H and K bands. Single-channel imaging is configured by setting DW to either Open, which passes all light to the Red channel, or Mirror, which reflects all iight to the Blue Channel. Although only 1 channel is illuminated, this mode provides maximum sensitivity if only a single wavelength is desired.
- Cass Rotator Fixed or Following: Selectable in the OT NICI component. Fixed mode causes the sky field to rotate between images, which helps to distinguish faint, nearby companions (which rotate with the field) from a primary's speckles (which remain fixed). Details on this so-called Angular Differential Imaging mode are described in Observing Strategies. In Follow mode, the Cass Rotator tracks the sky so the field doesn't rotate. It is used for imaging of extended targets.
- In standard imaging mode, dither patterns can be used to remove sky and detector artifacts, limited only by the accessibility of the AO guide star in the patrol field. Dithering is performed automatically by the Sequence Executor, including opening and closing the AO loops, so it is very efficient and reliable. However, its use should be carefully considered: in ADI mode the field rotation already provides a dithering effect, and moving the star around the field potentially changes the PSF which degrades the ADI subtraction.
- In coronagraphic mode, dithering is more problematic for the following reasons:
- In order to maintain the star under the mask, the two must be moved together;
- the mask moves along an arc traced by the wheel in which it is mounted;
- the mask position is not repeatable to sub-pixel accuracy;
- the mapping between the mask position and AO steering mirror is known only to limited accuracy.
- In coronagraphic mode, therefore, dithering requires a re-acquisition on the coronagraph spot at every step in order to maintain accurate centering. Due to the low observing efficiency, dithering is strongly discouraged for coronagraphy. The ADI mode is far more efficient and reliable for faint companion searches. For some applications where there is no choice, dithering can be permitted, but it must be at a low frequency (e.g. once every 15 minutes).
There are two additional principal modes which have not been officially commissioned.
- AO guiding on one target with a different target under the coronagraph spot: This mode has not been tested and is expected to be complex due to field rotation issues. Please consult with the instrument scientist to discuss feasibility.
- Extended AO guide star: For proper AO functionality the guide target should be a point source; guiding on extended targets up to 0.8 arcseconds diameter may be possible but is only offered on a shared-risk basis.
Sensitivity: Sensitivity and contrast strongly depend on the quality of the AO correction (Strehl ratio). The values listed on these pages are generally derived from observations taken with bright (V < 11) AO guide stars and in good seeing conditions. There is still little experience with very faint stars and worse than median seeing.