Instrument Configuration

The primary user configuration decision is over the coronagraph focal plane stop and pupil mask, the observing wavelength (Y, J, H or one of two filters in the K band) and the desired contrast and inner working distance (size of the Coronagraphic mask). Coronagraph configurations are: direct imaging (for imaging or polarimetric imaging), Coronagraphic (combined with either polarimetric imaging, or spectroscopy). Note that the Y filter is a design goal; you should state clearly if the Y filter is critical to your programme.

The tables below list the properties of the standard GPI coronagraphic configurations. GPI will automatically select appropriate apodizer, focal plane masks, and Lyot stops for each wavelength. In GPI's apodized-pupil Lyot coronagraph, these three are carefully matched to each other - performance would be severely degraded if e.g. the Y-band focal plane mask was used at K band.

Notes:

Direct imaging (without coronagraph) is an option for all filters - to specify, use e.g. "K1-direct".

Spectral resolution varies slightly across the detector; typical values are given above. Each individual spectrum is 12-18 pixels long. The coronagraph mask diameter sets a hard limit on the inner working angle, but typically full performance is only achieved ~1 λ/D beyond the mask radius; the contrast curves given in the Contrast page should be used to predict performance at a given radius.

In addition, coronagraph masks are available for a non-redundant aperture masking mode, more details can be found here. 

In polarimetric mode, the light is not spectrally dispersed - the whole waveband is split into two polarization channels.

For your science target, you will need to be able to specify its magnitude both in the I band and in your observing waveband. This information, combined with estimates of r₀ and t₀ from prior, internal, WFS telemetry, is used to pre-configure the AO system, including loop gains, bandwidths, and to select the sub-aperture size for the fast WFS. Wavefront sensors, wavefront controllers, and calibration system all operate autonomously.

In the direct imaging mode (without coronagraph) the CAL unit is not available. Note that the contrast curves are based on the assumption of the use of a coronagraphic mask.

The choice of masks is dictated by your science goals.  A full set of possible combinations will ultimately be provided. The tables above give a representative list; in special cases, free mixing of mask combinations may be possible. Please contact the instrument scientist if you feel you need a non-standard combination which is not listed.The links below go to design documents which contain more details. Specific items are discussed in the following list:

1. Compromise masks for smaller and larger Inner Working Area OCDD Section 8
2. Saturation and brightest observable objects: FPRD Section 5.5.2.3.4 and 5.6.1.4

Science Camera Configuration

The IFS science instrument has few moving parts. The basic parameters are operating wavelength (set by filters) and polarimetry mode selection. The science camera incorporates the following mechanisms:

Cold pupil mask. As described above.

Filter wheels. Blocking filters are used to isolate diffraction orders and to control the background reaching the detector array. These will closely approximate the standard Y, J, H filters, plus two custom filters (K1 and K2) to span the two-micron window. Filter bandpasses are listed in the tables above, and are shown graphically in the figure below. As stated above, provision of the Y filter is a design goal.

Polarization analyzer. An optional polarizing beam splitter (a Wollaston prism), introduces an angular deflection between ordinary and extraordinary rays and produces two images with orthogonal polarization states on the focal plane array. Together with an external rotating half-wave plate, the analyzer permits measurement of the Stokes parameters.

Science Detector Configuration

On-chip integration time will be set automatically, by two considerations. First, the integration time must be long enough that speckle noise or photon halo shot noise must dominate over detector read noise and dark current.  Second, the on-chip integration time must be sufficiently short that trailing of images does not significantly degrade detectability of faint companions at the outer working distance. 

For bright stars, the H2RG will support subarray readout modes allowing very short exposures. In full-readout mode for spectroscopy, the detector will just enter saturation on H magnitudes brigher than 2.