1. Overview of GHOST¶

Note

This section of documentation is a summary/transcription of the GHOST Concept of Operations Document (ConOps) as at November 2015. It should be reviewed and updated once the instrument is actually built and commissioned.

1.1. Description of the Instrument¶

GHOST is a fibre-fed echelle spectrograph. It has a wide variety of observing modes tuned to a range of science cases.

GHOST comprises two positioner arms, “IFU 1” and “IFU 2”. The layout of each focal plane is shown below. Each comprises multiple micro-lens arrays (black hexagons) feeding fiber bundles for targeting science fields at standard (large hexagons in IFU 1 and 2) or high (small hexagons in IFU 1) spectral resolution, peripheral guide fibers surrounding the science fields (red hexagons), and dedicated sky fibers at a fixed offset position a few arcseconds from the science fields (cyan hexagons). Each positioner can access different halves of the 7.5 arcminute field (as shown in Figure 4), with a 16” common region of overlap.

Warning

Find image

The instrument has two distinct observing modes. In ‘standard’ mode, up to two targets can be observed simultaneously using the large fibre bundles on IFU1 and IFU2. In ‘high resolution’ mode, a single target can be observed using the high-resolution bundle on IFU1. A ThXe calibration lamp can also be supplied simultaneously with the observations. Sky fibre bundles are provided for ‘standard’ mode on IFU 2, and for ‘high-resolution’ mode on IFU 1.

Mode	Standard Resolution	High Resolution
Spectral coverage	363-950nm, simultaneous	363-950nm, simultaneous
Spectral resolution	50,000	75,000
Radial velocity precision	600 m/s	10 m/s
Multiplexing	Dual targets/beam switching (82” minimum separation)	Single target
Patrol field	7.5’ semicircle (16” overlap between IFUs)	7.5’
IFU plane size	1.2”	1.2”
IFU element number and size	7 $\times$ 0.4”	19 $\times$ 0.2”
Sky fibres	3 $\times$ 0.4” (on IFU 1)	7 $\times$ 0.2” (on IFU 2)
Approx. limiting Vega magnitude [1]	17.4 - 17.8	17.0 - 17.4

[1]	Can achieve S/N ratio of 30 in 1 hour at 450 nm.

The GHOST control software positions the IFUs as required during observations. A mask in the slit injection unit blocks light from the unused fibre bundles.

1.1.1. Instrument Operating Modes¶

The following table is an overview of the instrument observing modes available to users, which are selecting via the Gemini Observing Tool (OT).

Configuration	Focal Plane	Resolution	Detector binning [2]	Guiding [3]	Approx. B mag.
Two-target	IFU1 & IFU2	50k	1x2	P2 & guide fibres	$< 18$
Two-target (faint)	IFU1 & IFU2	50k	1x4	P2	$\geq 18$
Two-target (very faint)	IFU1 & IFU2	43k	2x4	P2	$\geq 18$
Beam-switching	IFU1 & IFU2	50k	1x2	P2 & guide fibres	$< 18$
Beam-switching (faint)	IFU1 & IFU2	50k	1x8	P2	$\geq 18$
Beam-switching (v. faint)	IFU1 & IFU2	43k	2x8	P2	$\geq 18$
High-resolution	IFU1	75k	1x1	P2 & guide fibres	$< 17$
High-resolution (faint)	IFU1	75k	1x8	P2 & guide fibres	$17-18$
High-resolution (PRV)	IFU1 & ThXe	75k	1x1	P2 & guide fibres	$<16$

[2]	Reported as (binning in the spectral direction) x (binning in the slit direction)

[3]	P2 corresponds to peripheral wave-front sensor (PWFS) guiding.

Two-target modes

In standard spectral resolution mode, two targets can be observed simultaneously using the standard-resolution fibre bundles on IFU1 and IFU2, and the associated sky bundle on IFU1. Targets are specified to the OT using absolute astronomical coordinates; a guide star is also needed for the peripheral wave-front sensor (PWFS) guiding. Care needs to be taken to configure the instrument such that the PWFS does not vignette a science IFU.

For standard two-target modes, targets are expected to be bright enough that the PSF edges can be used for guiding via the guide fibres attached to the science bundles. For faint/very faint mode, guiding is by PWFS only. Guide fibres can also be disabled in standard two-target mode if crowded fields cause the guiding to be inaccurate.

In faint/very-faint mode, a larger detector binning is used to reduce the impact of read noise.

Beam-switching modes

In regions of low target density (i.e. where there is a single target within the GHOST field-of-view), the two standard-resolution IFUs may be beam-switched to provide continuous target observation, whilst alternating each IFU between the target and an offset sky position. This facilitates accurate sky subtraction by differencing sequential frames, avoides the resampling of bright sky lines or detector artefacts, and elimiates the effects of potential flat fielding errors and differential fibre throughputs. This is particularly useful for faint targets and the ‘red’ camera, where there are numerous time-variable sky lines.

The Gemini OT will automatically set diametrically opposed offset conditions for sky measurements, to allow beam-switching to be accomplished using telescope motion alone. However, in the case that this is inappropriate (e.g. crowded fields, or where the PWFS may vignette a science detector using the default configuration), it is possible to explicitly specify sky positions. This inflicts a time penalty, as the IFU positions will need to be reconfigured.

In the faint and very-faint modes, larger detector binning is used, and guiding via the GHOST fibres is disabled. Guide fibres may be used in the standard beam-switching mode, although like the two-target mode, this can be disabled if necessary.

High-resolution modes

High-resolution modes use the high-resolution science fibre bundle on IFU1. A high-resolution sky fibre bundle is on IFU2, and can be positioned independently of IFU1 for simultaneous sky observations. The use of a single science field provides maximum flexibility for the positioning of IFUs so as to avoid vignetting by the PWFS, and maximizes the patrol radius for selecting PWFS guide stars. Spectral binning in the spectral direction is not used in this mode, to fully sample the spectral PSF. A factor 2 binning along the slit is optimal.

Warning

This factor 2 binning isn’t reflected in the table!

The high-resolution science fibre bundle has six peripheral guide bundles, for guiding using the extended PSF of bright targets. This can be disabled as required, and is disabled by default in faint mode. Eight-pixel binning in the slit direction is also used in faint mode.

For targets requiring the best possible wavelength calibration, a precision radial velocity (PRV) mode is provided. A fibre agitator is used to reduce modal noise introduced to the fibres by stress, strain or imperfections. A ThXe calibration source is may also be fed into an additional high-resolution fibre which is passed to the spectrograph for calibration simultaneous to observations. This source is cycled on and off with a given duty cycle, giving total counts within a given exposure time to be similar in magnitude to the science fibres (and avoiding saturation).

1.2. Description of the Data¶

Note

Will actually need some, you know, data to do this completely.

1.2.1. BPM Flag Encoding¶

The bad pixel mask (BPM) flag encoding used for GHOST is derived from that used for GHOS, and is summarized below:

Bit	Pixel value	Meaning
0	0	No conditions apply (i.e. good data)
1	1	Generic bad pixel (e.g. region occulted/not illuminated; hot pixel; bad column)
2	2	Highly non-linear pixel response
3	4	Saturated pixel
4	8	Cosmic ray hit
5	16	Invalid data (e.g. all data rejected during stacking)
6	32	Not used.
7	64	Not used.
8	128	Not used.
9	256	SCI pixel value has been replaced via interpolation
10	512	SCI pixel value has been replaced, but not via interpolation