WEAVE LIFU overview

The WHT's new multi-object spectrograph WEAVE is mounted at the WHT prime focus, behind a new field corrector (incorporating an atmospheric-dispersion corrector), which provides good image quality over a 2-degree field of view, at zenith distances up to 65 deg.

WEAVE allows science observations in any one of 3 focal-plane modes:

• LIFU (large integral-field unit) - the subject of these pages.
• MOS (multi-object spectroscopy) - up to ~ 1000 fibres placed anywhere in the 2-deg field of view
• mIFUs - up to 20 mini-IFUs placed anywhere in the field of view

In addition, a Focal Plane Imager (FPI) can be deployed anywhere in the field. It delivers images with size 2.4 arcmin x 2.0 arcmin, and the scale is 0.1 arcsec per pixel. There are no filters, and the FPI is not intended for science imaging. It is used during astrometric calibrations, for MOS acquisition, and for other troubleshooting and quality-control purposes.

The main parameters of the MOS, mIFU and LIFU focal-plane modes are compared below:

	MOS fibres	Mini-IFUs (20)	LIFU
Diameter of field over which deployable (deg)	2.0	2.0	On-axis
Diameter of individual fibres (arcsec)	1.3	1.3	2.6
Area of individual fibres (arcsec2)	1.33	1.33	5.31
IFU field of view (arcsec)	-	11 x 12	90 x 78
Fibres per IFU (and filling factor)	-	37 (0.50)	547 (0.55)
Minimum separation on sky (arcsec)	60	60	-
Tumbler position (deg)	0 or 180	180	90
Configuration time (minutes)	55	20	~ 1
Spectroscopic resolution (low-res mode)	5000	5000	2500
Spectroscopic resolution (high-res mode)	20000	20000	10000

The 547 fibres of the LIFU array cover a hexagonal field 90 x 78 arcsec²:

Each fibre has a diameter on the sky of 2.6 arcsec, and each yields one spectrum on the WEAVE (blue-arm and red-arm) science detectors. The spectroscopic resolution is R ~ 2500 with the low-resolution VPHs, and R ~ 10000 with the high-resolution VPHs.

The sky background is measured using 8 additional bundles of fibres, at a radius of 150 arcsec from the centre of the main array. More details can be found on the page about the geometry of the LIFU array.

The array has covering factor 0.55, so many observers will require dithered observations.

Acquisition and guiding are carried out with the aid of an off-axis imager.

Spectroscopy: light entering the spectrograph is split by a dichroic (wavelength 5900 Å) between the blue and red arms. Any of 3 VPH (volume phase holography) diffraction gratings can be inserted in the blue arm (one low-resolution, 2 high-resolution). In the red arm, one of 2 VPH (low-res or high-res) can be inserted. The detector in each arm comprises 2 6k x 6k CCDs, i.e 12k x 6k pixels. Each exposure on each arm delivers one raw .fit file containing the image plus headers and ancillary tables.

The observations are carried out via queue-scheduled observing blocks (OBs), which include the usual arc and flat calibrations.

The observed raw data are passed to the Cambridge Astronomy Survey Unit (CASU) for automatic pipeline processing, and subsequent automatic spectroscopic analysis and archiving. Open-time data are obtained and processed in the same way as WEAVE survey data.

In LIFU mode, the throughput (from sky to detector) is ~ 0.25 in lo-res mode and ~ 0.15 - 0.2 in hi-res mode. An exposure-time calculator is available.

The LIFU mode of WEAVE was commissioned in early 2023, and the pages in this tree incorporate the most recent commissioning results with regard to acquisition, guiding, spectroscopic resolution, detector properties and throughput.

Survey and open-time science observations began in October 2023. For press releases about WEAVE first light (late 2022) or the WEAVE inauguration (30 October 2023), click on the relevant picture below:

The LIFU pages were written by Chris Benn, Lilian Dominguez and Cecilia Farina (the WEAVE instrument-scientist team), but bring together results, calculations and figures from many people who have worked on the design, construction and commissioning of WEAVE (project pages here).

The commissioning team at ING comprises: Cecilia Farina, Chris Benn, Eduardo Mantero, Ian Skillen, Javier Mendez, Lilian Dominguez, Lucia Suarez, Marc Balcells, Nada Ihanec, Neil O'Mahony and Yeisson Osorio.