WEAVE - instrumental overview

Below are summarised the expected observing capabilities of the WHT's new multi-object fibre-fed spectrograph WEAVE.

The LIFU (large integral-field unit) mode of WEAVE was commissioned in early 2023, and LIFU science observations began in October 2023. WEAVE/LIFU is currently offered for open-time observations. Commissioning of the MOS and mIFU modes of WEAVE will resume after the spectrograph-alignment investigation (ongoing as of Sep 2024).

As a result of the commissioning tests (WEAVE-team members can see a summary of key results here), there may be small changes to some of the numbers given on this page.

WEAVE is mounted at the WHT prime focus, behind a new field corrector / ADC, which provides good image quality over a 2-degree field of view, at zenith distances up to 65 deg.

Focal-plane modes
The instrument can be used in any of 3 focal-plane modes: MOS (multi-object spectroscopy), mIFU (mini integral-field units) and LIFU (large integral-field unit). The key parameters of each mode are summarised below:

In MOS mode, each of up to ~ 1000 individual fibres can be positioned anywhere within the field of view, with each fibre intercepting a circular area of sky of diameter 1.3 arcsec.

There are actually two sets of MOS fibres, one for each of two focal-plane plates A (960 fibres) and B (940 fibres), and WEAVE is designed so that configuration of one of the two sets of fibres (e.g. for plate A) takes place during a 1-hour observation with the other set (e.g. on plate B). Then the tumbler is rotated by 180 deg so that plates A and B swap positions.

In mIFU mode, up to 20 fibre bundles (each 11 x 12 arcsec² on the sky) can be positioned anywhere within the field of view, on plate B. For any given observation, the MOS and mIFU modes cannot be mixed.

In LIFU mode, a single IFU is positioned at the centre of the field of view. The hexagonal array of 547 fibres provides a (hexagonal) field of view of 78 x 90 arcsec². The fibres have circular cross-section with diameter D = 2.6 arcsec on the sky, and the centre-to-centre fibre separation S = 3.4 arcsec. The filling factor in the focal plane is π x (D/S)^2 / (2√3) = 0.55. Standard patterns are available for carrying out dithered observations. For more information about LIFU mode, see the LIFU pages.

The scale in the WEAVE focal plane is 17.8 arcsec mm^-1 for MOS/mIFU modes and 15.4 arcsec mm^-1 at the LIFU fibre array.

Spectrograph modes
WEAVE's fibres feed a dual-arm (blue + red) spectrograph housed on one Nasmyth platform of the WHT. A 5900-A dichroic splits the light between the blue and red arms. Dispersion is effected by inserting one of three VPH gratings in the blue arm, and one of two in the red arm, giving five spectroscopic modes. In low-resolution mode, WEAVE covers the wavelength range 3660 - 9590 A.

For any given observation, low- and high-resolution modes can't be mixed, e.g. it's not possible to observe at low resolution in the blue arm and high resolution in the red arm.

Note that because of the larger fibre size, the spectroscopic resolution of a LIFU observation is half that of a MOS or mIFU obsevation.

Note also that in high-resolution mode, observers have a choice of two possible VPH gratings, for wavelength range 4040 - 4650 A ('blue') or 4730 - 5450 A ('green'); they can't be deployed simultaneously.

The inter-CCD gap in the wavelength coverage (tabulated above) arises from the join between the two 6k x 6k low-fringing EEV CCDs comprising the science detector on each arm.

Each MOS/mIFU spectrum on the CCD has FWHM ~ 3 pixels (6 for LIFU spectra) perpendicular to the dispersion direction. The detector readout noise is expected to be about 3 electrons rms per 15-micron pixel. The CCD readout time for the default mode (slow, binning 1x1) is ~ 150 sec. No detector windowing is currently possible.

Signal-to-noise
The WEAVE exposure-time calculator can be used to estimate the signal-to-noise ratio per A for each focal-plane (fibre) and spectrograph mode, and as a function of sky brightness and seeing. Some results from the calculator can be found in this table provided by Scott Trager.

Autoguiding
Autoguiding for MOS and mIFU modes is provided by positioning guide-fibre bundles (diameter 4.8 arcsec) at the positions of up to 8 guide stars (mag <~ 15). For LIFU mode, a separate guide camera is provided.

Sky subtraction
The sky background is measured in MOS mode by assigning 5 - 10% of the science fibres to sky, in MIFU mode by assigning one of the mIFUs to sky, and in LIFU mode by sampling sky through 8 dedicated peripheral bundles.

Observations
Observations with WEAVE are specified as observing blocks (OBs), with each OB containing all the information required to make a 1-hour observation, which might include e.g. 3 autoguided science exposures, an arc and a flat. The 1 hour includes any observing overheads. The overheads for target acquisition, CCD readout and calibrations are expected to be <~ 10%.

WEAVE observations are carried out in service mode, by ING staff, with the aid of an OB scheduler. The scheduler tool optimises the use of the night by matching OBs with required observing conditions, and by taking into account the RA, science priority, change overheads etc.

Data reduction and analysis
Data-reduction and analysis (and archiving) are carried out automatically by the WEAVE pipelines at CASU in Cambridge, and the raw and processed data will be made available to the PI teams through the WEAVE archive. The results of a quick-look data reduction are visible to observers at the telescope.

Availability of time on the WHT during the WEAVE years
This is described in the document linked here. An average of 70% of the time on the WHT will be used for the eight designated (pre-planned) surveys (~ 15 million spectra) with WEAVE, while the other 30% is for 'open-time' community observations. During 2024 and 2025 at least, open-time observations will be carried out with WEAVE only, and observations with older instruments (e.g. ISIS, ACAM) or visiting instruments will not be possible during this period.

WEAVE data policy
The approved WEAVE Data Policy is described in this document.