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Technical information




Other information

Chris Benn
WHT manager

William Herschel Telescope


After the commissioning of WEAVE, 70% of the time on the telescope will be used to carry out dedicated surveys with the new instrument. On current plans, the remaining 30% of the time is available to the community for observations with the Cassegrain instruments, with visiting instruments or with WEAVE. Given the complexity of mounting and dismounting WEAVE, the current plan is to change to non-WEAVE instruments not more than once every ~ 9 months.

The current common-user instruments on the 4.2-m William Herschel Telescope are:

  • ISIS - single-slit spectroscopy, R < 10000, 4' slit, spectro-polarimetry
  • LIRIS - IR spectroscopy, R < 4000, and imaging (including imaging polarimetry), 4' field
  • ACAM - optical imaging, low-resolution spectroscopy, 8' field

Visiting instruments are mounted at the Cassegrain or Nasmyth (GRACE) f/11 foci or at the f/3 prime focus. The enclosure at the Nasmyth focus includes an optical bench. Recent visiting instruments include AOLI, CANARY, EXPO, FASTCAM, GHaFaS, HiPERCAM, PAUCam, PNS, SAURON and ULTRACAM.

The multi-object spectrograph AF2, prime-focus imaging and adaptive optics (NAOMI) are no longer offered.

The time-application procedure is documented here.

It takes less than a minute to switch incoming light between straight-through Cassegrain (usually ISIS or LIRIS) and folded-Cassegrain (ACAM) foci, and a few minutes to switch between Cassegrain and Nasmyth foci. Switching between Cassegrain and prime foci (which involves installing or removing the secondary mirror) is a daytime job.

Targets with significant proper motion (e.g. comets) can be observed with differential tracking.

The median seeing at the WHT is 0.8 arcsec during April - November. The dark-of-moon sky brightness at high ecliptic latitude is similar to that at other good dark sites, V ~ 21.9 mag arcsec2.

The CANARY laser guide star is in use at the WHT during a few nights per semester. During these nights, the risk of collisions with the pointing of other telescopes can be queried via the laser traffic control system pages.


2023 February:
LIFU commissioning is almost complete, and this mode of WEAVE will be offered for observations in 2023B.

2022 September 29: On-sky commissioning of the LIFU mode of WEAVE begins, in parallel with pre-commissioning tests of the fibre-positioning system (for the MOS and mIFU modes).

2022 summer: WEAVE sub-systems assembled on the WHT, pre-commissioning tests begin.

2020 February 17: Start of pre-WEAVE standdown for preparation of telescope, and integration and testing of WEAVE components. This will be followed by on-sky commissioning and science verification. On current plans, the standdown, commissioning and science verification will require most of 2021 and 2022. The WEAVE science surveys should begin after that. (Updated September 2022.)

2019 November 4: Update on the items below: due to a delay in delivery of part of WEAVE, part of 2020A has been returned to science.

2019 May 28: Update on the item below: due to a delay in delivery of part of WEAVE, ~ 3 months of 2019B has been returned to science.

2019 February: In 2019B, the amount of time available for observing with the WHT will be very restricted, due to the upcoming assembly, integration and on-sky commissioning of the new MOS fibre spectrograph WEAVE from 2 September 2019 until spring 2020. The remaining 2019B nights, August 1 - September 1, will be used to finish off existing high-priority science programmes requiring the current instrumentation. Recent information about the timescale of the developments under way at the WHT (and INT) can be found on the ING instrumentation-update pages.

2017 March: The lower dome shutter has been repaired. Observations at elevation < 25 deg will no longer be vignetted by the dome (plot).

2017 February: AF2 and the prime-focus imager (16-arcmin field) will not be available in 2017B.

2017 January: Vignetting at low elevation: the WHT lower dome shutter is currently stuck in a raised position, and observations at elevation < 25 deg will be vignetted (plot).

2014 August: As a result of the ongoing rationalisation of instrumentation at the WHT, the adaptive-optics suite (NAOMI/INGRID, NAOMI/OASIS) will not be offered after 31 January 2015, i.e. from 2015A onwards.

2013 February: Differential tracking with autoguiding is now available, but is not compatible with some dither/nodding scripts (as often used with e.g. LIRIS). Observers interested in dithered, autoguided, observations of a moving target should contact ING.

For older/retired news items, click here.

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Contact:  (WHT Manager)
Last modified: 17 February 2023