The aim of the WHT quality-control programme is to characterise and monitor all aspects of telescope performance, in order to detect as early as possible any potential problems (thereby maximising S:N for observers, and minimising downtime).

Before-and-after performance characterisation is of particular importance in connection with two major changes planned at the WHT over the next few years: commissioning of a major new instrument (WEAVE multi-object spectrograph) at prime focus in 2019, and commissioning of a new telescope control system in 2018.

Key performance-characterisation results are summarised below, with links to additional quality-control and performance information.

Optical performance:

• Optical quality (aberrations): FWHM < 0.3 arcsec at all focal stations
• Dome seeing: negligible when the primary mirror is less than 3 degC warmer than the air in the dome (> 80% of nights)
• Mirror reflectivity: typically 0.82 - 0.89, depending on when the mirror was last aluminised
• Geometrical disortion at prime focus: scale varies by a few % over radius < 30 arcmin

Pointing and tracking performance:

• Pointing: pointing rms ~ 1.5 - 3 arcsec, depending on focal station and position on the sky
• Blind-offsetting: accurate to <~ 0.3 arcsec rms over offsets of 20 arcmin
• Open-loop tracking (i.e. without autoguiding): ~ 1 arcsec rms over 10 minutes
• Closed-loop tracking (i.e. with autoguiding): <~ 0.05 arcsec on timescales of minutes, <~ 0.3 arcsec on timescales ~ 1 hour (flexure etc.)
• Rotator tracking: < 0.2 arcsec at the edge of the field of view, at all focal stations
• Oscillations: there's occasional evidence of oscillations ~ 1 arcsec, period ~ 1 minute, at Cassegrain, under investigation
• Azimuth, elevation and rotator slew speeds: all ~ 1 deg/sec

• Time lost to technical problems: ~ 2% over the last 10 years
• Time lost to bad weather: ~ 20% on average, with mid-summer and mid-winter averages of ~ 5% and ~ 40% respectively (average usable hours per night = 6.8, varies little with season)
• Observing overheads: instrument dependent, but typically ~30% for visitor-mode spectroscopy, less for imaging
• Use of time not scheduled for observing: typically ~ 8% of time at the WHT is for quality control, instrument setup, standdowns and commissioning