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Description of UKIRT
UKIRT is a 3.8m classical cassegrain telescope with a thin primary
mirror utilising an "English" yoke mounting. It has been the subject of a
concerted Upgrades Programme since 1990. It currently provides:
- Sky access from +60° 07' to -40° (equinox of date).
- All-sky absolute pointing with RMS errors ~ 1."3.
- "Encoder" offsets over a few degrees to ±0."3.
- Cross-head offsetting up to 3.'5 off axis; currently to <0."1.
- Image stabilisation to <0."1 (in winds <45 mph) by
a fast tip-tilt secondary mirror.
- Tip-tilt control by a CCD Fast Guider, on G or K stars
with V > 18. m6 in dark conditions.
- Auto-focus using the fast guider; focus actively maintained using
a thermal and elastic model of the telescope.
- Active correction of primary figure and alignment by a lookup
table. This, and optical performance, are regularly monitored.
- Active and passive dome ventilation and internal air circulation
Between 13 February 1998 and 29 September 1998 the delivered K-band image
quality (on exposures >tens of seconds) was systematically monitored and corrected for
undersampling. The results showed:
- Median FWHM ~0."433;
- Best quartile FWHM <0."25;
- Best directly measured FWHM = 0."171 (Strehl Ratio ~25%);
- About 10% of the time the image FWHM may be <0."15
(NB: the FWHM of a diffraction limited image is ~0."11 at K).
- In September 1998 the median corrected FWHM was
0."265
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