THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1999

4. Approved Programme

UKIRT's approved development programme provides for maintenance of UKIRT's outstanding imaging performance after the completion of the upgrades programme, reduction in telescope emissivity, and the development of new instruments. In the longer term potential development strategies focus on UKIRT's role in support of the large new telescopes now coming on line.

4.1. Completion of the UKIRT Upgrades Programme

The outstanding imaging performance demonstrated in the seeing monitoring programme in 1998, which yielded a median image FWHM of 0.433" between February and October 1998, and an unparalleled median FWHM of 0.265" over the month of September, has not been matched in 1999. Though images of 0.3" to 0.4" have occurred regularly, the expectation of performance has been in about 0.5" and the quarter-arcsec images of 1998 are now suspected to be a legacy of the El Nino weather pattern of 1997-1998. Nevertheless efforts have continued to ensure that the intrinsic performance of the telescope is maintained and improved.

4.1.1. New Secondary Mirror

The replacement secondary mirror, provided like its predecessor by the Max Planck Institute for Astronomie, Heidelberg, was installed on 14 June. Tests immediately confirmed that the production process for the new mirror has been successful. Because it was deliberately made oversized and ground down to the specified diameter after figuring, the mirror shows no signs of a turned-down edge. Acid-etching of the rear surface for stress-relief after lightweighting has removed all trace of print-through from the latter process. New mounting pads had been designed to be athermal, to avoid the temperature-dependent distortion caused by the original design; this too has been successful and no trace of the R³ and R⁵ trefoil distortions caused by the thermal stress are seen.

The main residual defect of the telescope optics is now spherical aberration. It appears that the primary mirror exhibits some residual spherical aberration (we have very little on-telescope test data for the primary mirror alone, since UKIRT does not have an accessible prime focus) and this may have been partially corrected by aberration of opposite sign induced by the thermal stresses in the old secondary. Currently most of the dynamic range of the active primary control system is devoted to correcting this aberration; about 180 nm RMS remains. The residual aberrations of the telescope now probably limit its deliverable Strehl ratio (ratio of central intensity in a delivered image to that in a perfect image) to around 0.77 at the K band. The current performance is therefore very close to the goal of the Upgrades Programme (an intrinsic Strehl ratio of 85% at 2.2 µm, corresponding to a total RMS wavefront error of 142 nm). The effects of this small residual wavefront error will only be discernable in the very best seeing; nevertheless further improvements are possible. Two spare sets of the new mounting pads were ordered; one of these will be installed in the ``old'' secondary mirror after it too has had its rear side etched for stress-relief to remove the print-through. <sup>1</sup>This should leave the ``old'' secondary with the turned-down edge as its only significant optical defect; it can then be employed as a temporary replacement for the new secondary while the latter is upgraded, probably by ion-figuring, to remove the residual spherical aberration. High-performance (low emissivity) coatings, even experimental ones, can be contemplated: there are obvious advantages to having a spare secondary!

4.1.2. Automatic Focussing

Most factors degrading telescope performance can be constrained to individual insignificance, so that no one defect is obviously governing the image quality. Focus, however, is normally determined directly from image quality. This virtually ensures that focus errors are the largest optical aberration, since they are identified by detection of appreciable image degradation. Consequently we have sought for some time to constrain focus errors as far as possible (in 1997 implementing automatic stabilisation of focus against temperature changes and telescope elasticity effects). Even so, checking focus at the start of a night is a time consuming process, and since the correcting model has its limitations, it must be repeated during the night if the best image quality is to be secured. An auto-focus facility, which allows fast and objective focus measurements which do not depend on image quality, was therefore implemented when the new secondary was installed. This is a facility of the Fast Guider, and substitutes a 2 x 2 lenslet array for the single re-imaging lens used for normal tip-tilt fast-guiding. This produces four sub-images, each formed by a quarter of the telescope aperture, on the guider CCD. The radial separation of the four sub-images is a measure of the focus setting of the telescope. This hardware function was in fact provided with the Fast Guider on its original delivery by the MPIA, when the intention was to implement adaptive focus correction of the images, i.e. the removal of seeing-induced focus errors (expected to be the largest component of tip-tilt corrected seeing). In the event the short-term focus excursions were often found to be too large to be accommodated by the dynamic range of the piezo-electric actuators and the attempt to implement adaptive focussing was abandoned.

As now implemented by N. Rees, the new facility determines the telescope focus at 60 Hz and averages the result over 32 s, before applying the averaged correction to the Z-position of the secondary through the hexapod positioner which supports the tip-tilt assembly and the secondary itself. This facility enables fast, objective, focus measurements to be made whenever convenient during the night. If a bright enough guide star is employed (the working limit is around V = 14), active focus and fast tip-tilt corrections can be applied continuously. This has not been a common mode of operation, but focus checks are now routinely done when pointing is checked on a ``nearstar'', as the whole process takes about a minute, and is unobstrusive to the observer. The corrections determined by the autofocus system are relative, and must still be calibrated for each instrument. This is done during engineering time, using analysis of image properties away from best focus, and is repeated at intervals of a few months.

4.1.3. Pure-Seeing Monitoring

A bonus offered by the auto-focus system is a parameter which is a sensitive measure of ``pure'' seeing. This is the RMS of the focus fluctuations measured by the autofocus system during the averaging period mentioned above. The resulting parameter, Z_rms, is analogous t o the output of a Differential Image Motion Monitor (DIMM), the standard site-assessment tool used world-wide, and is independent of the telescope optical performance. Z_rms is therefore useful for investigatio n of local seeing effects and their dependence on conditions, without the potential complication of telescope defects, tracking errors, etc. ²

4.1.4. Emissivity Monitoring and Improvement

A concerted effort to improve the monitoring of telescope emissivity with CGS4 was undertaken in 1999. After careful verification of the effects, e.g. of taking sky measurements during dark time or at dawn, and calibration measurements with dome and DVS open and closed, an automated script was introduced for rapid, efficient measurement of emissivity at the end of nights on which CGS4 is used. This employs two measurements with the echelle grating and takes only a few minutes. Emissivity measures are now available on a roughly weekly basis, providing a denser database of measures than previously available. The results are adequate to allow monitoring of the evolution of the dichroic performance, for example. Efforts to use the 150 l/mm grating at times when the echelle is not in the spectrometer have been less successful, and the results are not yet considered reliable. The overall emissivity measurements have been supplemented by the use of a reflectometer/scatterometer, with which the accessible surfaces (primary mirror and dichroic) are monitored, and changes resulting from CO₂ cleaning can be quantified. ³

4.1.5. Primary Mirror Cooling

The primary mirror cooling system made considerable progress. A trial run late in the year demonstrated overall functionality but revealed several problems with the delivery of cooling power, with coolant piping integrity and control issues. These are being corrected.