UKIRT Annual Report 1999
THE UNITED KINGDOM INFRARED TELESCOPE
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
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.
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
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.
1This 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
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
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, Zrms, is analogous t
output of a Differential Image Motion Monitor (DIMM), the standard
site-assessment tool used world-wide, and is independent of the telescope
optical performance. Zrms 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
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
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.