Description of UKIRT
Top-end and Secondary Mirror systems
Primary Mirror and Active Figure Control
(this describes both Cassegrain and wide-field mode)
NB: See also the bottom-end systems
which control the tip-tilt secondary and its postioner.
UKIRT's primary mirror is about 1/3 as massive as was normal at
the time of its construction in the mid-1970s. A key element of
its design is therefore the mirror cell, which is very massive and
rigid, with webs over a metre deep. The cell is suspended on
Serrurier trusses from a midsection which carries the declination
axis, and which in turn supports the topend via a second set of
trusses. An outline drawing of the telescope structure can be
The rectangular centre section is of unusual design: two
substantial beams to east and west (the "Dec Beams") are linked by
two thin tension members (the "tie beams") on their north and
south ends. Tension in the tie beams is maintained by tension in
the declination axis, which is maintained at ~20 tonnes by two
inflated stainless-steel toroids located in the yoke.
The thin tie beams were intended to permit use of a coude focus
with minimum obstruction in those attitudes when the central
structure must needs obstruct the beam; in the event the coude
focus was never used for observing, but the structure of the
centre section is exceptionally light and efficient. A view of a
solid model of UKIRT in its building, showing this structure, is
The telescope is equipped with a number of temperature probes
which allow both steelwork and adjacent air temperatures to be
measured. These are used for diagnostic and for control purposes.
In particular, the temperatures of the steelwork are used together
with a model of the telescope's elastic behaviour to predict
changes in the telescope's length are fed back as corrections to
the hexapod positioner so as to keep the instrument in focus
at all temperatures and attitudes.
Top end and Secondary Mirror systems
The telescope top-end, along with the secondary mirror, its
tip-tilt actuation system, its precision positioning system and
the associated bottom-end fast guider systems, was supplied by the
Max Planck Institüt für Astronomie (MPIA) in
Heidelberg. The top-end comprises a substantial tubular steel ring
which bolts to the fixed top-end ring above the upper Serrurier
trusses of the permanent telescope structure. It carries four
vanes which rise as they converge on the central boss. The vanes
are of 16mm stainless steel and are surfaced with
constrained-layer damping to reduce the transmission of vibrations
from the fast "tip-tilt" secondary mirror to the telescope
structure and eventually the fast guider sensor, which could cause
The converging vanes meet the square central boss, to which they
are bolted, tangentially; consequently the vanes are not
oriented exactly in the cardinal directions and diffraction
spikes should not be used for accurate image orientation.
Secondary mirror positioning system: the Hexapod
The secondary mirror and its actuation system are in turn
supported by a hexapod precision positioning system manufactured
by Physik Instrumente of Waldbronn, Germany (PI). The six legs,
containing precision screws driven by DC motors, are extremely
stiff and can position the secondary mirror with an accuracy of ~2
microns in translation and a few arcsec in rotation.
The original tip-tilt secondary mirror was delivered with the new
top-end systems in August 1996; these (see below) produced an
immediate improvement in image quality and telescope performance by
the elimination of image blurring due to telescope vibrations.
However the lightweighted mirror suffered a turned down edge,
significant print-through of the lightweighting pattern
(dramatically apparent in the out-of-focus image, right), r3
and r5 trefoil, and some spherical, aberrations caused by
thermal distortion due to the mounting system. Overall the defects
of the secondary were believed to reduce the limiting deliverable
Strehl ratio of UKIRT's images (all else being perfectly adjusted)
to around 50%.
Accordingly, in 1998-99 the MPIA procured a new secondary. This
shows none of the above defects (see out-of focus image, left).
Like its predecessor, the new mirror is made of Schott Zerodur
low-expansion vitreous ceramic, selected for very high optical
uniformity, and was also figured by Präsizion Optik of
Crailsheim, Germany, by testing through the back. However this
mirror was made ~14mm oversize, so that the edge could then be
ground off to eliminate turn-down. Again, like its predecessor,
the figured mirror was then 60% lightweighted, in this case by
diamond grinding the hexagonal pattern into the back. This was
carried out at ZBNM Technisches Institüt, Jena, Germany. In
the other critical departure from the previous process it was then
stress-relieved by acid-etching at Karl Zeiss, Oberkochen,
Germany. At the MPIA the mirror was fitted with three re-designed
athermal mounts, fabricated of Invar and Zerodur by PI, before
being shipped to the telescope. It was installed on 14 June 1999.
The following lists the specifications for the new secondary
mirror. The only difference between the old secondary mirror (K1)
and the new secondary mirror (K2) is the central hole size. For K1
this was 85.0 mm.
||313.4 +/- 0.1 mm
||92.0 +0.2 -0.0 mm
|Radius of curvature
||1723 +/- 2 mm
||-1.326 +/- 0.005
||30-50 scratch and dig
|- over entire diameter
|| <110nm P-V
|- over any 100mm diameter region
|| <70nm P-V
|- over any 50mm diameter region
|| <35nm P-V
The secondary mirror is equipped with a high-bandwidth
articulation system designed and manufactured by PI. The mirror is
attached to three piezo-electric actuators and can be rapidly
tilted in any direction to move the image by up to ±17" on
the sky. It is normally driven in antiphase with a
similarly-mounted momentum-compensating mass which corrects for
90% to 95% of the disturbance torques generated, and has a
bandwidth well in excess of 100 HZ. Its principal function is fast
"tip-tilt" image stabilisation, correcting image movements of
order an arcsec caused by telescope mechanical vibrations (e.g.
"wind shake"), drive errors and atmospheric seeing-induced image
movement. The tip-tilt system is controlled by a Fast Guider at a
telescope focus, which measures the position or the image of a
guide star up to 100 times per second and sends corrections to the
secondary control system to stabilise it. This system can also be
used for chopping between two points on the sky up to 20" apart.
(See Acquisition and guiding
systems for more details of the Bottom-End
systems which control the secondary's actuation and positioning
Primary Mirror and Active Figure Control
in both Cassegrain and Wide-Field Mode
The thin primary mirror is relatively flexible, and consequently
amenable to control of its large-scale optical figure by quite
modest forces. The Table summarises its properties.
UKIRT Primary Mirror
Outer edge thickness
Inner edge thickness
The components of the support system are
shown schematically in this figure.
The mirror is supported axially by 80 pneumatic "bellofram"
frictionless pistons, and radially by 24 lever arms glued to the
sides. The belloframs can be controlled as one (the original
arrangement) or as three separate sectors (the default arrangement
since the support system was upgraded in 1994). In addition there
are three radial positioners (located tangentially to the edge),
three axial positioners equipped with load cells and controlling
the sectors of belloframs by a servo loop which adjusts their load
Around the edge of the mirror are 12 force actuators. These
pneumatic devices can apply up to ±500 N with a resolution
of 1 N in an axial direction. By this means the primary can be
bent to correct the following low-order aberrations:
... where these limits are P-V wavefront errors.
- astigmatism up to ±6.9 microns
- triangular coma (trefoil) up to ±1.8 microns
- quatrefoil and cinquefoil up to ±0.5 and ±0.2
- spherical aberration up to ±0.5 microns
The last-mentioned aberration is corrected by generating a
substantial change in overall curvature. This is an r2
function, but the process produces some aliasing into r4
spherical aberration along with considerable defocus from the r2
effects. The latter is easily corrected by refocussing.
The amplitudes listed are those
achievable if the entire available authority of the actuators is
devoted to the single aberration; in practice several aberrations
must be corrected at once and at the time of writing residual some
spherical aberration (about 180 nm RMS on the wavefront) cannot be
All instruments on UKIRT are mounted at broken Cassegrain foci
facing one of four exit ports of the Instrument Support Unit
(ISU), attached to the bottom of the mirror cell. The ISU contains
a the rotating tertiary mirror which normally carries an
IR-reflecting dichroic coating, which directs the IR beam to the
to the selected port, while allowing visible light to continue
downwards to the acquisition and guiding systems. Fine adjustment
of the tertiary mirror in rotation and in tilt allows accurate
alignment of the instrument acceptance cone with the secondary
mirror. (Alternatively put, it can be adjusted so that the
delivered f/36.4 IR beam emerges emerges horizontally from the
Two tertiary substrates are of BK7 glass figured to a high
optical quality, and a spare of selected float glass is available
for special-purpose coatings if need be. No optical defects
originating in the tertiary mirror have ever been identified.
The normal dichroic coating was designed by Carl Zeiss
(Oberkochen). It is a proprietary multilayer silver-dielectric
combination "formula 2". When in good condition (it is quite
fragile) its properties are as follows.
float-glass substrate with a gold-on-chromium coating is also
available at the telescope.
- Transmission ~50% (TBC) of incident "CCD visible" light.
- Reflection: >50% longwards of ~850 nm, >97% longwards
of 1 micron.
- Emissivity: Probably ~2% longwards of 2 microns.