In Martini a 6-segment adaptive mirror is placed downstream of the telescope focus at a conjugate of the turbulent layer. An off-axis toroid mirror reimages the telescope focal plane further down the GHRIL table. A dichroic placed about 300 mm upstream of the reimaged Martini focus diverts the infrared light to WHIRCAM via the f-converter. The visible light is transmitted to the CCD wavefront sensor where the corrections to be applied to the adaptive mirror segments are measured. The converging f/11 beam reflected by the dichroic falls on a convex mirror (M1: 100 mm radius of curvature) mounted 50 mm upstream of the reimaged focus and tilted at 5° to the axis. This mirror collimates the beam and reflects it onto a concave mirror (M2: 500 mm radius of curvature) which is mounted 200 mm from M1 and tilted so that it sends the beam to WHIRCAM parallel with the beam from the dichroic. The concave mirror converts the beam to f/55 and forms a focus 250 mm from its front surface, which is in the same plane as the reimaged focus from Martini but translated across by 25 mm.
The 100 mm r 
convex mirror gives an image scale on WHIRCAM
of 0.05 ''/pixel. It can be substituted by a 160 mm r mirror (M1a)
placed 175 mm from the dichroic to give an image scale of 0.08 ''/pixel.
The pupil images formed inside the cryostat by these f-converters have
diameters of 3.3 mm and 5.2 mm respectively, and for maximum sensitivity, these
should be matched by cold stops. The rear filter wheel currently contains
stops with diameters of 7, 5 and 3 mm which can be rotated into position just
ahead of the fixed cold stop. The stops do not lie precisely at the position
of the pupil images and it has not yet been possible to evaluate which stops
give the best performance. Furthermore, the small pupil image sizes make the
alignment of the instrument very critical and it is probably wise to use
oversized stops until the alignment accuracy has been assessed.
The concave f-converter mirror can be aligned to the optical axis of WHIRCAM by positioning the mirror 250 mm in front of the WHIRCAM focal plane (which is 125mm from the front of the snout) and running movie mode after selecting the nbL filter with one of the small stops (say 5 mm for the 0.05 ''/pixel option). The mirror can then be tipped so that it reflects back into the cryostat and adjusted so that it is centred by altering its height (which should be very close to 150 mm above the GHRIL table) and translation so that the signal is minimised. When it is aligned, out-of-focus images of the corners of the mirror may be faintly visible.
The Martini beam from the dichroic can be aligned using the Martini laser beam injected into the telescope focal plane. The beam from the dichroic is centred onto M1 by tipping and tilting the dichroic with the micrometer adjusters. From M1, the beam is directed to the centre of M2 by tilting the M1 kinematic mirror mount with hex wrenches. Finally, the beam from M2 is aligned with the WHIRCAM optical axis by pointing it at the dimple in the centre of the cryostat window cover; this is achieved by tilting M2 with the hex wrench adjusters.
The mirrors are glued into holders which are held in kinematic mirror mounts
supported on posts and bases (Thorlabs).
The f-converter mirrors were supplied by Technical Optics (Isle of Man).
Their surface accuracy is 
and they are coated with a protected
gold coating.