A.

Sources and telescope simulator

The extended source is a 1280x1024pxs OLED screen (mono or polychrome) able to send different extended or moving sources, with a spectrum between 450nm and 700nm. Additional pinholes are also used for alignment purpose and performance characterization in the case of star pointing. A set of objectives and collimation lenses allows to transfer the beam and adapt its size to the aberrations generators.

Two different solutions are proposed to send calibrated wave front errors to the active optics system. The dynamic solution, adopted for MADRAS characterization, is based on a magnetic ALPAO DM88 [4], which performance is better than the requirements. The static solution adopted for the RASCASSE experiment consists in a rotating wheel carrying different phase screens (SILIOS technologies [5]) with calibrated WFEs from 40 to 200nm RMS. Deviation from specifications is lower than 4 nm RMS for each masks.

The different SILIOS phase screens allow sending WFE with only form content, only mid or high spatial frequency contents, polishing errors and different combinations of these WFEs. This strategy is fundamental to characterize the WFS performance on low order modes estimation with and without high order modes. The amplitude of WFE are defined by the TAS system studies and directly etched on a glass polished transmitting plate. Additional laser cut pupil masks can be added close to the pupil plane to simulate the telescope spiders and secondary mirrors obscuration.

Fig. 1

Left: Illumination unit, Right: telescope simulator and zoom on the spider pupil

B.

The MADRAS active mirror: closed loop performance.

The design and performance of the space active mirror are extensively described by Laslandes et al in [1]. The active mirror is based on boundary actuation, ie the actuators influence is transmitted through the edges of the mirror in order to avoid any actuator print through effect (see Fig. 2).

Fig. 2

The MADRAS CAD and equipped prototype

The Zerodur mirror is hold by an Invar warping harness and actuated with classical PZT. With a useful diameter of 90mm, a total volume of 80x200x200mm³ and a mass of 4.0kg, the system is able to generate 24 modes with a correction performance of 5nm RMS per mode and less than 10nm RMS on random phase maps.

Fig. 3 right shows the closed loop performance of the active mirror on each mode, while the specifications and FEA results are presented on the left. Opto-mechanical interfaces can be adapted to receive and characterize any active mirror from 20mm to 200mm in diameter.

Fig. 3

Left: RMS of modes to be generated and performance obtained after FEA optimization. Right: Closed loop performance.

C.

The RASCASSE wave front sensors

RASCASSE aimed to design, realise and integrate two types of wavefront sensors, and compare their performance in the case of extended moving scenes for different scenarios. The scenarios are defined by the luminosity parameters, the contrast in the scene, and the content and amplitude of the WFE to be measured. The bench was designed to provide a 1x1deg² FoV with an optical quality of 60nm RMS and a WFE variation lower than λ/100 over the entire field. Each optical component have been characterized using Fizeau interferometer (accuracy<1nm RMS, repeatability<1 nm RMS). The static wavefront error induced by all lenses is less than 10 nm RMS.

The pupil WFS is a customized Shack Hartmann WFS, designed for the specific scenarios. It is made of a field stop, a pupil relay and a 10x10 μlenses matrix. (Fig. 4 Right)

Fig. 4

Left: The phase diversity channel. Right: The Shack Hartmann channel

The focal plane WFS is based on the phase diversity method, and uses a lateral shift splitter to feed the camera with two twin images, with a focus difference of 1.2λ. The Phase diversity channel is telecentric to provide a constant magnification on the two images (Fig. 4 Left).

Both WFS use an ORCA2 camera with a water cooling system. Fig. 5 shows the images obtained with an entrance grid of 7x7 point sources for the phase diversity, and a point source for the SH channel. Initial performances were characterized using an extended scene: grid of 7x7 points covering all the field of view. Initial aberrations are less than 60 nm RMS as specified. Variations on the field of view are around λ/25 with a major contribution coming from filed curvature.

Fig. 5

Left: Phase diversity image obtained with a grid of 7x7 point sources; Middle: Shack-Hartmann image obtained with a point source; Right: Total WFE over the field.

D.

Automation of optical configurations, acquisitions and data storage

Specific GUIs were developed for both benches. The MADRAS GUI allows calibration of the system and closed loop control as well as a display of actuators commands, WFS signal and imaging cameras.

All operations on the RASCASSE test bench have been automated for remote control:

E.

Long term stability

The uncertainties on long term were determined using representative sequences, including rotation of the phase wheel and translation of the collimation lens. Several sequences of more than 24 hours with more than one hundred changes of optical configurations were done.

For this long term characterization, an extended scene was used: grid of 7x7 points covering all the field of view. Using these acquisitions, the uncertainty at 3σ was estimated for astigmatism, coma and focus.

In addition, stability of the image position was checked by controlling the distance from the field of view center. Concerning aberrations, the uncertainty meet the requirement of λ/100 (Tab.1) Variations of distance from the center of field of view is less than a pixel, so that the image deformation can be neglected.

Tab. 1

Synthesis of uncertainties