CIAO : an on-the-shelf adaptive optics system for astronomers

. Since 1990, adaptive optics are used in astronomy to remove the effects of atmospheric turbulence, and then retrieve diffraction-limited images, even in bad seeing conditions. Thanks to its strong knowledge in Shack-Hartmann wavefront sensing and deformable mirror, Imagine Optic has developed a simple and affordable adaptive optics system for astronomers. We present the current prototype as well as first experimental results on both natural stars and extended sources, with the main goal of allowing an effective correction in all sky conditions regardless of the object.


Introduction and context
Adaptive optics has proved to be an indispensable tool for astronomers observing from the ground.The techniques developed in this field now make it possible to overcome the harmful effects of turbulent air masses on the wavefronts reaching telescopes.
If adaptive optics remains limited today by seeing conditions and the amount of light, the fact is that they are powerful systems and highly demanded by astronomers.However, these systems are expensive and often custommade for specific telescopes, so only a few can afford them.
CIAO, for Compact Innovative Adaptive Optics, is designed to be interfaced on [0.5 -2] meter class telescopes, therefore being more easily adaptable and affordable for the astronomical community.

An affordable on-the-shelves system
CIAO can be used on various fields, with standard components: • For science applications: to inject the light from a star in a single mode fiber • For amateur astronomers: to increase the number of stacked images in the lucky imaging technique & to allow high resolution even in bad seeing conditions • For satellite communication applications: to increase the coupling efficiency in a single mode fiber • For Space Surveillance Awareness: to increase image resolution By the way of a semi-reflection mirror or a dichroic mirror, a part of the light is sent to the wavefront sensor, and the other part is directed to the scientific path.
The system can modify the transmitted wavefront at the scale of 10 nm RMS: the quality of the acquired images is therefore increased thanks to the deformable mirror which compensates for aberrations due to atmospheric turbulence in real time.
CIAO can be fixed on a telescope through the eyepiece interface and creates a new focal plane on which a scientific camera can be installed.A LabVIEW software drives the deformable mirror using data from the wavefront analyser.

Experimental results on natural stars and extended sources
The system has already been tested in different conditions and on various objects.For its first observation in 2017, CIAO has been installed on the one-meter class telescope of the Pic du Midi observatory.To test the system in a classic setup with natural sources, its first targets were stars.Once the concept has been validated, and thanks to satisfactory results, CIAO was adapted to do adaptive optics on extended objects.
Under seeing conditions of 0.71 arcsec, Mars was targeted despite its low apparent size.A series of short exposure images was acquired, and combined with the lucky imaging technique, the gain was clearly visible on the resolution of the images.The microlenses formed spots small enough for them to be separated and for their barycenter to be calculable.For this observation, the loop was running at 200Hz.Recently, in February 2023, the CIAO system was interfaced on the 355 mm diameter C14 telescope and observed Aldebaran, with a seeing of 2.2 arcsec.The loop was running at 300 Hz.  Figure 4 shows the signal obtained proportional to the number of photons at the output of the single-mode fiber : therefore, the detection threshold can be increased by a factor of 10.
Coupled to lucky imaging, CIAO is particularly well adapted to planet imaging, to study their atmosphere and to increase the number of possible nights where high resolution can be obtained.It is therefore possible to remove static aberrations of the telescope coming from misalignment and thermal effects, and to retrieve diffraction-limited images.
Based on a simple architecture and on-the-shelves components, CIAO is a smart and ideal solution for astronomers who are looking for an affordable system to do high resolution imaging, or couple the light from the telescope into a single mode fiber with high efficiency.
The use of on-the-shelves components makes it possible to have a high-performance adaptive optics system even on extended sources, with a significant gain.The current prototype continues to be modified and improved to ensure its operation in all sky conditions whatever the object as long as it is bright enough.

Fig. 1 .
Fig. 1.The CIAO system and its components.CIAO is based on a piezo-electric deformable mirror and a Shack-Hartmann wavefront sensor, a variation of the standard product HASO4 FAST developed by Imagine Optic, which can sample the wavefront up to 1200 Hz.The 40 actuators of the deformable mirror can change the wavefront up to 2000 Hz.The optical system between the object plane and the image plane is optimized in the visible spectral band, for a field of 8x8 mm².By the way of a semi-reflection mirror or a dichroic mirror, a part of the light is sent to the wavefront sensor, and the other part is directed to the scientific path.

Fig. 4 .
Fig. 4. Light from Aldebaran in a single mode fiber.