Bringing the Visible Universe into Focus with Robo-AO

Light from astronomical objects must travel through the earth's turbulent atmosphere before it can be imaged by ground-based telescopes. To enable direct imaging at maximum theoretical angular resolution, advanced techniques such as those employed by the Robo-AO adaptive-optics system must be used.

The overall aim of this procedure is to compensate for the degrading effects of atmospheric turbulence on astronomical imaging. As light waves pass through the atmosphere, they're distorted. The compensation described here begins by focusing a high power laser beam in the direction of an astronomical object to create a bright artificial reference source.

Next, the shape of the returning laser light wave is measured with a wavefront sensor. Next, a command is sent to the deformable mirror in the optical system to correct the shape of all incoming light waves, including those from astronomical objects. Ultimately, the adaptive optics correction recovers diffraction limited images with an angular resolution of around 0.1 arc seconds at visible wavelengths on a 1.5 meter telescope.

One main advantage of this technique over existing methods like those performed at large telescope facilities is that our Robo AO system operates a visible wavelengths. The other main advantage is that it operates in a very efficient, fully robotic automated mode. Using these capabilities, we will search for close companions to nearby stars, most importantly, those known to host exoplanets.

We will perform precision measurements of the motions of stars in nearby stellar clusters, and we will search for and obtain images of transient events such as supernova. A key use for this system is in support of transient planet surveys like NASA's Kepler mission. If Kepler sees a transiting planet candidate, it might be a real planet or it might be an astrophysical false positive.

So robo looks at these systems to try to rule out false positive scenarios by looking for blended stars close by, and this is a key step in confirming that the thousands of Kepler candidates are actually planets. In addition to surveys that require an extremely large number of targets, the robotic nature of robio is perfectly suited to rapid response. Transient science, for example, imagine a transient goes off on the nucleus of a galaxy.

The exquisite spatial resolution of robio can tell you whether this is a star being tightly ripped by the central black hole, or just a supernova explosion that happened to go off nearby. Yet another example, much closer to whom in our own solar system, is long-term high spatial resolution monitoring of weather and volcano in in planets and their moons. The idea of equipping modest sized telescopes with a robotic adapto system is intended to dramatically enhance existing infrastructure in a cost effective way.

Install the components of the robo AO system on the telescope during daylight hours. Here, the adaptive optics are mounted, then the electronics, and finally the cabling is installed. Note that the laser is always mounted to the telescope with the system properly calibrated safe times for operation identified, and with the redundant shutter closed turn on the ultraviolet laser check, the conditions are safe to open the telescope dome once it is dark enough for observing.

This includes being in a safe range for humidity, dew point, depression, precipitation, wind speed, and airborne particles. Open the telescope dome and point to a star overhead that is relatively bright, one with apparent magnitude less than five. Refocus the telescope by adjusting the telescope secondary mirror until the star is approximately at its best focus.

Estimation of this, using a live image from one of the science cameras is sufficient. Point the telescope towards the selected target. Frame the objects in the field of view of the science cameras by adjusting the telescope direction as necessary.

Confirm that the uplink tip tilt mirror laser is centered in its range. Once this is done, open the internal and redundant laser shutters to allow the laser beam to propagate. Record one second of data from the Wavefront sensor camera, approximately 1200 frames while the procal cell optical shutters is turned off.

Now calculate a median image from this data. This will be used as a background frame to subtract any electrical or optical bias from images captured by the Wavefront sensor camera. Next, activate the pcal cell triggering system so that laser pulses from 10 kilometers are transmitted to the Wavefront sensor.

Adjust the uplink tip tilt mirror until the Shaq Hartman pattern of laser images appear in the Wavefront sensor camera. Leave the uplink tip tilt mirror in position with the pcal cell off momentarily. Record a new Wavefront sensor background image.

This is necessary as the optical background changes slightly as the laser is pointed in different directions by the uplink tip tilt mirror. Now start the high order adaptive optic system. The positions of the laser images from the Wavefront sensor are used to determine a mirror shape that will flatten the non planar light waves entering the telescope before they enter the science camera.

A weighted average of the position measurements is also used to maintain the pattern of laser images on the center of the Wavefront sensor. Begin observing in the visible range by choosing the desired filter. Set the angle of the atmospheric dispersion corrector prisms to minimize the residual atmospheric prismatic dispersion.

Next, set the frame size and exposure time of the electron multiplying CCG camera so that there is a minimum frame transfer rate of approximately 10 hertz and ideally 30 hertz. This capture rate typically reduces the intra exposure image motion below the diffraction limited angular resolution. Adjust the electron multiplication gain on the E-M-C-C-D camera such that the maximum intensity of the target is approximately half the well depth of the detector or at a maximum value of 300 for fainter targets for faint objects.

Slow the frame rate of the E-M-C-C-D camera until there are approximately five to 10 photons being detected in the core of the image point spread function. This ensures that there are enough photons for post-facto registration processing. Record a continuous set of images from the E-M-C-C-D camera for the calculated total integrated exposure.

When the observing is complete, close the telescope dome and point the telescope to the flat screen. Turn on the dome flat lamp Record a series of full frame images of the flat field illumination produced by the dome flat lamp on the screen for both the E-M-C-C-D and infrared cameras. For each astronomical filter used during the observation.

The flat field intensity at each pixel represents the combined quantum efficiency of the telescope. Adaptive optic system filters and camera turn off the dome flat lamp and switch to the blocking filters in front of each camera. Record a series of dark images on both cameras using the same range of exposure times and image formats that were used during the observations.

The dark frames are used to remove bias due to dark, current and electronic noise from recorded data. Finally, park the telescope and turn it off. The Robo AO laser adaptive optic system is used to compensate for atmospheric turbulence and produce diffraction limited resolution images at visible and near infrared wavelengths.

The left panel shows a typical image of a single star seen in red light through uncompensated turbulence. The image width is about one arc. Second, the right panel shows the same star.

After adaptive optics correction, the image width decreases to 0.12 arc seconds slightly larger than the 0.1 arc. Second perfect image width for the setup used. Note, the first airy ring can be seen faintly around the core of the image.

This much improved angular resolution enables the discovery of binary and multiple star systems. The increased resolution also allows for the detection of much fainter stars in dense fields as shown in this near infrared image of the messier three GLO cluster. On the left is the uncompensated image.

On the right is the robo AO corrected image. Features of solar system objects can also be seen with greater clarity. Here on the left is an uncompensated snapshot of Jupiter.

On the right is the corrected image showing more easily identifiable cloud features, and the transiting moon gani me indicated by the arrow. Once mastered the setup of a laser adaptive optic system on a new astronomical target can be achieved in just a few minutes. In practice, robo whale operates automatically with the procedures controlled by a robotic sequencing system that performs all of the setup steps presented here in less than a minute.

With this automation system, robo is able to complete up to 200 observations per night After its development. This technique paved the way for researchers to perform the largest ever imaging survey for close companions, covering thousands of nearby stars, and allowing a careful and detailed comparison of multiplicity properties across the entire main sequence and beyond. After watching this video, you should have a good understanding of how to use an adaptive optic system to correct and measure for the effects of atmospheric turbulence and recover diffraction limited imaging from the ground.