Located on the Canary Island of La Palma, the Swedish 1-meter Solar Telescope (SST) is the world’s leading facility for high resolution observations of the Sun. It is operated by the Institute for Solar Physics(ISF), which is part of Stockholm University’s department for Astrophysics. Research at the institute primarily aims to gain knowledge about the outer layer of the solar atmosphere, which is dominated by magnetic fields. How do magnetic fields arise? How are they formed and ultimately destroyed or removed from the solar surface? How do they affect the Sun‘s outer atmosphere? How do they give rise to solar storms and the radiant energy that the Sun emits?
These questions are explored using observational data registered with the Swedish Solar Telescope. The telescope system looks at 60 x 60 arc-seconds of the Sun, which equals 43,320 x 43,320 km on the solar surface. This is an area that is more than three times bigger than the Earth’s surface but represents only 0.03% of the Sun’s surface.
The telescope uses adaptive optics to reduce the effects of atmospheric distortion. Atmospheric distortion is caused by the Earth’s atmosphere, which bends the light in random directions. It is the reason why stars seem to twinkle and why the Sun seems rippled at sunset. Without adaptive optics, the Swedish Solar Telescope would generate blurry images.
The Concept of Adaptive Optics
The adaptive optics system within the Swedish Solar Telescope, which was funded by the Swedish Research Council, consists of a Shack-Hartmann wavefront sensor and a deformable mirror. The Shack-Hartmann wavefront sensor is a glass plate with many lenslets etched on it, which subdivide the pupil of the telescope in 85 segments. Each segment delivers an individual image of the Sun. When the atmosphere disturbs the image, it causes the image to shift, and this shift is different for each segment. The shifts are measured and translated into commands to the deformable mirror, so that it takes a shape that compensates for the distortions.
Schematic showing the principle of Shack-Hartmann wavefront sensors. Left: If there is no atmosphere, incoming light rays remain parallel. The wavefront is plane. Right: Where there is atmospheric distortion, the light rays bent by the atmosphere hit the lenslets at different angles. The wavefront is corrugated. The deformable mirror reacts continuously to compensate for the shifts.
The wavefront sensor beam. From the bottom left to the top right: first is the Mikrotron camera, right in front of it is the Shack-Hartmann lenslet array, halfway the optical rail is the re-imaging lens, and at the end of the rail is a movable field stop. A little further is a broadband filter to select green light, behind it is a stack of prisms and a beamsplitter which divert part of the light to the wavefront sensor.
The Need for High-Speed Equipment
The problem is that the atmosphere changes quickly, so this has to be done very accurately and at a very high frequency. The adaptive optics system at the Swedish Solar Telescope has to correct the deformable mirror at least several hundred and preferably more than 1,000 times per second. This requires high-speed equipment.
The EoSens® CL high-speed camera by Mikrotron was installed in 2011 and is used to record the image formed by the Shack-Hartmann wavefront sensor. The image to the right shows a close-up of the Mikrotron camera with the Shack-Hartmann sensor in front. On the CMOS sensor itself you see a reflection of the image that the camera sees; a honeycomb pattern that is created by the Shack-Hartmann sensor.This image is composed of many small images of the Sun, each produced by one segment of the pupil. As the image is being sent to the computer, it is already being processed. By the time the last lines of the camera image are being received, the computer has already calculated the phase variation of the whole pupil. It then only has to calculate how to shape the mirror to produce an inverse phase variation. Within one second, 2,000 images are extracted, pre-processed and measured.
Image formed by the Shack-Hartmann wavefront sensor recorded by a Mikrotron EoSens® CL high-speed camera. You can see how its lenslets subdivide the pupil of the telescope in 85 segments. Each video is made of 1,000 images. The camera was running at 2,000 frames per second (2 kHz). The telescope was pointed slightly to the side of a small sunspot, which is the black spot in the sub-images.
The image shows the interface observers at the Swedish Solar Telescope see. The green boxes indicate the sub-images of the lenslet array. The small red crosses show the shifts in these sub-images as calculated by the software. The shifts are then translated into commands to the deformable mirror.
Fast Data Transfer
To ensure the highest bandwidth possible, the researchers at the Institute for Solar Physics use the following features of the EoSens® CL:
- The images are transferred using Full CameraLink®.This robust and powerful interface is widely used and makes integration into the existing setup easy. It enables high-speed data transfer in three configurations. The fastest variant, „Full,“ is set.
- The EoSens® CL is equipped with 8 taps. A tap is a data path transferring the image data.
- The pixel clock frequency is set at 80 MHz. With every pixel clock, the digital value of one pixel is transferred. CameraLink® supports a pixel clock range of 20 to 85 MHz.
- A region of interest consisting of 432 x400 pixels is selected. This means more than 2,400 images per second can be read out before the CameraLink® bus is saturated.
- In practice though, the frame rate is set to 2,000 frames per second. If the camera is run at the limit, any glitch or delay in processing will immediately cause a frame to be missed. Reducing the speed ensures the system runs flawlessly 24 hours a day.
- The in-frame counter is activated to detect missed frames and to check whether the frame grabber is correctly synched with the incoming frames.
The EoSens® CL offers a 10-bit per pixel output. “Although this is a nice feature to have, we are doing image processing in real time,” says Guus Sliepen, Research Engineer at the Institute for Solar Physics. “8-bit per pixel allows certain optimizations which are necessary for us not to exceed the available CPU power.”
Provision of Raw Data
While some features of the EoSens® CL are put to good use, others are not. All image enhancement tools within the EoSens® CL can be disabled. At the Swedish Solar Telescope, both the digital gain and the fixed-pattern noise (FPN) correction are switched off to ensure the EoSens® CL delivers raw data. “The reason we do not want the camera to do any corrections is because our optical system itself is not perfect, and also introduces variations in gain and offset for each pixel,” explains Guus Sliepen. “So we measure the dark field and flat field for the whole optical system, and apply the correction using our software.” One exception is the black level offset. It is raised accordingly to ensure that, even in total darkness, the pixel values are always above 0.
When asked about the camera’s best features, Guus Sliepen mentions its uncompromising speed and excellent performance. He, however, also highlights on its easy handling. “It does not require firmware updates and proprietary tools,” he says. “The serial interface ASCII is also easy to use.” He further compliments the camera’s well thought out design. “It is very solid and compact and its screw holes are very well placed, making it easy to mount.” He sums it up by saying: “The EoSens® CL camera is the easiest CameraLink® camera I have worked with.”
The Sun is unusually quiet, meaning that its activity is at a moderate level. This can be evidenced by the low number of sunspots. The granules, caused by convection, are blobs of rising and falling gas. The bright spots formed in the canyons between the granules are solar faculae. The video shows the image quality obtained with the Swedish Solar Telescope. With smaller telescopes, bright spots cannot be resolved. And thanks to adaptive optics and the excellent location of La Palma, a sharp image is acquired during the whole movie.