The Swedish 1-m Solar Telescope (SST) is a refracting solar telescope at the Roque de los Muchachos Observatory in La Palma, one of Spain’s Canary Islands. Run by the Institute for Solar Physics of Stockholm University, the SST is the largest optical refracting telescope in the world able to observe 60 x 60 arc-seconds of the Sun, which equals 43,320 x 43,320 km on the solar surface or approximately three times the surface of Earth.
SST’s 85-electrode Adaptive Optics (AO) system, which enables astronomers to examine finer details of far fainter astronomical objects than is otherwise possible from the ground, is essential to pushing the frontier of solar physics. The Earth’s atmosphere causes light to bend in unpredictable ways; the AO reduces this distortion and blurring.
To deliver an image quality of unsurpassed quality for the AO, astronomers selected the Mikrotron EoSens 1.3MCL-CM camera with 1280 x 1024 pixel resolution, 14 μm pixel size, and global shutter. By using a CameraLink Full interface, the entire area of the camera’s 4/3″ 1.3MP sensor can be read out at 506 frames-per-second (fps). However, by configuring the image region to only 440 × 400 pixels, the Mikrotron camera was able to acquire the visual data for the AO at up to 3723 fps.
After installation, all internal image processing features of the Mikrotron camera were turned off, with the exception of the black level offset, which was adjusted so that the minimum intensity of an exposed frame was greater than zero when light was blocked from reaching the sensor. This is necessary to ensure that proper dark field correction could be performed.
The Mikrotron camera is connected to a frame grabber without any on-board memory installed, permitting incoming data to be written to the RAM of an Intel-based workstation PC, with only negligible delay. Running Debian GNU/Linux as its operating system, the PC comprises an ASUS Sabertooth X58 motherboard with an Intel Core i7 X990 CPU, along with 3 GB of DDR3 memory. The CPU has 6 cores, each running at 3.47 GHz.
The latency of the servo loop, which is the time from image exposure until the actuation of the deformable mirror the AO controls, was kept as low as possible for optimal performance. As soon as the last pixel of a sub-image has been written to RAM, its processing begins, so readout from the camera and image processing of the sub-images can proceed in parallel. Computational delay from image processing is only 72 microseconds after reading out the last row of pixels from the camera.
The SST has been able to acquire the highest resolution images of the Sun of any telescope. Its adaptive optics system is largely responsible for this. In addition to providing answers to pressing concerns like solar magnetic fields and the dynamics of the upper solar atmosphere, the SST continues to help astronomers comprehend how stellar spectra form.
Learn more about Mikrotron cameras at http://www.svs-vistek.com.
