Image stabilization, in a system fast enough to compensate for atmospheric turbulence, qualifies as the lowest order of true adaptive optics. The control loop needs to be much faster than the turbulence varies, so a control loop with at least 100 frames per second is normally required. (Slower frame rates can correct for drive errors over the entire field of view, but will not be fast enough for diffraction-limited imaging.) Depending on the characteristics of the atmosphere and the size of the telescope, atmospheric tip-tilt correction will improve images over a field of view perhaps 1 arcminute in diameter. This is useful for general planetary imaging. A similar technique is called “Lucky Imaging”. This involves taking a long series of short exposure images (normally less than about 50 msec), and saving only the best images. The idea is that the atmospheric turbulence would be small during some exposures, resulting in some good frames. All of the best images, typically 5% to 15% of all the images, are then combined in software post-processing to reduce noise and give a high resolution final image. For Jupiter and Mars, the image series should be completed in a few minutes, or the planet rotation will complicate the combining.
It turns out that low-order adaptive optics is sufficient in a small telescope (optimally 10” to 24”) to add another level of improvement. Correcting defocus and two astigmatisms (along with image stabilization) will effectively double the aperture that can be used for lucky imaging. While major observatories are spending large budgets to create a complete adaptive optics system, their goals are different. Sky time is so precious on these observatories, that their adaptive optics systems are designed to work most every night, and on most exposures. For smaller telescopes, used by colleges and advanced amateurs, it is usually acceptable to get partial adaptive optics correction, and one can combine lucky imaging to generate a very good final result. The combining of lucky imaging with low-order adaptive optics (the AO-5) is the novel idea proposed by Stellar Products.
A prototype AO-5 was built in 1994, based on a 66MHz PC with a parallel port CCD camera. After 20 years, faster computers and better CCD cameras can be combined into an updated design! During the last half of 2014, Dr. Bruns configured an AO-5-type wavefront sensor with four subapertures and made atmospheric measurements using a 16” telescope. Analysis of the wavefront sensor data was recently completed. The results were plotted to show the probabilities of getting diffraction-limited imaging for a 16” aperture under a wide variety of seeing conditions, exposure times, and control loop bandwidths. The detailed results are being included in a new technical paper. To summarize, however, it is clear from the analysis that the probability of getting a single lucky exposure is negligible, unless some kind of adaptive optics is used. If fast tip-tilt image stabilization is included, the telescope might produce diffraction-limited images a few percent of the time, on a very good night. By adding defocus and two astigmatisms (the full AO-5), the probability increases to near 50% on good nights, and stays at about 10% on mediocre nights. This means that enough lucky images can be captured to make very high resolution images on most (but not all) nights. This is the key result and the motivation for continued development of the modern AO-5.
The AO-5 optics and electronics are now being re-designed to take advantage of the current fast cameras and computers. The goal is to make it lightweight enough for 10” aperture telescopes, but useful for telescopes up to 24”. The wavefront corrector will use the original design, based on moving lenses, instead of using a deformable mirror. Using lenses gives a very smooth correction, and individual actuators can be turned off without affecting the other corrections. An internal calibration source is used with the AO-5 software to partially automate and simplify user operation.
Since atmospheric turbulence limits the field of view of the full correction, only small diameter targets can take advantage of the adaptive correction. Also, the limiting magnitude is determined by the subaperture size. For a 16” telescope, targets down to magnitude 9 should be useful. This includes the Galilean moons, Mars (when far from Earth), Uranus, Neptune, and many close double stars. Getting resolutions of 0.3 arcseconds with a smooth corrector is the design goal. Updates to this page should be posted every few months. For any other questions, please contact Dr. Bruns using email@example.com.
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