| The second generation SBIG Model AO-8 adaptive optics system sharpens the images taken with any ST-7, 8, 9, 10, or 2000 series USB CCD camera. It does so by actively correcting the light path seen by your telescope to eliminate image wander on the CCD detector due to low order local atmospheric effects (turbulence). It also corrects for residual periodic drive train errors in your mount, wind-caused telescope vibrations, and other erratic motions of the optical system that would otherwise be too fast for an autoguider or telescope drive corrector to respond to effectively. The AO-8 lets you obtain the ultimate in image resolution that your telescope and observing site can achieve, by using adaptive optics technology similar to that used to sharpen the images taken through the world’s largest ground-based telescopes.
The AO-8 mounts between your telescope and your SBIG CCD ST-series USB camera. It is controlled by the guiding CCD built into the camera. The AO-8 gets its power and commands directly from the ST-series camera via a short cable that plugs into the camera's I2C port. No other external cables are required for power or control. The AO-8 has an I2C output port as well so it is fully compatible with other I2C accessories that you might add to the system, such as a filter wheel or autofocuser.
Light coming into the AO-8 from your scope passes through a multicoated 6mm thick plane/parallel glass correcting plate in the AO-8 before it reaches the CCD camera’s imaging detector. This plate is mounted in a lightweight paddle that can be tipped and tilted by two geared stepper motors connected to it. Tilting the plate alters the path of the light rays passing through it. By monitoring a guide star with the smaller tracking CCD built into SBIG cameras, or with the SBIG Remote Guide Head, the AO-8 stepper motors tip and tilt the correcting plate to make fine adjustments to the light path approximately 10 times per second. This holds the image fixed on the same pixels of the CCD detector during the exposure. A feature image below shows the paddle holding the glass correcting plate element and the stepper motors within the AO-8 housing.
The glass correcting plate of the AO-8 design has a relatively large range of motion, roughly +/- 40 pixels in all directions. If you are imaging at approximately 2 arc seconds per pixel (a good value for deep space imaging), this means the AO-8 can correct for up to about 80 arc seconds of periodic error in the telescope mount. Since most modern mounts with periodic error correction (PEC) are capable of reducing any residual periodic error to well within this range, it is possible to take long exposures with the AO-8 without making any guiding corrections to a well-aligned mount at all. Guiding the light path in this manner is also far more accurate than can be achieved by issuing corrections to the telescope drive. The tilt of the correcting plate during operation does not lead to any image rotation or measurable defocusing at the edges of the frame, even when relatively large ranges of correction are required from the beginning to the end of the exposure.
The AO-8 is a closed loop system, which means that it checks the position of the guide star after every move of the correcting plate and makes appropriate adjustments to the next move. This results in a series of small, fast, and very precise moves that continue over the course of a long exposure. This is possible because the guiding chip is located behind the AO-8 and can measure the results of each move it makes.
Previous adaptive optics designs sometimes had problems when using a filter wheel and narrow band filters. Light from potential guide stars was attenuated by narrow band filters, resulting in guide stars that could be difficult to find or too dim to use. All current SBIG USB cameras support a separate and optional Remote Guide Head that essentially places the on-board guiding CCD anywhere in front of the filter wheel that the user decides. This Remote Guide Head duplicates all the functions of the built-in tracking CCD, including the fast readout function required to control the AO-8. By using the Remote Guide Head in an off-axis guider (such as the Astrodon #MOAG) in front of the filter wheel, you can still have all of the benefits of the AO-8, no matter what filter is being used, even filters that make starlight nearly invisible to the imaging CCD. Because of its compact design (it takes up only 2" of back focus), the AO-8 lends itself to the use of off-axis guiders.
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