The Starlight Xpress USB Adaptive Optics device provides an effective method of removing the effects of rapid guiding errors from CCD images. All but the most expensive telescope mounts suffer from rapid gear errors during guiding and such errors are very difficult to correct when the only control method available is to send speed corrections to the drive motors. A device that can adjust the image position by rapidly deviating the optical path, can correct for such errors very quickly and without the associated settling time issues.
A common method of shifting an image for AO purposes is to use a tip-tilt mirror to re-direct the beam through a variable angle. This works well, but deviates the optical path through 90-degrees and takes up a considerable back focal distance. Its motion sensitivity is also affected by the distance between the mirror and the CCD. A straight though device is more convenient and optically shorter, so the original Starlight Xpress AO unit was designed with this in mind back in 2004.
A secondary advantage of the straight through design is that it is possible to construct a system that has a well defined optical deviation for a defined input signal. This means that the sensitivity of the system in pixels shift per input step is essentially constant and is independent of the optical system used and the distance between the CCD and AO.
The AO element is a multi-coated AR bloomed plane-parallel optical window with a thickness of 13 mm and a diameter of 60 mm. This element can be tilted by up to approximately +/- 3-degrees.
Converging light from the telescope objective lens or mirror passes through the window on its way to the CCD chip, but is essentially unaffected when the window is perpendicular to the beam. However, when the window is tilted, the converging beam is displaced by an amount which can be defined as approximately 0.075 mm per 1-degree of tilt. The maximum image deviation is therefore approximately +/- 0.15 mm in both the X and Y planes. This corresponds to about +/- 23 pixels on the CCD of an SX-825 camera.