The AO-L and AO-8 are second generation adaptive optics systems from SBIG specifically designed to enable an SBIG camera user to
obtain the ultimate in image resolution that his/her telescope and site can achieve. The AO-L is designed for the STL series
cameras. The AO-L can also be used with any ST-7/8/9/20/2000 USB camera with the appropriate adapter kit. The smaller AO-8 is a
replacement for the former AO-7 and can only be used with ST-7/8/9/10/2000 USB cameras. Both of these units get power and
commands directly from the camera via a short cable to camera's I2C port. No other external cables are required for power or
control. Both units have an I2C output port as well so they are fully compatible with other I2C accessories that you might add to
the system, such as a filter wheel or our soon to be released autofocuser.
TIP-TILT HIGH SPEED GUIDING
SBIG has exploited the second guiding CCD detector in our self-guiding cameras to stabilize stellar images, enhancing resolution.
These new AO systems use a tip-tilt transmissive element to correct for image wander due to low order local atmospheric effects
and for correction of mount errors, wind vibration and other erratic motion of the optical system that is otherwise too fast for
an autoguider or telescope drive corrector to respond to effectively. By monitoring a guide star with the smaller tracking CCD
that is built-in to SBIG cameras, or the Remote Guide Head, the AO makes fine corrections at approximately 10 times per second to
hold the image fixed on the CCD during the exposure.
The system is closed loop, which means that it checks the position of the guide star after every move and makes adjustments on
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 guide chip is located behind the AO device and can measure the results of each move it makes. In the
past, with the AO-7, this presented some difficulties when using narrow band filters because light from potential guide star
candidates was attenuated by the narrow band filter to one degree or another resulting is guide stars that could be difficult to
find or too dim to use. However, the new AO design and new camera improvements eliminate this problem. Because of their more
compact design, the new AO systems lend themselves to the use of off-axis guiders that can place a pick-off mirror or diagonal in
front of the filter wheel. Also, all new USB cameras support a Remote Guide Head that essentially places the on-board guiding CCD
anywhere in front of the filter wheel the user decides and this Remote Guider duplicates all the functions of the built-in
tracking CCD, including the fast readout function required to control the AO. This means that by using a custom off-axis guider,
one can still have all of the benefit of the AO no matter what filter is being used, even filters that make starlight nearly
invisible to the imaging CCD.
Another benefit of the new AO design is that they take up far less backfocus. The former AO-7 required about 3.5 inches of
backfocus and placed the camera at right angles to the optical system. Inserting an off-axis guider or other pick-off mechanism
added even more backfocus. However, the AO-L and AO-8 require only about 2 inches of backfocus (when attached directly to the
camera) and leave the camera square in the optical path. Additionally, the transmissive element of the new AO design has a
relatively large range of motion. In the AO-8 for example, the range of correction is roughly +/- 40 pixels. Assuming one is
imaging at approximately 1 arcsecond per pixel, this means the AO-8 can correct for up to about 40 arcseconds of periodic error in
the telescope mount. Since most modern mounts with PE error correction are capable of reducing any residual periodic error
within this range, it is possible to guide long exposures with the AO-8 without making any guiding corrections to the mount at
all. Guiding in this manner is also far more accurate than can be achieved by issuing corrections to the telescope drive. The
tilt of the element 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 beginning to end of the exposure. If the image drifts slowly during
the exposure due to Periodic Error or slight misalignment of the mount, the AO can continue to operate without having to issue any
corrections to the mount to re-center the guide star. This larger range of motion is one more advantage of the AO-8 over the
earlier AO-7 design which worked best when making corrections within a narrower range, requiring mount calibration and
interaction.
The AO-8 systems can be installed by the user to any dual sensor ST-7/8/9/10/2000 USB cameras. The AO-L can be installed by the
user to any STL camera, and to any dual sensor ST-7/8/9/10/2000 USB cameras. A USB camera equipped with the optional Remote Guide
Head is required to use either AO with an off-axis guider. This remarkable system has had a profound effect on CCD imaging by
reducing the atmospheric turbulence, wind induced vibrations, and eliminating the remaining periodic errors in most telescope
drives. Some of the best, high resolution images we have seen produced by amateur astronomers have taken advantage of the AO
technology introduced by SBIG.
|
