Autoguiders are one of the most vital tools in any deep-sky imager’s arsenal – essential for extremely crisp images of the stars.
Conventional autoguiders move your mount to correct tracking errors, but Orion’s new Adaptive Optics Guider (AOG) tackles the problem in a very different manner.
Instead of moving the mount, the AOG moves a small optical glass centred in the light path, deflecting the light by diffraction and keeping the image in the same place on the camera’s sensor at all times.
This has some major advantages over conventional guiding, as it is easier and faster to move a small piece of glass than it is to move a whole mount.
Corrections are made very quickly and, with a suitably bright star, several times each second.
The AOG also acts as an off-axis guider, which means that it channels light to a guide camera from the same telescope as the imaging camera, using a small prism to ‘pick off’ a sample of the view.
Using the same telescope for this avoids ‘differential flexure’, a technical term for unwanted movement.
The Orion AOG is well made from machined aluminium.
There’s a good range of adaptors supplied to enable focusing using all sorts of cameras, including a 2-inch nosepiece, a male ‘T’ thread adaptor and rotation locking ring for the camera side, a female ‘T’ thread adaptor for the telescope side and 30mm, 17mm and 7mm ‘T’ adaptor extension tubes.
We noticed a distinct ‘droop’ in the optical train using the supplied 2-inch nosepiece with our Sky-Watcher 80mm ED refractor, so opted to use Sky-Watcher’s own ‘T’ adaptor to ensure a more substantial mounting.
Generous length cables are supplied for power, USB 2.0 and ST4-mount-compatible guider connections, and the whole outfit is supplied in a padded hard case.
To control the AOG you need to attach a fast frame-rate camera, in addition to your imaging camera.
We used an Orion StarShoot AutoGuider.
The 42.5mm depth of the AOG makes it unsuitable for use with instruments with a short back-focus like Newtonian reflectors, but it was a good match for our refractor.
Unfortunately, we were unable to use our field flattener with the AOG in place, as it was impossible to achieve the critical sensor-to-mounting-face spacing required.
In common with all off-axis guiders, obtaining focus with both the imaging and guide cameras is fiddly at first.
However, using a daytime view of trees, we achieved focus.
We found the best method was to position the guide camera in the centre of its focus travel, obtain focus using the Crayford focuser on the scope and measure the extension of the focus tube.
Next, we adjusted the focus until the imaging camera was in focus, noted the increase in the focus tube extension and chose the nearest adaptor to this measurement to space the imaging camera correctly.
Final accurate focus of both cameras was achieved by focusing the imaging camera using a Bahtinov mask and the scope’s focuser, and then accurately focusing the guide camera using the fine adjustment on the AOG.
Locating suitable guide stars is much more difficult with an off-axis guider than it is with a separate guiding telescope.
Careful planning is needed to ensure suitable orientation of the pick-off prism and imaging camera to get a guide star in view.
However, the software is easy to use and once we’d found a suitable guide star, calibration of the guider and mount was achieved first time.
Mount calibration is required because the actuators that move the optical glass only have a limited amount of movement, so if the tracking is poor through inaccurate polar alignment or excessive periodic error, the mount is commanded to make additional corrections.
The AOG takes over immediately the corrections are back within its operating range.
We were very impressed by the well-formed stars captured during our test sessions, although we did lose a couple of frames to tracking errors.
Throughout, the click of the actuators as corrections were made gave us confidence that all was working correctly.
The AOG works well cancelling out smaller errors.
However, if your mount doesn’t track particularly well, you may find that a more conventional autoguider would suit you better.
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This review appeared in the May 2010 issue of Sky at Night magazine