Starlight Xpress SXV-AO unit - Manual (Page 2)

Handbook for the Starlight Xpress AO unit
Issue 1 21/8/2005
The most common method of shifting an image for AO purposes is to use a ‘tip-tilt’
mirror to reflect 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 SX unit was
designed with this in mind. 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 12mm and a diameter of 40mm. This element can be tilted by up to
approximately +/- 3 degrees, by rotating four small stepper motors at the periphery of the
aluminium carrier plate. 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
0.075mm per 1 degree of tilt. The maximum image deviation is therefore approximately
+/- 0.15mm in both the X and Y planes. This corresponds to about +/- 23 pixels on the
CCD of an SXV-H9 camera.
The AO unit is usually controlled by serial data from an RS232 port. This port can be one
of those provided on most PCs, or the serial interface of an SXV camera (if supported by
the software package in use). A USB to serial adaptor can also be used on a PC without
native serial ports. The serial data input of the AO is an RJ11 socket and so a lead with an
RJ11 to 9 pin ‘D’ style socket is provided to enable connection to a standard serial port.
The computer port should be set to 9600B, 8 bits data, 1 stop bit, no parity.
A typical application of the AO unit for off-axis guiding:
The following diagram shows how the AO unit may be used with the optional off-axis
guider (OAG) and an SX CCD camera. This is likely to be the normal configuration for
most imagers, as it offers accurate guiding which is free of flexure and mirror shift
problems. The OAG consists of a short aluminium barrel with a pick-off prism which
feeds light to an SXV guide camera, mounted at the end of a short extension tube. The
guide camera position is designed to be close to the correct focal distance when a
Starlight Xpress camera is mounted on the main optical output of the OAG and brought
into focus. Any small errors may be corrected by loosening the two set screws in the
threaded collar and sliding the camera into the correct position.
The OAG is attached to the AO unit by two set screws which engage with a grooved ring
at the output of the AO assembly. Please note that the guider barrel should be oriented so
that it projects along the line which joins the North and South motors. When located
properly, the two RJ11 sockets on the AO will be at the bottom of the unit, diametrically
opposite to the guider barrel. The long axis of the CCD in the guide camera should be