Starlight Xpress SXV-AO User Manual

Handbook for the Starlight Xpress AO unit

Issue 1 21/8/2005

Handbook for the Starlight Xpress SXV-AO unit

Thank you for purchasing an SXV-AO active optics unit. This device should give you

much improved guiding accuracy with almost any telescope and mount. Please read the
following manual which will explain how the device operates and the best ways in which
it can be applied to your optical system.

The principle of operation:

The SXV-AO 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 which 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.

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

Handbook for the Starlight Xpress AO unit

Issue 1 21/8/2005

oriented so that it is parallel to the AO backplate. This can be seen in the picture at the
front of this handbook, as indicated by the orientation of the guide camera socket.

The imaging camera is attached to the rear of the OAG, using a suitable adaptor ring.
Two types are supplied – one with a ‘T’ thread for the SXV-H9/9C and M25C and one
with an M42 thread for the rest of the SX range. The orientation of the camera is adjusted
by loosening the two set screws and rotating the ring into the required position. As the
camera is not involved with collecting guiding information, it may be set at any angle.
However, the pick off prism may cause shadowing if the long axis of a large chip is set in
the vertical plane.

Both the AO unit and the OAG have recesses which will take a 48mm filter. When using
the OAG, it is an advantage to use the recess inside the camera mounting ring, so that the
guide camera sensitivity is not compromised by any filters that you may add. This is

Handbook for the Starlight Xpress AO unit

Issue 1 21/8/2005

especially useful if you image with narrow band filters, such as H-alpha. A light pollution
rejection filter, such as an IDAS P1 or P2 might be best added at the output of the AO
unit, where it will improve the guide star contrast, as well as improving your camera
images. Note that the recesses are not threaded and so you should be careful that a filter
does not fall out and get damaged when dismantling the unit.

N.B. Although the SXV-AO was designed for operation with an SX camera, it is
possible to attach other imagers with an appropriate adaptor and still use the AO via one
of our USB2 interface boxes. The USB2 interface provides a convenient way to read and
control the SXV guide camera, even when no SX main imager is in use. You will need to
organise the alternative camera spacing so that it may be brought to focus simultaneously
with the guide camera.

Connecting up the AO electronics for off-axis guiding:

The method of connection will vary slightly, depending on the guiding software in use
and the equipment available. The SX software and AstroArt allow you to use the SXV
port divider box for serial control of the AO unit and so you might prefer to use this
method to cut down on the number of cables required. At the time of writing, Maxim DL
does not offer an option to connect via the divider box and so a serial cable from the PC
COM port will be required if you use Maxim. PCs with no COM port can use a
commercially available USB to serial interface for the connection.

1) Connect the serial to RJ11 cable (supplied) into the AO unit input socket.

2) Connect the RJ11 to guider port cable (supplied) from the AO output to the guider
input on the mount. This cable is not essential, but it allows the software to
‘bump’ the mount when the errors become too large for the AO unit. Most lower­cost mount users will find it necessary for long exposure times.

3) Connect the guide camera to the socket on the SXV imaging camera, or the socket
on the SXV interface box (used with older parallel port imaging cameras, or when
imaging with other makes of camera). Include the port divider box in this cable, if
you intend to use this for serial control (Fig 1).

4) Connect the serial to RJ11 cable to the appropriate port divider output, or to the
RS232 cable from your PC (Fig 2).

5) Connect the AO power lead to the power supply block.