Handbook for the SXVF-H35 Issue 1 August 2007
19
Using the built-in serial ports
The SXVF-H35 incorporates two fast serial ports for use with external accessories.
The ports are available on 5 pins of the 18 way connector that is provided for the
autoguider and may be accessed by plugging in a ‘serial port divider box’. The divider
box and cables are available as an accessory and may be chained in series with the
autoguider cable, when the guider is in use, or may be used on its own.
The two serial connections are in the form of standard RS232 PC style plugs and
provide TX, RX and Ground connections at RS232 levels. Access is via commands
sent through the USB connection and, at the time of writing, is limited to any serial
controls that are provided by the SXV software. It is expected that more functions will
be added as the software is upgraded.
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Using the add-on autoguider:
A very useful accessory is the add-on autoguider head, which takes its power and
control signals directly from the SXV camera, via the 18 way socket on its rear panel.
The autoguider is only 1.25” in diameter and has a video style ‘CS’ mount thread in
its nose, so video lenses may be attached. The guider may be used with either an off-
axis prism assembly mounted in front of the SXV camera, or with a separate guide
telescope, rigidly mounted alongside your imaging telescope. I personally use it with
an 80mm aperture F5, inexpensive refractor as a guide ‘scope, but a shorter focal
length lens will make more guide stars available in any given region of sky (See the
picture below).
To use the autoguider, first orient it so that the connector plug is roughly parallel to
the declination axis of your mount. This is not absolutely essential, as the training
routine will learn the angle of the head and compensate for it, but it is easier to
understand the motion of the guide star if the guider frame is aligned with the RA and
Dec axes. Now connect the head to the SXV camera, using the 18 way connector lead,
including the port divider box, if it is to be used.
The recommended way of connecting the autoguider output to the mount is to use an
RJ11 telephone lead between the socket on the SXV camera and the autoguider input
of your mount. This output is ‘active low’ (i.e. the control relays pull the guider inputs
down to zero volts when applying a guide correction) and matches most of the
autoguider inputs on commercial mounts. If ‘active high’ inputs are needed, or a very
low control voltage drop is essential, then you will need to add a Starlight Xpress
‘relay box’ between the guider output and the input to the mount. Please contact your
local distributor if a relay box is required. Some mounts (Vixen, for example) use a
similar guider input socket, but have re-arranged connections. Details are given on our
web pages at the end of the ‘STAR2000’ section.