This procedure is the same whether the telescope or the probe is to be
moved. Route the autoguider probe image to one of the TV monitors. If the
autoguider has not been turned on (resulting in a blank screen) press
the button below the fourth mini-monitor on the extreme top right of
the Control Desk (red = on; white = off). This switches the High Tension
power supply on. The HT supply is interlocked with the main dome lights:
if the latter are switched on, the HT is disabled in order to protect the
guide probe; conversely, it is not possible to power the guide probe with
the dome illumination on. The autoguider display should come up with a
frame and an `idle' message (Figure 
). If it does not,
press the SHUTDOWN button on the Autoguider Panel. If there are still
no signs of life and the autoguider panel
appears to be active, call the Duty Technician.
Figure: The Autoguider Display
If the autoguider is alive, press the FIELD button
on the autoguider panel. This causes the autoguider
to scan a field of about 22 and to lock onto the brightest
pixel (in software; the probe does not move),
which in all likelihood is the intended guide star (if the proper procedure
was followed to move the probe to the guide star position).
Note that the outer parts of the scanned area are heavily vignetted at Prime,
but not so at Cassegrain. An image will appear on the 3232,
3-bit (gray-scale)
autoguider sub-display (figure ), with the position of the
brightest pixel marked with a cross in the diagram below.
If the image occupies a single pixel, the intended guide star is probably very weak or strongly vignetted. In such a case, first press WAIT to stop the integration, and subsequently repeat the FIELD integration. Random events look very similar to faint stars. If, however, an image is found consistently in the same place, then it is probably real.
The ``magnitude'' of the star indicated on the autoguider display is alleged to be roughly . A magnitude 14 is acceptable for guiding.
If you are happy with the guide star selection, press ACQUIRE; if
not, press WAIT and pause for thought (see section
, furtheron).
Pressing ACQUIRE passes the coordinates of the guide star measured by the probe to the TCS computer and requests that now either the probe or the telescope be moved, depending on the acquisition mode selected, such as to position the star at the centre of the autoguider field. This is done either by sending a message over the interprocessor link to the Instrumentation Computer (for probe movements) or directly by moving the telescope. When the positioning of the probe/telescope is completed, the autoguider will do another field integration. It then switches from raster to cross scan and starts to measure the position of the star with greater accuracy. This is the ``pulling-in'' phase, as is indicated on the monitor. Messages are sent to the telescope computer, which attempts to move the probe/telescope to minimise the error. When the error drops below a preset threshold, scanning ceases and the x and y profiles of the star are displayed with a message `OK to Guide?'.
At this point, press GUIDE unless the profile is double-peaked, indicating a narrow double star or out of focus image, or if the object is very weak. Wait until the autoguider display is completely redrawn and then press the LOOPENABLE button on the A&G panel which should turn green. Note that the loop is locked by the action of switching from LOOP DISABLE to LOOP ENABLE. Wait until the cross (the measured star position) is centred in the 2 arcsecond diameter circle. You are now ready to observe.
The coordinates of the guide star will be recorded in the automatic observing
log and displayed on the MIMIC display. If the same star is observed later,
the probe can be sent to the guide star following section .
The autoguider signal should be monitored during observing in case the
signal becomes weak (indicated by the message: CLOUDY). On the right-hand
side of the autoguider display (figure ) are histograms of the
last 128 guiding errors in x and y
On the left-hand side are the current state (GUIDING),
warnings (e.g. CLOUDY), an approximate visual magnitude, a measure of
the relative transparency (RT, relative to the measurement of the first scan)
and a measure of seeing estimated from the profiles (beware: not equal to
or linearly related to the true seeing! It is a function of both seeing and
brightness of the guide star).
The cross should always remain within the circle. If it consistently strays, you have probably lost the guide star. Check whether there is any signal.
On source change press WAIT; the autoguider is automatically switched to the LOOP DISABLE status. If you need light in the dome (also with tungstens, unless these are very dim), turn off the HT.