An ST-4 CCD camera was mounted on Turpol in July 2000 in order to improve the
direct centering of faint objects in the diaphragms. Using this facility,
objects as faint as V ~ 17 magnitude can be directly viewed and centred at any
time when not integrating with Turpol. Typically a 9th magnitude star is
easily seen with gain = 1 and 1-3 s integration time. Sources fainter than
V ~ 14 mag should need gain = 4 and 10-20 s integration time. For such long
integration times it is advisable to subtract dark frames. IMPORTANT:
Because there is no shutter in ST-4, dark exposures must be taken while
integrating with Turpol, such that the flip mirror of Turpol acts as a
shutter.
In order to use ST-4 the periscope mirror
must be put in the position VIS (for visual; because the CCD camera is mounted
on the eyepiece). The other position, called TV, is used for viewing the
diaphragms with the guide probe (using the TCS). NB! Do not use force on
the switch! Turn the switch just lightly against the end-stop. See
Section 5.4 in the Turpol Manual for more information about the periscope.
This is normally done by staff.
- In order to view the size of the diaphragms, turn the diaphragm
illumination up a bit. This switch is found on the CAMAC crate in the
control room. Note that minimum is at about 11 o'clock and maximum at
about 2 o'clock. The knob turns freely back and forth along the
largest distance between these two positions, but should not be
forced across 12 and 1 o'clock. (It seems that this knob has lost
zero position..., comment by AAD 12/8-2007.)
- Switch on the portable PC and cd ST4 where you start the CCD
program by typing ccdtrack. Go to Camera/Scope to do
the setup. Here you set the gain, and whether to subtract darks or not.
For bright stars gain = 1 and no dark subtraction is needed.
- If you have faint stars, then go to Grab and set the
integration time for the dark and set dark=yes. Then, in order to
make a dark exposure, set Turpol to integrate mode in order to use
the flip mirror as a shutter. While integrating, make a dark exposure
with ST-4, and then stop the fake integration.
- Use Focus and set display to quarterly. Use auto-contrast. Select
the integration time. Press enter. First a full frame image is taken and
gives you a small rectangle for you to center. Press enter again, and
continuous exposures of the diaphragm are now displayed, and you should
be able to see the diaphragm edges, as well as the star, if it is at the
right position. If the star is faint, turn down the diaphragm
illumination to minimum.
- In order to center the star, stop auto-guiding on the TCS (ag-off),
set the offset size to 1 arcsec (o-s 1 1), or even smaller, and
move the telescope until the centre is found. Then start guiding by
using the relaxed guiding (ag-rel),
which moves the box (i.e. not the telescope!) until centred on the star
and then starts autoguiding (wait until it says autoguide on the
TCS monitor).
