After finishing the sky flat fields, it is necessary to calibrate the
pointing of the telescope, modifying the pointing model accordingly. To do
this there is a semi-automatic routine using seven stars. Firstly you need to
activate the TV by typing on the ICS:
SYS>tv on
On the JKT mimic on the DAS you will see the TV flat move into the
telescope beam. When this is completed, locate the TV with the blue circle
on its screen in the JKT control panel. Directly beneath there is a
small panel of controls relating to this TV. Turn the gain knob right down
until it clicks off (marked "INCREASE" under "GAIN CONTROL"), make sure that
the left scan switch is switched up and that the right scan switch is switched
down (marked "H" and "V" respectively under "SCAN REVERSE"), and then switch
on the TV power via the small black button (marked "ON" under "POWER").
Finally switch on the actual TV via the grey button marked "POWER".
Now, on the TCS, type:
USER>calibrate
This comand will start the seven star calibration routine and the
telescope will move to the first star. Take the keyboard from the TCS and move
it in front of the TV screen. When the telescope has moved to the first star
and is tracking, slowly turn up the gain on the TV using the gain knob. The
TV has about a five second delay in reacting to an increase in gain so be
patient. Eventually a star should appear on the TV screen. Using the arrow
keys on the TCS keyboard, centre the star in the blue circle and press the
"Do" button on the keyboard (marked above by "HANDSET").
At this point it is IMPORTANT to note that the TV is very sensitive
and can be PERMANENTLY DAMAGED by over illumination. As a result you
should turn down the gain while the telescope is moving to a new target.
The above procedure needs to be repeated for each of the seven stars in
the routine. At the end of the routine the TCS will ask:
Were all the stars centred? (Y/N)
Answer "Y" and the TCS will display the calculated set of pointing
parameters. Please enter these into the JKT log book. The sky sigma value
should be about 20 arcsec. If the sky sigma is above ~25 arcsec, check in
the log book the sky sigma value for the previous night and if the previous
night's value is better than the sky sigma calculated for the present night,
then, on the TCS, type:
USER>cal last
This command will use the previous night's pointing model instead of the
recently calculated one.
Finally, remember to switch off the TV using the grey button marked "POWER"
and the red button on the panel (marked "OFF" under "POWER"). Also, turn the
gain knob right down until it clicks and on the ICS type:
SYS>tv off
This command will deactivate the TV and on the JKT mimic on the DAS you
will see the TV flat move out of the telescope beam.
Focussing
After calibrating the pointing, it is necessary to focus the telescope on
a star or group of stars. In order to speed this process up, a couple of
scripts have been written.
Firstly, you need to send the telescope to a field that contains one
or more stars. A standard Landolt field is always a good choice or possibly
one of the targets that you will be observing later on. Whatever the field
you choose, you will need to enter the object and its coordinates into the
telescope catalogue. You may then send the telescope
to this field by typing on the TCS:
*USER>gocat object
Note that "object" represents the name of the field to which the
telescope is to be sent.
When the telescope is in position, change the filter to a wide band
filter and use the glance command in order to determine an exposure time
such that the stars in the field to be focussed on have a decent number of
counts and are not overexposed. The form of the commands to be typed on the
ICS are as follows:
SYS>filter m
SYS>glance n
Note that "m" refers to the number of the slot in the filter wheel
containing the filter or the name of the filter that was entered in the ICS.
Note that "n" refers to the exposure time of the glance image.
Now you may call the focussing script by typing on the ICS:
SYS>focusrun
The script will then prompt for the following:
Focus start position: The focus is usually between 39.60 and 40.00. Using this and the
previous night's focus from the log book, choose a lower limit for the focus.
Note that you must NEVER specify the current focus position as the
start focus position as this will crash the computers.
Focus step size: This is always 0.05.
Number of focus steps: Set the upper limit for the focus by defining this number.
Exposure time (s): This is the exposure time in seconds that you determined earlier for the
stars in this field.
When the script has finished taking the specified number of exposures,
it will then print instructions as to what to do next on the ICS screen.
Load the ing package in IRAF by typing in the IRAF window:
cl>ing
The IRAF prompt will change to ing>. Now type the following
at the IRAF prompt:
ing>jkt_starfocus n
Note that "n" refers to the number of focus steps carried out by the
focusrun script previously.
The second script will now display the field of stars in the ximtool
display. Mark a couple of stars to be focussed on using "m" and when you
have finished press "q". The script will now plot a graph of FWHM against
focus position and a graph of ellipticity against focus position. It will
also calculate the best fit focus position for the minimum FWHM/ellipticity.
Press "q" again to quit the graph.
Finally, in order to set the focus position to the suggested value, type
on the ICS:
SYS>focus x
Note that "x" refers to the required focus position (mm).
