Follow the steps below to focus the telescope for ALFOSC.
- Set the focus to the default value for ALFOSC
by typing foc-pos 23230 on the
This is also done by the script
is automatically set to zero for foc-del when setting foc-pos.
- Load the focus field catalogue by using the command read alphot.cat
in the TCS.
- Point the telescope to a focus field.
- Start guiding. It is recommended to use autoguiding.
- Setup ALFOSC for focusing with the pyramid. This can be done with
the script focuspyr-setup [-filter]
where the optional argument
-filter leaves filter wheels as they have been setup by you,
the default option without this argument sets all filter wheels to OPEN.
The setup can also be done manually:
- Take out any filters from the light beam in case you want to measure
the foc-pos value.
Otherwise, for measuring the foc-del value, put in the
- Set the aperture wheel to the Open position.
- From the grism menu, choose Focus Pyr.
- Make a smaller window by typing
in the sequencer window.
- Take an exposure of at least 10 seconds by typing
the sequencer window. For the focus fields typical exposure times are 10
seconds without a filter. In case the filter offset has to be measured,
15-30 seconds for broad band filters, and significantly longer exposure
times for narrow band filters are required. The focus image will be shown
in the DS9 window.
in a sequencer window. The focus pyramid
images will be analyzed automatically, and the calculated telescope focus
offset will be displayed in the talker and sequencer windows. If you wish
to analyse the focus pyramid image interactively, do the following step:
in a sequencer window. (This will automatically
run focuspyr-auto in the image as well so offsets can be compared).
Put the cursor on the upper one of the four images of one focus star in
the display. Type a followed by q to measure the focus offset.
Repeat this sequence on several stars to get a better estimate of the offset.
With the cursor in the DS9 display, type q to escape from the focus
measurement mode. The number of steps to move the telescope focus will be
- Change telescope focus by typing foc-pos <new-value> on
the TCS, or
in the sequencer.
- Repeat the focusing procedure at least once until a stable focus has
- When finished:
- Move the focus pyramid out of the beam.
- Reset the CCD readout window and binning to the desired size. To
obtain full frame window without binning, type resetxy
in the sequencer window