ALFOSC: Using the Focus Pyramid:

We encourage the observers to check the focus a couple of times during the night. Below we describe how to set and monitor the focus in general terms, and then specifically for our different instruments.

• The easiest way to focus the telescope is to use script alfosc.focus
• In the evenig when it is dark enough, type in the sequencer window e.g.
  alfosc.focus
  this will setup ALFOSC for focussing using alfosc.focuspyr-setup, move the telescope to the optimal field (-a-option), take a ten seconds integration, analyse the focuspyramid image, find the best focus, adjust the telescope focus, if needed, take a new integration, analyse the focuspyramid image,find the best focus, adjust the focus and take more integrations until the "best telescope focus" difference between subsequent frames is less than 13 focus units or the maximum number of images is reached.
  Note1, if seeing is bad you might have to increase the integration time (-t-option).
  Note2, if seeing is bad, sometimes no pyramids are found automatically. You can analyse the image using alfosc.focuspyr script or use the default ALFOSC focus and focus the telescope later when the seeing has improved.
• if you need to focus during the night use
  alfosc.focus -t 12 -a nearest
  or if you pointing at a suitable field already:
  alfosc.focus -t 12 -a noacq
  
• Remeber to take the focus-pyramid from the beam and reset the CCD windowing
• If you wish you can use less automatic way to find the best telescope focus:
• Preset the telescope to focus star field (e.g. read-cat focus.cat or read-cat focus_bright.cat ).
• Use the image acquisition software recommended for the instrument you are using.
• Change the telescope focus by typing foc-pos value on the TCS where value is the new focus value.
• If you, for any reason, cannot focus the telescope, use the default focus specified in Table 1. You can also change the telescope focus by a few units below and above the current value to manually find the best focus. Type expose time in the sequencer window (where time is the exposure time in seconds) to obtain an image of the focus field. The exposure time should be set to 10 to 15 seconds in order to fully sample the seeing frequencies. Use the 'imexam' command to analyze the image in the DS9 window. Press the key r to view the radial profile and to find the fwhm of a star (the pixel scale for each instrument can be found on the instrument pages).

For focusing with ALFOSC, a focus pyramid is used, an optical device which splits the light beam into four segments, each of them producing an image of the star on the CCD. The distance between the four images is linearly dependent on the focus of the telescope. By measuring the distance between the four images of a star, the correct focus of the telescope is given independently of seeing variations. Note that the internal ALFOSC focus has a default value of and is found to be very stable. For ALFOSC polarimetry the recommended focusing procedure is a bit different from the one described below.

If you wish to observe with several filters, determine first the telescope focus with no filter in the beam, and use this as a reference focus value. Set the correct telescope focus value for no filter by typing foc-pos value (make sure that foc-del is set to zero while evaluating the focus). Then find the appropriate filter focus offset to be applied for each filter and set foc-del offset always before using that filter. If no offset is available, determine the focus once more according to the suggested steps below - this time with having the filter in the beam - and note down its offset.