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General information

Practical notes

Background knowledge

  • purposes of wavefront analysis in astronomy
  • generating a plane wavefront:
    • laser produces parallel light
    • but this is coupled into the fibre using a lens
    • so we need a collimator
  • CCDs:
    • physical background:
      • Photoelectric effect
      • process of reading the CCD
      • amplifying the signal
      • analog/digital (A/D) unit, produces 12-bit output -> finite resolution of electrons
      • efficiency (electrons/photon)
    • quantities and effects:
      • bias
      • flat-field variation
      • dark current
      • ...

Technical aspects

  • don't try to follow the manual word by word, especially:
    • for determining Shack-Hartmann gradients (see below)
  • working directory on PC: /home/fprakt/INDIVIDUAL_DIR
  • flat-field LED: keep voltage <= 3.3V!
  • representation of an image on screen:
    • this TFT screen displays 24-bit colors
    • as grey tones are made of equal values of R/G/B, it can distinguish only 8-bit (i.e. 256) shades of grey
    • but the camera delivers a 12-bit image
    • this is why we need to adjust the color scale when displaying an image
  • usage of objectives:
    • if you use the big objective, you can fix it to the rail with the left screw and still make fine adjustments using the micrometer screw
    • remove the objective before taking flat-fields (otherwise, not the entire CCD will be illuminated; even though in this way, you cannot correct for potential intensity variations due to the objective)

Hints for viewing FITS files

  • from shell: ds9:
    • view -> tile
    • color maps:
      • adjusting the scale: hold down right mouse button and drag up/down or left/right
      • try different scale modes
      • try color button
      • resetting: color -> reset color map
  • within IDL: atv, IMAGE where IMAGE is a two-dimensional array

Determining Shack-Hartmann gradients (in IDL)

  • repeat for reference and disturbed wavefront:
  1. atv,IMAGE
  2. as an optimal procedure, perform a dark correction
  3. getting the spot locations:
    1. choose an order of clicking the spots
    2. click mousemode -> ImExam
    3. click near 1st spot
    4. click button save photometry file (or the like) - but don't try to change filename or folder. This generates atvphot.dat file in current folder with header.
    5. close dialog
    6. repeat for all spots, including 1st:
      1. click near spot
      2. close dialog
  • read resulting atvphot.dat file using readcol
  • use the IDL code given in the corresponding section of the manual, but be careful: it's probably only useful for you starting from dx=xref-xastig on the 3rd page and excluding the unneccessary velovect commands

These notes are incomplete, so please make changes and additions based on your own experience!

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Page last modified on June 27, 2011, at 02:52 PM