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LabCourseF36WavefrontAnalysis

General information

• contact person: Stefan Hippler
• "complete" manual
• also have a look at the homepage for further information not contained in the "complete" manual!

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!