Astrophotography Skills – Exposing Images

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Outline of what it will contain:

  • Shoot in Raw!
    • A rule: never throw away data
    • DSLR
      • JPG conversion in camera throws away data
      • Shoot raw and you get to control what data are used and when; data you don’t use aren’t thrown away, and you can try other decisions.
      • This is not just an ideological debate; for astrophotos, your camera’s built-in JPG conversion will make wrong decisions. You need the raw files.
    • CCD
      • Most produce FITS files, which are a kind of raw
      • If yours can produce JPG, don’t
    • The Never throw away data rule will come back later, too, when we talk about post-processing.
  • Taking multiple exposures
    • Why do it?
      • Is it equivalent to one long exposure?
      • Noise reduction
      • Error recovery
    • Selecting exposure length
    • how many exposures
    • Rotate filters or combine sets?
  • Temperature stability
    • Noise from your camera varies with temperature
    • If you have a temp-regulated CCD
      • Choose a maintainable temperature
      • Choose a temperature for which you already have a collection of darks
    • DSLR or non-regulated CCD
      • Keep notes on the temperature
      • Try to collect dark frames that roughly correspond to the temperature
      • DSLR: if doing multiple exposures, don’t use built-in noise reduction – use separate dark frames.
  • Don’t forget Flats
    • Darks and bias can be collected separately
    • Flats need to be done during image session, before anything moves
  • Order of operations
    • Darks and Bias Frames
      • With temperature regulation, do these another time
      • Otherwise, fit them in at a point of average temperature
    • Focus on a suitable star near target
    • Find and frame target
    • Start autoguiding and wait for it to stabilize
    • Take multiple exposures
    • Take flats
    • If filtered monochrome,
      • take all exposures for a given filter together
      • take flats with each filter, before changing filters

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