
psphot description article (outline)

- background

- process

  - image preparation

    - load image
    - measure sky level & noise
    - construct noise image
    - construct mask image

  - PSF determination 

    - initial object detection (find peaks)
    - measure object moments & basic properties
    - classify on basis of moments
    - select likely PSF stars
    - test PSF model functions
    - select best PSF model function
    - aperture correction  
    
  - apply PSF to sources

    - use fit result to select PSF-like objects
    - fit and subtract PSF-like objects


- aperture photometry vs psf photometry

  - aperture is always the reference
  - how do we choose an appropriate aperture
  - 
