Closer Look at a Star Image


Examining a starís image closely reveals that it is not a simple circle. Starlight arrives at the top of the earth's atmosphere as an extremely narrow beam of light. The earthís atmosphere and optical surfaces cause the point source to broaden. By the time the beam of starlight reaches the surface of a CCD imager it has been turned into a Point Spread Function which looks like a hill. FWHM is a measure of how wide the hill is at half its height. In this image of NGC 2903, 5x4 minutes on a 17" Reflector, two stars are selected, one properly exposed and one overexposed. Using imexamine in IRAF, a surface plot was created for each star from stacked but otherwise unmodified FITS data. Each square in the surface plot represents one pixel.

Correctly Exposed Star

This image of NGC 2903 highlights one star properly exposed. The star's surface plot shows the star's image is spread across a circle of about 8 pixels with the peak as a single pixel. The hill, representing the star's pixel values, is symmetric and steep indicating good seeing and optics. Once an image has been stretched, star images should still show this pattern although the hill will generally be lower.

PointSpreadFunction Proper Exposure

Overexposed Star

In this image of NGC 2903 an overexposed star was selected. Overexposure is evidenced by the flatness of the top of the star's surface plot. As a star continues to overexpose, the top of it's image surface widens and eventually merges with other widening nearby star images. Stretching will not help as all pixels at the top of the surface have the same value. This surface plot also shows that the star's excess light has spilled over into adjacent pixels. This effect is called blooming and produces spikes of light on every bright star in the image.

PointSpreadFunction Overexposed