The primary mirror of the Hubble Space Telescope (HST) has a slight spherical aberration, which means that light rays from a point source do not all focus at the same point. This results in a slight blurring of images. The amount of spherical aberration is about 1/50th of a millimeter, which is small enough that it would not be noticeable in images of everyday objects. However, it is significant enough to affect the quality of images of distant objects, such as stars and galaxies.
Solution:
The spherical aberration of the HST was corrected in 1993 with the installation of the COSTAR (Corrective Optics Space Telescope Axial Replacement) instrument. COSTAR consists of two small mirrors that are positioned in front of the HST's primary mirror. These mirrors compensate for the spherical aberration, allowing the HST to produce sharp images of distant objects.
Diffraction Spikes
Diffraction spikes are caused by the interaction of light with the edges of the HST's secondary mirror. When light passes by an edge, it is diffracted, or spread out. This diffraction creates a pattern of bright spikes that extend outward from bright objects in images. The number of spikes and their angle depends on the shape of the secondary mirror. The HST's secondary mirror is octagonal, so the diffraction spikes form an eight-pointed star pattern.
Solution:
Diffraction spikes cannot be completely eliminated, but they can be reduced by using a smaller secondary mirror. The HST's secondary mirror is about 1/20th of the size of the primary mirror. If the secondary mirror were smaller, the diffraction spikes would be fainter and less noticeable.
Image Artifacts
The HST also produces a variety of image artifacts, such as ghosts, scattered light, and cosmic rays. Ghosts are caused by multiple reflections of light within the telescope. Scattered light is caused by light that is reflected off of dust particles or other objects in the telescope. Cosmic rays are high-energy particles that can damage the HST's detectors, creating bright spots or streaks in images.
Solution:
Image artifacts can be reduced by using a variety of techniques, such as baffling, filtering, and image processing. Baffling is used to block stray light from entering the telescope. Filters are used to block out specific wavelengths of light, such as ultraviolet light, which can damage the HST's detectors. Image processing is used to remove or correct image artifacts.
