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Hubble Space Telescope
COS Imaging Observing Strategies

Essential Facts

Configurations and Image Quality

The NUV imaging mode requires the observer to make only two optical element selections. First, either the PSA or BOA is selected as the aperture. The second selection required is MIRRORA or MIRRORB. MIRROR B provides an attenuation factor of approximately 25 compared to MIRRORA, and produces a doubled peak in the image. Ground testing shows that the peaks are easily separable for an isolated point source but may present difficulties for extended sources or crowded fields. Similarly, the BOA includes a neutral density filter and the optical properties of that filter degrade the image compared to what is possible with the PSA.


COS is very sensitive, and the imaging mode concentrates an object’s NUV flux into a diffraction-limited image rather than dispersing the light.

For information on imaging strategies with COS see chapter 6 of the COS Instrument Handbook.