Zeroth-Order Ghosts:

click for PS Zeroth-order ghosts result from a simple reflection from the grating, when the grating is close to face-on with respect to the camera. Since the grating is a rather poor mirror (especially in this configuration) the ghosts are usually of low intensity, <1%.

Light from the collimator enters from the right, and is dispersed by the grating, forming an image at each wavelength on the detector. Reflected or scattered light from strong spectral features, such as very intense night-sky lines, passes back through the camera, becoming parallel beams at the grating where a portion is reflected as if the grating were a simple mirror. This "zeroth-order" light is then re-imaged on the detector. The projection of the grating normal is a common point of symmetry for all "primary-ghost" pairs.

The simple picture above is verified in data from LRIS with the 831/8200 grating. A sequence of exposures was taken at different grating tilts (GRANGLE), and the location of ghosts and primaries measured (in the plot, each horizontal line is a different exposure). As the grating angle changes, the measured point of symmetry moves across the CCD. In some cases, the ghost spectra are truncated at a particular pixel location -- this is because the corresponding primary images are missing, because they fall off the detector. The gray band marks the expected region of ghosts, determined by the upper and lower edges of the CCD reflected about the symmetry point. In one frame, an obviously out-of-focus ghost appears outside the expected range; this is due to an extremely strong primary imaged (out-of-focus) on the mounting beyond the edge of the CCD. click for PS

click for PS A section of one of the LRIS images showing ghosts (arrows; many others are also present). (Frame 153, April 1999)

Last modified: 06 feb 2001

Andrew C. Phillips / Lick Observatory