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
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.
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