  In this article we compare the cross-correlation and breakfinder techniques
applied to the measurements of redshifts from low-resolution spectra. We assume
spectra obtained from multinarrowband imagery, a technique for multi-object
spectrophotometry. Comparing the cross-correlation with the breakfinder, we
find that neither is intrinsically superior to the other. They have comparable
precision for early type galaxies but the cross-correlation is clearly superior
for later type galaxies. On the other hand, unlike the breakfinder, the
cross-correlation is sensitive to instrumental errors as well as the shape of
the templates used. We carried out simulations with two types of global
spectral errors: a slope error and sinusoidal errors. They can introduce
serious systematic errors in redshifts measured with the cross-correlation. The
breakfinder is a spectrally local test and does not suffer from this type of
error. We therefore conclude that the two techniques are complementary and that
they should both be used to help flag objects for which they give abnormally
discrepant redshifts.

Astrophysics