ALFOSC spectroscopic observations: observing and reduction
strategy to best correct for fringes
The current ALFOSC CCD#8 is 'fringe sensitive'. See this document for an
example of the fringe amplitude for grism #4. For higher-resolution
grisms the fringe amplitude will be correspondingly higher.
Based on a discussion with Vallery Stanishev, we specify below some
observing guidelines and points of notice regarding the removal of
fringes in red ALFOSC spectra. As a rule of thumb, the fringe
amplitude can be suppressed by a factor 10 by using standard
data-reduction steps, provided that sufficient calibration data were
For spectra redwards of 6000 Å take 2 flats before and 2 after
the science spectrum (with the telescope pointing to the target).
These flats can be used to beat down the fringes. The flats have to
be taken with the telescope pointing at the target in order to
minimise flexure effects.
However, it is found that often quite large residual fringing remains
after normal fringe correction. This is likely due to the fact that
the calibration unit illuminates the ALFOSC optics differently than
the telescope does.
One can try to shift the flat a bit (~+/-1 pixels along the dispersion
axis, the shifts are not integer pixels, have in mind) before
flat-fielding, and in many cases this can result in almost full
removal of the fringes from the target spectrum, at the expense of
slightly increased noise.
It should be kept in mind, however, that if a very blue object is
observed, the second-order contamination will make removing the
fringes almost impossible even if one shifts the flat. The reason is
that the blue light does not fringe, but fringing is introduced when
dividing by the red lamp flat.
As fringes only affect the red part of the spectrum, only the red part
(>6000 Å) of the flats should be shifted: keep the blue part
un-shifted. Flats for the blue can be taken in the afternoon.