Faint Object Spectrograph and Camera
Fringes in Spectroscopic
Background: The observer was using the grism 10 with 1" slit
and 2x2 binning. He took some spectroscopic dome flats in the afternoon.
He found that, using the domeflat to flatfield the science frame still
results in residual fringes of the level of about 2% in the red part in
the science spectrum. This is not acceptable to his requirement,
since his objects are very faint.
We had a discussion whether the residual fringes were due to:
We did the following experiment: we took an an internal lamp exposure straight
after the science exposure. This is then used to flatfield the science
exposure. The result is that there is no noticable residual fringe pattern.
Hence, scenario 1 above is responsible.
there is a slight wavelength mismatch between the dome flats and
the science frame, causing a small shift in the fringe patterns in the
2 frames, which results in an imperfect flatfielding.
this may be caused by the different colours between the domeflat
lamp and the sky.
The below demonstrates the difference in fringe-removal between flatfielding
using a spectroscopic dome flat, and an internal lamp flat taken at the
location of the science object.
When an observer is doing spectroscopy in the near infrared, to get good
fringe removal one should take internal lamps at the same telescope position
as the object. This only takes a 2 minutes but makes a real improvement.
The flexture of the telescope does cause some shift in wavelength (which
should be quantified further). For good wavelength calibration (e.g. for
accurate radial velocities) HeAr arcs should also taken at the same telescope
location as the object.
We should offer a new template offering spectroscopy with offsets along
slit (akin to the templates on FORS and EMMI) which allows the observer
to nod the telescope and so be able to median combine science frames to
contruct a "super sky (fringe)" pattern.
||This is an extracted spectrum of an object, normalised and sky-subtracted,
but not flat-fielded yet. This shows the very pronounced fringes in the
red part of the spectrum. The grism is no. 10 and slit width is 1", binnig
||This is the previous spectrum, now flat-fielded by the spectroscopic
dome flat. Note the residual fringe pattern of 2%
The object spectrum, flat-fielded by an internal lamp "flat" taken
at the same telescope location as the object. The residual fringe pattern
is not present at a significant level.