ALFOSC: horizontal versus vertical spectrum orientation
The grisms can be rotated in ALFOSC to give spectra that run either
horizontal or vertical on the detector.
Definition: a horizontal grism gives a horizontal spectrum on the detector.
To use a horizontal grism, you need a vertical slit.
Definition: a vertical grism gives a vertical spectrum on the detector.
To use a vertical grism, you need a horizontal slit.
Cross-dispersed Echelle mode (grism#9 and #13 with cross dispersers)
can only use vertical slitlets. The Echelle orders run horizontally
Grism#17 can only be mounted horizontally, and needs a vertical slit.
For all other grisms the default setup gives vertical spectra, and
need horizontal slits.
When to opt for horizontal spectrum orientation?
Although the default long-slit setups give vertical spectra, there are
some projects that can benefit from a setup giving horizontal spectrum
For a windowed CCD the readout time is much shorter if the window is
oriented horizontally. See
this table for readout times for windows of different dimensions
and orientations. So for time-resolved spectroscopy it is advantageous
to use a horizontally oriented grism with a vertical slit.
Additionally, there are offset vertical slits
which allow you to tune the wavelength region that falls on the CCD.
There are no offset horizontal slits.
However, as the CCD clocks downwards (vertical direction), any column
trap or bad column will always cross the horizontal spectrum and leave
some wavelength bins useless. For vertical spectra this can be
avoided by positioning the star on a good column, so for most projects
the default setup is preferred.