For open use observations, pre-imaging for target fields are taken
around
one month before MOS spectroscopic observations.
Images taken by FOCAS are currently required to assign MOS slit
position.
Observers are requested to inform the final coordinate of the target
fields
to the support scientist after considering effects of a CCD gap and
spectrum
distribution on dispersion direction which are described below.
FOCAS has two 2KX4K CCDs. Because of this structure, there is around
5 arcsec gap between the CCDs.
The observers can adjust coordinates to avoid that targets fall onto
the
gap, of which position can be found here.
A problem of asymmetry in a spectroscopic image arises especially in
300B configurations.
For example, with the 300B + L600 setting, 3500-6000A region can be
observed.
The blaze wavelength 5500A will be just below the slit, which means
that
the spectra will extend more to the blue side if we put the slit at the
centre of the FOV (5500-3500=2000A toward blue, 6000-5500=500A toward
red).
In another word, you have more room for slits at western part of the
your
target coordinates in case of PA=0 deg.
You can change the spectral image distribution by shifting the slit
position.
Note that each spectrum will occupy about 2000 pix in wavelength
direction,
which is as big as about half of the available length of the CCD (4000
pix
in Y).
Information on the blaze angle, wavelength coverage can be found here.