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Two steps are required in order to align the fibre optic system. First, the
slit and the prism are aligned together and, next, the slit - prism
subset is aligned with the optical axis of the spectrograph.
The slit - prism alignment is conveniently performed in the laboratory. The
focal plane bundle end is illuminated with a laser beam; the output light
at the other end of the bundle enters the prism. The screws at the kinematic
seats are adjusted in such a way that the output light from the slit
does not become vignetted by the two slots of the prism holder.
In order to align the slit - prism set with the axis of the spectrograph,
ISIS must not be mounted on the telescope. In this process, basically, the
bundle slit must be aligned and centred on the detector, and also the
optical axis of the central fibre of the bundle must be centred with
the collimator's axis. The following steps are required:
- The Dekker must be placed in position zero with the command
DEKKER 0.
- The bundle slit is placed in position on the spectrograph. In order
to do so, the software command SLIT_DOOR OPEN is used to open
the door giving access to slit units. The command MSLIT gives
access to the unit supporting the slit mounting. This
mounting is put on the unit by introducing the bundle through the
ISIS window above mentioned.
- The diffraction grating is removed, using the CHANGE procedure
from ICL. From this position,
one of the collimator mirrors can be see. Either the blue or red
arm can be used for the alignment.
- The focal plane bundle end is illuminated with white light. From the
grating position, the image of the fibre slit can be seen on the collimator
mirror. Its centering and orientation can be checked this way.
- ISIS long slit can be positioned with the software command LSLIT.
Looking at the collimator mirror the movement of both the fibre slit and
long slit can be followed. Then, differences in their centering and orientation
can be seen.
- For a correct centering, the position of the fibre slit must coincide
with that of the long slit. This can be achieved by sliding the slit
mounting on the slit unit.
- The white light illumination of the bundle is removed and, instead, the
focal plane end of its central fibre (number 31) is illuminated with a
laser beam.
- The output light cone illuminates the collimator mirror. Since the FRD
of this cone is larger than the mirror, the mechanical mirror holder
becomes illuminated as well.
- The light cone can be centred on the collimator by moving the screws
on the kinematic seats of the slit supporting plate. This way, the alignment
between the optical axis of the fibres and the one of the spectrograph is
obtained.
The alignment process described up to now can be checked by means of a method
which is similar to the autocollimation one. This method is as follows:
a flat mirror is placed at the diffraction grating position, facing the
collimator mirror -- the grating encoders position for the red arm to reach
this situation is 35500. When a laser beam illuminates the focal plane end of
one of the fibres at one slit edge, the output light is reflected by the
collimator mirror. This beam will be reflected by the flat mirror and returned
onto the collimator mirror, which will focus it
(if the alignment is good) onto the opposite edge on the slit from where
the light started.
Previous: Fibre slit mounting
Up: Installation and Setup for Fibre observing
Next: Guide fibre bundle focussing
Previous Page: Fibre slit mounting
Next Page: Guide fibre bundle focussing