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Up: Positioning
Previous: Grating
cell
Install the second table extension at the telescope end of the optical
table, in line with the slit axis. Use alignment posts to position another
laser onto this axis (two holes on the bench should be on - or very close
to - this axis. They accurately set the height of the slit axis laser and
the direction to within a reasonable tolerance ). Remove relay mirror R
. With the collimator C
in position, the two laser beam should be coincident at the ISIS dummy
grating surface.
Install the collimator target onto the front face of C
- positioning so that the collimator axis is centred onto the target. The
position of the slit axis laser is shown as 25.0mm offset from this. If
necessary, adjust the position of the slit axis laser until it is. Iterate
with the position of the slit laser so that its height from the pole at
the input to the collimator and the coincidence at the dummy grating surface
are correct. In addition, note the position of the slit axis where it passes
the surface of R
- this should be 15mm from the edge of R
. When you are happy with the slit axis laser position, check that the
laser beam tracks along the centre line of the R
when the grating cell is rotated.
Assume 150 
m (2.5 arcsecs) fibres. Install the complete slit assembly into its position
using the template from RGOC. The fibres are grouped in modules of 9 fibres,
with a total of 14 of these modules forming the curved slit.
Adjust the kinematic seats of the fibre support plate until it is parallel to the slit axis (best done by setting the gap between the fibre support plate and its mount to be the same all the way round in the first instance). set the slit translation (height) adjustment and slit focus (longitudinal) adjustment to midrange.
Fibre module 7 and 8 should be symmetrically placed about the slit axis.
To set the slit height correctly however, we need to remove the fibre just
above the centre line. Call this module 8, then module 7 (FM 
) is just below the slit axis. With the translation adjustment of the slit
in mid range adjust the height of the slit unit from its baseplate until
the slit laser beam just clears the top of FM
. Adjust the complete baseplate until the laser beam is centred above the
central column of holes in FM
.
Illuminate FM
with white light, (it may be necessary to block off all but one fibre for
the following). The f/8.2 output from the fibres should be centrally
placed on the slit axis on the collimator target. Adjust the tilt until
it is. Remove the collimator target from the front of C
. Rotate the grating cell containing the dummy grating until the white
light beams are directed back towards the slit assembly - they should come
back to the position of the (removed) fibre module 8. The grating encoder
reading for this autocollimation setting is approximately 6650 millidegrees.
Illuminate the top fibre module. Its image should fall on the bottom fibre
module. Adjust the rotation of the fibre plate until it is correct. Repeat
with the lower fibre module. Check the position of FM
relative to the slit laser beam.
To set the slit to the autocollimation position, illuminate FM
with white light. With the slit focus at midrange, adjust the longitudinal
position of the slit unit until the image is at best focus - position blocks
to enable the complete slit unit to be slid along the optical axis. When
this is achieved, iterate on the slit height and beam direction as before
if necessary. Clamp the baseplate securely to the optical bench.
Illuminate the top fibre module. Its image should fall on the bottom fibre module. Check this is also in reasonable focus. Repeat for the bottom fibre module. Lock all adjusting screws on the kinematic mounts. It is best that the slit unit is in autocollimation with the slit translation and slit focus adjustments in midrange so that once the spectrograph is aligned any environmental changes are easily accommodated by these computer driven adjustments.