ACAM sticks out from the WHT Cassegrain cluster at right angles
to the telescope axis, and at low elevation there can be significant
For example, when the telescope is at elevation 15°, and
the Cassegrain cluster is rotated to arbitrary angles, the y-position
(dispersion direction) of arc spectral lines moves on the CCD by ± 5 pixels (± 16
The flexure was characterised in more detail by measuring the
changes in y position on the CCD
of arc spectral lines
(1) at fixed elevations, as the Cassegrain cluster is rotated,
and (2) at fixed rotator position angles, as the elevation is
1 - Flexure as the Cassegrain rotator PA is changed, elevation fixed
The telescope elevation was fixed (at 15°,
30°, 60°, 75° and 90°), and the mount position angle varied from
-225° to +225° (in steps of 45°, black line in Figure 1),
and then back from +225° to -225° (blue line). The maximum
shifts are found at the lowest elevation, where there is a strong
dependence on mount PA, with the largest shifts at PA = -45°
-225°/135°. These position angles correspond to ACAM sticking out
to the right and left of the Cassegrain cluster (as viewed from
behind) respectively. If ACAM
is pointing up (PA = 45°) or down (PA = 225°/-135°), the
shifts are much smaller. Note the hysteresis: the shift depends not only
on the position, but on the direction in which one has rotated
to get there.
Shifts in arc-line y-position on the CCD versus
the Cassegrain rotator (mount) position angle (PA),
for fixed telescope elevations
15°, 30°, 60°, 75°, 90°.
measured while increasing mount PA from -225° to
Blue: mount PA decreasing from +225° to
2 - Flexure as the elevation is changed, Cassegrain rotator PA fixed
The rotator (mount) position angle
was fixed at PA = -225°, -135°,
-45°, +45°, +225°, and the telescope elevation varied from
90° to 15° (black line in Figure 2) and back from 15° to
90° (blue line). The results are consistent with the above:
when ACAM is
sticking out right or left, the shift in the y-position of an
arc-line on the CCD ranges
from 0 pixels, at 90°, to ± 5 pixels, at 15°.
When ACAM is pointing up or down (mount PA = 45°, 225° or -135°)
almost no flexure is observed when the elevation is changed from
90° to 0°.
Shifts in arc-line y-position on the CCD versus telescope elevation, for fixed
Cassegrain rotator (mount) PA = -225°, -135°, -45°,
Black: measured while decreasing
telescope elevation from 90° to 15°.
elevation increasing from 15° to 90°.
3 - Conclusions
In view of the above, we recommend taking arcs
before or after the observation of each target, particularly if the
Cassegrain rotator PA is near one of the critical angles (PA=-45°,
-225°, 135°) and the telescope elevation is below 60°.
During a typical half-hour exposure, at any elevation, the
flexure measured above is unlikely to cause shifts > 1 pixel in