AAO Newsletter October 1996 - Page 15
Figure 9: TTF chargeshuffle long-slit imaging of a D lamp (top)
and a Cu He lamp (bottom), before (top) and after (bottom) plate parallelism
has been established. The lines are discrete images of the focal plane
slit. Note the slight curvature in the lines over the 10' field at the
largest plate spacings.
spacings. A potential stumbling block to squeezing the plates to the
limits is that we need to be sure that the plates really are parallel,
otherwise they are in danger of touching at critical points.
With conventional etalons, the interference pattern easily visible with
the naked eye can be used to judge whether the plates are parallel or need
to be adjusted. In the limit of very small spacings, this is no longer
possible as the field is essentially monochromatic. Our method is to place
a long slit (aligned with a row of the CCD) in the focal plane and to illuminate
the optics with a monochromatic source. The TTF is stepped through
roughly 100 spacings synchronized with the charge being stepped by one
slit width at the detector. This results in a long-slit spectrum at the
detector. We place masks in the pupil plane that expose different quadrants
of the TTF and repeat this for all quadrants. For security, we start
this at the largest plate spacing (see Fig. 9). If the plates are not parallel,
the four long-slit spectra show differences in phase, intensity, structure
and so forth. This method is vastly more efficient than conventional approaches
and allows the plates to be tuned at any gap spacing in a matter of minutes.
J. Bland-Hawthorn (AAO) & H. Jones (MSO)
Service observing of programs requiring up to two hours integration continues to be available on the RGO Spectrograph, FORS, UCLES, IRIS, prime focus imaging and occasionally on Taurus. General service queries and applications for time should be emailed to "service@aaoepp.aao.gov.au".
During the period of 2dF commissioning, telescope time has been diverted
away from service observing. Nevertheless some nights or half nights have
been scheduled, often with little notice, so it is still worthwhile to
submit proposals. Proposals remain current for a period of up to two years.
