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CTIO Instrumentation (1Dec93)
(from CTIO, NOAO Newsletter No. 36, 1 December 1993)

We continued to make steady progress with the implementation of ArCon
CCD controllers and in the general upgrading of the 4-m prime focus.
The use of our Tektronix 2048 x 2048 CCD with ArCons, and the general
visitor use of the Large Format Prime Focus CCD (LF PF CCD) camera, are
described by Alistair Walker in a separate article.

Another related project which has achieved important milestones is the
4-m Prime Focus Atmospheric Dispersion Compensating (PFADC) corrector.
This is effectively a four-element Wynne-type corrector with
dispersion-correcting prisms incorporated. It was designed by Richard
Bingham (U. College, London). It is now in routine use (with the ADC
prisms in their neutral position) with the LF PF CCD and Argus, and
clearly delivers subarcsec (0.5 arcsec or better) images which are
better than any ever obtained with our other prime-focus correctors.
The control system for the ADC prism positions will not be ready for
about three more months.

There was also considerable progress with the series of projects
related to upgrading the f/7.8 secondary at the 4-m, but the refiguring
of the mirror itself has fallen distressingly far behind schedule. The
problem is that the optics company that measured the mirror's figure
for us took three months instead of the contracted one month. We thought
we had built an ample safety margin into our observing schedule, but we
had not allowed for such a gross over-run. The result is that we have
had to cancel the scheduled f/7.8 runs in early December (we will have
to use prime focus and f/30 IR instruments instead). We very earnestly
apologize to the observers who lost their time.

However, the final measurements (which we believe to be of high
quality) show that just the high frequency errors on the mirror's
surface contributed 0.6-0.7 arcsec to the 80% encircled image diameter,
so it was clearly necessary to obtain these measurements in order to
properly correct the mirror's figure using Kodak's ion polishing
technique. We will also be removing a similar contribution to the image
size due to spherical aberration, and so we continue to hope for a very
significant improvement in the image quality at this focus.  The mirror
is now at Kodak for the ion polishing step, but will then need to be
remeasured before we bring it back to Chile.

Other projects brought to a successful conclusion during this period
were a general refurbishment of the ASCAP single-channel photometers,
and the construction of an improved focus system for the CCD system at
the Schmidt telescope (see accompanying article).

Jack Baldwin

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