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4-m Image Quality Results (1Sep95)
(from CTIO, NOAO Newsletter No. 43, September 1995)

The image quality at the 4-m f/8 focus continues to be subarcsecond on
many nights. This is shown in the accompanying figure, which presents
all data taken with the f/8 side-port seeing camera between 1 January
and 15 May of this year. It can be seen that there was a slow decline
in the median image quality between January and March, and then a
sudden improvement again in April. It may or may not be coincidental
that we recollimated the secondary mirror in April. The median value
for all five months combined is 1.0", while it is 0.9" for April and
May taken together.

                         F/8 Direct Imaging

The figure shows that there are many nights of 0.7"-0.8" imaging to be
had at the f/8 focus. We want to call readers' attention to the
existence of the f/8 direct imaging system. It is used with our best
Tek2048, giving a scale of 0.16"/pixel and a field 322" on a side. The
usual CTIO filter selection is available (3 x 3 inch or larger). The
Shectman autoguider performs very well at this focus (we have taken 30
minute guided exposures with it which produced 0.7" FWHM images), and
starting in August we hope (at long last) to offer a working image
analyzer system that the night assistant can use to tweak up the
active optics on a bright star near fields of special interest (but at
the cost of 10-15 minutes observing time).

The potential drawback is that we don't have much experience using
this system. In the past there have been obvious problems with light
baffling, which produced rather awful background patterns when the
moon was up. We recently covered up some embarrassingly huge holes in
the main stovepipe baffle, and the background light problems are
obviously greatly improved, but we got weathered out on the recent
engineering run when we wanted to carefully check the present
performance of the system. The other caveat is that we have never
carefully checked the image quality outside the central 1024 x 1024
pixels of the CCD.

In spite of these disclaimers, persons interested in getting the best
resolution that we can offer over a small field should consider
applying for time at the f/8 focus. You should consider 2 x 2 binning
on the CCD, which would give 1/3" pixels.

                            Prime Focus

We have recently spent a large amount of time getting the new prime
focus corrector properly aligned with the rest of the telescope---
except that it is really the other way around. Due to a particularly
perverse design, we actually have to tilt the 4-m primary mirror to do
the alignment, and then spend another two (clear) nights recollimating
the f/8 and f/30 secondaries to make up for it. But we have managed to
get the best images with the PFCCD/ADC down to 0.8", a significant
improvement over the previous 1.0" floor. Note that at 0.8" FWHM the
images are very marginally sampled (0.43"/pixel with the Tek2048).

                           [Figure not included]

Jack Baldwin, Brooke Gregory, Gabriel Perez, John Filhaber

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