The University of Hawaii operates an infrared camera with a 128x128 HgCdTe detector array on loan from JPL's High Resolution Imaging Spectrometer (HIRIS) project. The characteristics of this camera system are discussed. The infrared camera was used to obtain images of the night side of Venus prior to and after inferior conjunction in 1988. The images confirm Allen and Crawford's (1984) discovery of bright features on the dark hemisphere of Venus visible in the H and K bands. Our images of these features are the best obtained to date. Researchers derive a pseudo rotation period of 6.5 days for these features and 1.74 microns brightness temperatures between 425 K and 480 K. The features are produced by nonuniform absorption in the middle cloud layer (47 to 57 Km altitude) of thermal radiation from the lower Venus atmosphere (20 to 30 Km altitude). A more detailed analysis of the data is in progress

Allen, D.

Hodapp, K.-W.

Ragent, B.

Sinton, W.

English

NASA Technical Reports Server

N90-21330
High Resolution Imaging of the Venus Night Side
Using a Rockwell 128x128 HgCdTe Array
K.-W. Hodapp, W. Sinton
Institute for Astronomy, University of Hawaii
B. Ragent
NASA Ames
D. Allen
Anglo Australian Observatory
Abstract
The University of Hawaii operates an infrared camera with a 128x 128 HgCdTe detector
array on loan from JPL's HIRIS project. The characteristics of this camera system will be
discussed. The infrared camera has been used to obtain images of the night side of Venus
prior to and after inferior conjunction in 1988. The images confirm Allen and Crawford's
(1984) discovery of bright features on the dark hemisphere of Venus visible in the H and K
bands. Our images of these features are the best obtained to date. We derive a pseudo
rotation period of 6.5 days for these features and 1.74 _m brightness temperatures between
425 K and 480 K. The features are produced by nonuniform absorption in the middle cloud
layer (47-57 Km altitude) of thermal radiation from the lower Venus atmosphere (20-30 Km
altitude). A more detailed analysis of the data is in progress.
I. INTRODUCTION
Allen and Crawford (1984) discovered bright features on the dark side of Venus at
short infrared wavelengths, where thermal emission from the upper cloud layers of Venus
would be too faint to be detected. Spectroscopic observations confirmed that this radiation
is seen in two relatively narrow features at 1.74 _tm and 2.3 ktm wavelength, which are gaps
in the absorption of CO 2 and H2SO 4, the main absorbers in Venus atmosphere. In these
spectral features, we see deeper into the hot atmosphere of Venus and the spatial structure
arises from patchy absorption in optically thin clouds in the middle cloud layer (47-57 Km
altitude) of Venus atmosphere. From measurements of the pseudo rotation period of these
features, their association with the middle cloud layer can be confirmed. The features visible
in the near-infrared are different from the structure seen in the UV from spacecraft, which is
associated with the upper cloud level at 70 Km altitude. Groundbased near infrared
observations can therefore make important contributions to our understanding of the
dynamics of Venus atmosphere.
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H. Infrared Array Characteristics
We were using a 128x128 HgCdTe Detector Array with a cutoff wavelength of 2.5ktm,
manufactured by the Rockwell International Science Center. The Detector Array is Indium
bump bonded to a Reticon readout structure. This Detector array was made available to us
by JPL's HIRIS project and, for HIRIS, was optimized for high read-out rates and high flux
levels of an earth observing instrument in low earth orbit. Further, this device was
optimized for the 150 K operating temperature expected in the passively cooled HIRIS
instrument. For astronomical imaging, we use the device at a read-out rate of 4kts per pixel
and achieve a read-noise of 2000 electrons. The peak quantum efficiency of the device itself
is about 30 % in K, 20% in H and less than 10% in J at an operating temperature of 100 K.
At lower temperatures, the Q.E. drops further, higher temperatures give a prohibitively large
darkcurrent. The device shows large scale variations of the quantum efficiency of a factor 2
peak to peak across the chip. The dark current is dominated by luminescence in the
multiplexer, reaching values as high as 4000 electrons per second near the output amplifier
of the multiplexer and 500 electrons per second near the center of the device. The full well
capacity is 800000 electrons, so that operation is always read noise limited. The device
works linearly up to 70% of full well, above which a smooth transition to saturation occurs.
The sensitivity of this detector array is clearly inferior to the performance achieved by other
IR-arrays currently in use for astronomy. Its strength is the large format, allowing imaging
of large fields of view without the need for extensive mosaicing.
IH. The Infrared Camera System
The 128x128 array is mounted in an uplooking liquid nitrogen dewar for use at the 2.2
m and 0.6 m telescopes of the University of Hawaii (Fig. 1).
Fig. 1: Cross section through the UH Infrared Camera dewar. The dewar is an uplooking
design with one LN 2 can. The optics are a straight refractive system with 1:1 reimaging.
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The dewar is currently equippedwith 1:1 reimagingoptics consistingof a CaF2 field lens
and a biconvexZnSereimaginglens. The cold stopcaneasily be exchangedfor useof the
cameraat different f-ratiosrangingfrom f/5 (0.6 m) to f/35 (2.2 m). Up to now, the camera
hasonly beenusedat the f/10 focus of the UH 2.2 m telescopewith a plate scaleof 0.55
"/pixel. The read-outelectronics,developedby JPL (Kaki, Bailey, and Hagood, 1988),are
mountedon the dewar.The 12bit datawordsare transmittedvia a high speeddata link to a
buffer board in a 12 MHz Compac286 computerwhich containssufficientmemoryto store
a complete frame. This buffer board also controls the timing of the integrations. After
readingthe data from the buffer, the computerthanhandlesdatadisplay, subtractionof sky
frames,preliminary analysisandmassstorageon disk andtape.
IV. Venus observations
As part of a collaboration lead by D. Crisp from JPL, involving UH, NOAO, and JPL's
Table Mountain Observatory, we obtained images of the dark hemisphere of Venus
approximately 2 weeks before and after inferior conjunction. A narrow band interference
filter centered on the 1.741.tm feature was custom made by Barr Associates and distributed to
all participating observatories. This narrow band filter was used for most of the imaging
work. However, we also obtained some images in broad band K, where these atmospheric
features are also visible, and, for comparison, a few images at a wavelength of 1.57 ktm,
where no radiation can escape through the middle cloud layer. We stopped the UH 2.2 m
telescope down to 0.55 m by an excentric full aperture mask mostly to keep sunlight from
hitting the primary mirror of the telescope. As a positive side effect, we thereby created an
unobstructed round telescope pupil eliminating the diffraction spikes of the secondary
support and thus enhancing the image contrast. The excentric aperture was oriented such
that sunlight would not reach the primary mirror during the daytime observations with Venus
being only 20 degrees away from the sun. To further reduce straylight levels from the sunlit
crescent of Venus, we placed a neutral density filter in the focal plane, covering the brightest
parts of the crescent while leaving the dark side of Venus unobscured. In the end of May
1988, prior to inferior conjunction, with Venus east of the sun, we only obtained images
under poor seeing conditions (2 arcsec or worse). In the end of June 1988, after inferior
conjunction, with Venus in the morning sky, we obtained data of very high quality on five
consecutive days. Each morning, the image quality was best in the first hour after sunrise
(0.5 to 0.7 arcsec seeing), resulting in diffraction limited image quality (0.8 arcsec), and later
deteriorated to typical mid-day seeing of 2 arcsec. The integration time in the narrow band
filter was 30 sec and 2 sec in broad band K. Sky frames were taken 2 arcmin away from
Venus and subtracted from the object frames. Skyflats were used for flatfield correction.
Between individual images, the telescope was deliberately moved by a few arcseconds, in
order to eliminate systematically bad pixels later. We typically took 20 frames per hour and
coadded the individual images in 30 min bins, which is a short enough time interval to
neglect motion of atmospheric features. For coaddition, the images were interpolated to
256x256 format, registered with a precision of one half the original pixel size, and coadded
with the systematically bad pixels masked off.
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V. Preliminary Results
The images, Fig. 2, show different atmospheric features on each day. Feature tracking
gave a pseudo-rotation period of 6.5 +/- 0.5 days, corresponding to equatorial wind speeds
of 68 m/s westwards. These wind speeds are consistent with Pioneer Venus and VEGA
Balloon measurements for the middle cloud region at altitudes of 47-57 Km (Preston et al.
1986). It should be noted that the IR cloud features are not identical to those seen on the
bright side of Venus in the UV. The UV features have a pseudo rotation period of 4 days
and are associated with the highest cloud layer at about 70 Km altitude. At 1.74 gm, the
brightness temperature of the brightest features is 480 K while the darkest features have
brightness temperatures of 425 K. We do not interpret this as temperature nonuniformities
in the middle cloud layer. Temperature nonuniformities of this magnitude in the middle
cloud layer have never been observed by in situ measurements and would be difficult to
understand theoretically. The brightness variations are interpreted as patchy, optically thin
absorption in relatively cold clouds (300-320 K, Sagdeev et al., 1986) against the
background of thermal radiation from lower layers of the atmosphere.
We plan to repeat observations of this kind during the next inferior conjunction of Venus,
which fortunately coincides closely with the flyby of the Galileo probe at Venus. We hope
to provide long time coverage of the atmospheric features on Venus while the spacecraft is
expected to obtain images with much better resolution for a relatively short time.
We thank the staff of the UH 2.2 m telescope for their support during these observations.
We thank E. Irwin, M. Wagner, and S. Massey for their help in designing and operating the
UH Infrared Camera and G. Bailey and S. Kaki for providing the drive electronics for this
device. The 128x128 infrared array is the result of work by the JPL Infrared Technology
Group in support of the Spaceborne Imaging Spectrometer Project Office at JPL under
funding by the NASA Office of Space Science and Application.
References:
Allen, D. A., and Crawford, J. W. 1984, Nature 307, 19
Bailey, G. C. 1987, Infrared Astronomy with Arrays, ed. C. G. Wynn-Williams and E. E.
Becklin, University of Hawaii, p. 52
Kaki, S. A., Bailey, G. C., and Hagood, R. W. 1988, preprint
Preston, R. A., et al. 1986, Science 231, 1414
Sagdeev, R. Z., et al. 1986, Science 231, 1411
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ORIGINAl: PAGE
BLACK AND WHITE PHOt_
Fig. 2 Images of the night hemisphere of Venus on five consecutive days. The images are
the result of coadding up to 12 individual 30 sec exposures, sky subtracted and flatfielded
images taken in a half hour interval. In the coaddition process, the images were individually
aligned and systematically bad pixels have been removed. The observation dates given in
Fig. 2 are the averages of the exposure dates of the individual frames coadded to produce
the images.
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High resolution imaging of the Venus night side using a Rockwell 128x128 HgCdTe array

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