Applications of two-dimensional vidicon photometry--Venus.

Applications of Two-Dimensional Vidicon Photometry:

Venus

by

Philip Henry Schaller, Jr.

Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science

at the

Massachusetts Institute of Technology

June, ₁₉₇₃

Department of_ ht nd Planetary Sciences, May 11, 1973

Certified _{by:_}

Accepted by:

Chairman, Departmental Committee on Graduate Students

TABLE OF CONTENTS

Acknowledgements ...

A. INTRODUCTION

B. GRAY SCALE EXPANSION

C. IMAGE REFLECTION AND

D. LAPLACIAN IMAGE . E. INTENSITY TRAVERSES F. CONCLUSIONS References ... Figure Captions Figures ... Appendix ... ... 2 g... 3

...

6

ROTATION ... 8 10 12 ... .... .14 .. g.... .. o... e e* 15

... ..

...

.. .. ...

.. ..

16

.... ....

...

....

....

...

19

... ....

...

....

....

54

ACKNOWLEDGEMENTS

I would like to thank my advisor, Thomas B. McCord, and all my friends at MITPAL for their help. Also,

thanks go to R. Beebe and others at the New Mexico Uni-versity for supplying their photographic plates for com-parison.

3

A. INTRODUCTION

The vidicon tube and the theory of its operation, and an operating camera and data system have been described by

McCord and Westphal (1972). The hardware and software

sys-tems have been described in detail by Kunin (1972). This thesis describes one application of the vidicon system to

as astronomical problem.

It has been known for some time that the planet Venus exhibits a featureless shroud of clouds in the visible

por-tion of the spectrum. In the ultraviolet portion, however,

features are visible (Boyer and Guerin 1969) (Scott and

Reese 1972). These features, and in particular the curious

"Y"-shaped cloud which rotates around the planet in

approxi-mately four days, are of extremely low contrast. Conventional

photographic pictures are printed on high contrast paper to bring out the low contrast clouds. The vidicon system

is ideally suited for this problem for three reasons.

1) The system has a wide dynamic range, incorporating

many 5ray levels.

2) The system is linear over this wide dynamic ranSe.

3) The data are already in digital form.

Analyses of the vlidican imaeas are simple using a computer.

The yidicon images presented in this thesis were taken

usinS the Kitt Peak National Observatory 84" telescope on

April 23, 24, 25, and 26, 1972. The first night the

mounted at the Cassegrain focus. Film photographs were taken the same nights at the New Mexico State University.

Thirteen vidicon images were selected for study in this thesis. These represent the best pictures at each of

the several wavelengths taken during each night of the run. On the first night (the only Coude pictures) the Y-shaped cloud had apparently rotated out of view. The second night many quality pictures were obtained at .358, .383, and .402 microns. The cloud is clearly visible on this night although

only one of the branches of the Y is seen. The third night only three pictures were obtained and were of less quality than the night before. The final night all pictures were taken through infared filters (see figures 1-13).

Figures 14 through 17 are the best pictures taken with conventional photographic plates during each night of the run. They are all at .37 microns. Correspondence with the vidicon pictures should be noted. They also show only one branch of the Y cloud and confirm the phase of the cloud during this time. The approximately four-day rotation can be observed, noting that the cloud was rotated out of view

on the night of the 23rd. These pictures, however, are not suitable for entering into the rotation rate controversy of 4 or 4.5 days (Boyer and Guerin 1969) (Scott and Reese 1972).

Various image processing techniques were developed in the course of this thesis research which were used to enhance the cloud features above the background planet. The remainder

5

of this thesis is devoted to a description of these techniques and their application to the Venus cloud problem. While the data were not taken for the purpose of this thesis, a good deal of it war, usable. Although no great scientific prob-lems were solved, it is hoped that this work will be the guide

B. GRAY SCALE EXPANSION

To make such low contrast features such as the Y cloud on Venus more visible, various image enhancements can be performed on the computer. The first of these attempts at contrast enhancement is gray scale expansion. The vidicon system has 256 gray levels. The actual feature may extend over only tens of gray levels. These gray levels are

line-arly expanded over the full 0 to 255 range. Points

else-where in the picture falling below 0 are set to 0 and those rising above 255 are set to 255.

As the contrast is enhanced (see figures 18-22), two

effects are noticeable. 1) The size of the image decreases

until under almost total contrast enhancement the image al-most disappears. This is due to less and less of the image

falling into the selected range of gray levels for total expansion. 2) Artificial contours are introduced by the quantization noise. When only a few gray levels are used to

cover the entire range of light levels, noticeable (to the eye) distinct changes in light level are visible across the

image. While the original pictures had eight bits of tray resolution, every contrast enhancement of a factor of two

reduces the number of significant bits by one. Thus these

contours are not inherent to the actual image (which is con-tinuous), but are introduced by the necessity of making

arbitrary decision levels in digitizing the data.

Since our object data is at one end of the gray scale, a logarithmic enhancement built into the film converter was

7

also tried. (see figures 23-27) The resulting images do not show much improvement. The vidicon system also contains a routine to contrast enhance an overexposed image without

loss of precision, as well as the normal overexposed image reducing routines.

C. IMAGE REFLECTION AND ROTATION

The usual method of mapping a feature which appears only at selected wavelengths is to divide the picture at one

of the selected wavelengths by the same scene at another, but featureless, wavelength. In this case, however, this cannot be done, The scattering function of the CO2 atmos-phere, which is superimposed on the feature, is a strong

function of wavelength and hence cannot be divided out. As noted before, only one branch of the Y seems to be visible on this occasion, The southern hemisphere can be divided by the northern hemisphere at the same wavelength

(using the same picture, obviously), and thus eliminate the CO2 scattering function. (This assumes a certain symmetry

about the function, of course.) The vidicon system now contains a routine to reflect a picture about an arbitrary axis in the plane of the picture. The problems of such a reflection on a discrete grid of points are obvious, but

the results are remarkably good. Note that the proper com-bination of two reflections can produce an arbitrary rotation about an axis perpendicular to the plane of the picture.

The reflected image of Venus is divided by the original and then contrast enhanced. (see figures 28-42) Two things are now immediately obvious. 1) There is a symmetry about the reflection axis. Where the image is dark on one side, it is light on the other side and vice versa. So in the same picture we have the southern hemisphere divided by the

northern hemisphere juxtaposed with the northern hemisphere divided by the southern hemisphere. 2) There is an edge effect to this process. The reflection, while good, is not perfect and the limbs of the planet in the two images.(or-ginal and reflected) do not match up exactly. Since the

light level changes rapidly across the limb, the division process greatly enhances the edge. Contrast enhancement makes it worse. The problems of contrast enhancement

10

D. LAPLACIAN IMAGE

Another method of enhancing low contrast features is that of the Laplacian. Being a second derivative operator, the Laplacian is not sensitive to overall reflectivity levels or their uniform variations. It yields only changes in re-flectivity variations. These are most often due to fine detail in the image.

The discrete Laplacian is performed on the image by subtracting the average level of the right nearest neighbors of a point from the level of the point. (Rosenfeld 1969) The resultant values are then contrast enhanced. (see figures 43-55) The absolute values of the Laplacian may also be used for display. (see figures 56-68)

Two items are noticeable in these pictures. 1) The ones taken at the Cassegrain focus show a considerable edge effect whereas the Coude pictures do not. This is due to our now enlarged raster (nine points instead of one) when taking the Laplacian. In the Cassegrain images the entire disk of the planet is approximately 32 points wide, while

the Coude images show a disk 128 points wide. Since the Coude image is of smaller scale and the limb varies more slowly, the effect of the larger raster is not noticeable.

One might try to circumvent this purely mechanical problem by enlarging the smaller picture in the computer before taking the Laplacian. Two methods of enlarging are

common and both have problems, a) Each point is mapped into, say, four points at the same gray level. Its

problem is graininess. The Laplacian pictures appear grainy enough without making them worse by this method. b) Each of the four points in the enlarged picture can be interpolated in gray level between the positions of the original gray levels. This is an expensive process and does not yield enough quality to make it worthwhile. Fre-quently, artificial contours are introduced and are enhanced by the Laplacian process. Both these methods suffer from

the problems associated with displaying a supposedly con-tinuous function on a discrete grid, (see previous section)

2) The second branch of the Y cloud is now discern-able. This has been unnoticed by any of the previously mentioned methods, which shows the power of this technique.

E. INTENSITY TRAVERSES

While the CO2 scattering function is a complication

for the purpose of observing the Y cloud, it is interesting in its own right. (see Appendix) This data is presented in the format of intensity traverses. The intensity of the image is scanned along a line perpendicular to the planet's terminator. All plots begin with the limb on the left and end with the terminator on the right. The organization of these 117 intensity traverses is first by image and then by

latitude, In order there are nine plots for each of the thirteen previously mentioned selected images corresponding to figures one through thirteen, respectively. In each set of nine, the plots start at the northern end of the planet and end at the southern end. Note that in some cases, the first or last plot misses the planet altogether. The fifth plot in the sequence is reliably along the equator. The lines of traverse are as close to being at equivalent

lati-tudes as is possible on a discrete grid, and very close in equivalent distance. A normalized distance scale, as well as the start and end points for each traverse, is given. Note that the traverses were taken before correction due to the mirror image reflection by the Cassegrain focus. This

is apparent in the absolute coordinates only. Also note

that the plots of the Coude images span about 128 points, making the Coude traverses appear smoother.

For interpretation, the plots at similiar latitudes, but through different filters, may be overlayed to show the

13

wavelength dependence of the CO2 scattering function. Small features, such as the Y cloud, seem to be masked by the

14

F. CONCLUSIONS

The first data from the two-dimensional vidicon photometer are at least as good as that obtainable from

conventional photographic film techniques. Further

pro-cessin to bring out desired features is readily and easily done through computer techniques. These techniques produce better and cleaner results With more and more sophisticated methods. The next method to be tried should be some form of high pass filtering in the spatial frequency domain. Noise filtering can easily be done along with this.

Further observations of Venus with the vidicon system

should span at least five nights in order to see one

complete cloud c.ycle. The image should be as large as

possible without reaching the edge of the target. Since the cloud feature spans just 10 or 20 gray levels out of 200, the exposure should be such that the brightest part of the image is near the top of the gray scale range. Overexposed images should be retaken, as it is at best difficult to retrieve the information. Images should be taken at several wavelengths during each night and

repeated on the other nights. Adequate loggin5 of the

setup of the observations and the data taken will ease

REFERENCES

Boyer, C. and P. Guerin (1969). Etude de la rotation retrograde, en 4 jours, de la couche exterieur nuageuse de Venus. Icarus 11,

338-355.

Kunin, J. S. (1972). A Technique for Two-Dimensional Photoelectronic Astronomical Imaging, with Applica-tion to Lunar Spectral Reflectivity Studies. Master's Thesis. Massachusetts Institute of Technology.

McCord, T. B. and J. A. Westphal (1972). Two-dimensional Silicon Vidicon Astronomical Photometer. Opt.

11, 522-526.

Rosenfeld, A. (1969). Picture Processing b Computer. Academic Press, 94-102.

Scott, A. H. and E. J. Reese (1972). Venus: Atmospheric

16

FIGURE CAPTIONS

North is to the top and east is to the right, unless

otherwise rioted. (Planetocerntric coordinates)

1 2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 Fi "f figure 18 4/24/72 4/23/72 4/24/72 4/25/72 4/23/72 4/24/72 4/23/72

4/26/72

4/26/72

4/26/72

4/26/72 4/26/72 4/26/72 4/22/72 4/23/72 4/24/72 4/25/72 gray levels -9 "n with logarithmic e lter: .358 microns .383 .383 383 .402 .402

.564

.906

"9 .948 1.00 1.05 "HB 1.10 microns "' .37 " "9 .37 9 "9 .37 " "9 .37 " 10 to 200 of figu 60 to 200 " 100 to 200 160 to 200 130 to 200 re 3 nhancement

Notes double

ex-posure

New Mexico State University

24 25 26 27 28 29 30 31 32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

Notes second branch

of Y cloud figure 19 with logarithmic enhancement

figure 20

figure 21 "

figure 22

same as figure 3

reflection of figure 28

figure 28 divided by figure 29

gray levels 10 to 245 of figure 30

"1 20 to 235 " "o 30 to 225 " "1 40 to 215 " "f 50 to 205 " "f 60 to 195 " "f 70 to 185 "f _{80 to 175} " o90 to 165 "t 100 to 155 "f 110 to 145 " "o 120 to 135 "

Laplacian of figure 1 Note: second branch

of Y cloud

Laplacian of figure 9 10 11 12 13

Absolute value of Laplacian of figure 1 2

3

8 9 10 11 12 13

53

54

55

56

57

58

59

60

62

64

65

66

67

68

20

Figure 1

Figure

3

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Figure

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Figure

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Figure 15

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Figure 27

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Figure 50

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Figure 55

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53

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