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The Halley Multicolour Camera

 

Five spacecraft comprised the “Halley Armada” which encountered the comet in spring 1986. The Japanese probes, Suisei and Sakegaki, did not penetrate the comet’s inner coma and did not carry experiments for studying the nucleus. The Vega 1 and 2 spacecraft made encounters on March 6th and 9th, 1986, respectively (Table 1). Just after midnight on March 14th, 1986, the Giotto spacecraft made its closest approach (596 km). The Vega and Giotto spacecraft all carried sophisticated remote sensing experiments for determination of nucleus properties.

 

The Comet Halley "Armada"

Spacecraft

Closest approach distance [km]

Date and time of closest approach

Fly-by velocity [km s-1]

Helio-centric distance [AU]

Phase angle of approach

[deg]

Best pixel scale obtained [m px-1]

Comment on imaging systems and data

Vega 1

8890

06.03.1986

07:20:06

79.2

0.792

134

Out of focus (FWHM=10 px)

Vega 2

8030

09.03.1986

07:20:00

76.8

0.834

121

Saturated on nucleus

Giotto

596

14.03.1986

00:03:02

68.4

0.89

107.2

38

Little 3-D information

because of reset 9s before closest approach

Deep Space 1

2171

22.09.2001

22:28:30

16.5

1.36

88.0

47

Some 3-D information through to ?? ; no colour

Table: Parameters of previous cometary fly-by missions.

 

Nick Thomas was involved in the data analysis for the Halley Multicolour Camera and took charge of the testing in 1990 which was designed to determine whether HMC was still functional.

You can see some of the HMC scientific publications in our LAPIS area.

Images from Giotto

The HMC data set has been analysed and the results published in book form as an ESA Special Publication. Below, we show a number of pictures from the book in their original digital form.

This area is still under construction but we are hoping to put some archive material about HMC on-line soon.

 

The Halley Multicolour Camera image of comet Halley. This is a composite image produced from 68 individual images. The resolution varies across the image. At the brightest point, the resolution is about 50 m.

 

 

The imaging principle. How HMC produced an image of the nucleus from the spinning Giotto spacecraft.
Artist's impression of the Giotto spacecraft. The long tube at the top is the straylight baffle of HMC.

 

 

The Adoration of the Magi by Giotto. Note the image of the comet.
Members of the original HMC team. From left to right: David Hughes, Fred Whipple, ?, Walter Huebner, Claude Arpigny.

 

 

Data processing

 

HMC was a camera on a spinning spacecraft. It worked by using a technique called Time Delay and Integration (TDI). Here a line sweeps across the sky and the image is built up by clocking the line image under a mask at the same speed as the motion of the line across the sky. The image on the sky is then part of an annulus. The CCD image returned to ground is rectangular and it must be transformed back into an annulus by a geometrical rectification program. The image shows an example of this.
The HMC images had a number of artefacts produced by the system which needed to be calibrated out by the image reduction process. a. The raw image b. Bias and dark current subtracted. c. The coherent noise in the image. d. The final result.
The images acquired near closest approach were difficult to calibrate. The CCDs were getting warmer and the coherent noise was varying with every image. A scheme was developed to extract the coherent noise to improve the quality of the data. This used adjacent images, scaled and rotated, to establish a mean image of the scene which could be subtracted from the real image allowing better identification of the coherent noise.
The dark current was substantial in all images. Here we see the efficiency of the dark current subtraction procedure that was developed after 3 years of study. Each sensor was a different colour. The blue filtered data (sensor D) was strongly influenced by dark current.

Images analysis

The comet was first seen clearly from about 124000 km.
The reflectivity (I/F) of the nucleus and dust at a phase angle of 107 degrees.
Features on the nucleus of comet Halley.
The nucleus outline and terminator is shown. The rotation axis of Wilhelm et al. (1987) is superimposed. The concensus is now that the rotation is more complex (Wilhelm assumed a pure spin).
The Halley Multicolour Camera image of comet Halley. This is a composite image produced from 68 individual images. The resolution varies across the image. At the brightest point, the resolution is about 50 m.
Observed features on the surface of comet Halley.

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