MÉTODO DEL ESTUDIO DE SUELOS

of sight, and 5.5 km in diameter on the horizontal circular disc (Figure 5.3). Meanwhile, the instrument makes measurements at several points on the circumference of a

circle with 301.4 km in diameter as shown in Figure 5.3.

The interference filter inserted between the Barlow lens and the IPD screens all but a narrow range of frequencies about the emission line being measured. The use of multi-layer coatings allows the filter to have a very precise spectral specification.

The most important factor to filter performance is the angle of incidence. The central wavelength of the bandpass is shifted due to a non-normal incidence such that (Vadnais

[1993])

s i n 0

[5.3]

incidence and n^ff is the effective index of refraction of the filter. For example, the central wavelength could be shortened by 8.7% (73.3 nm) for a 30° angle of incidence, when the central wavelength is at 843.0 nm and the n^ff is

'1.5' as for normal glass. This could be a serious problem for the F P I's entire performance, so this is the reason why the Barlow lens is placed prior to the filter. To measure the 843.0 nm emission line, a 1.0 nm FWHM interference filter centred at 843.2 nm at normal incidence is used (Vadnais [1993]) . The filter is chosen such that the centre wavelength is slightly redder (longer wavelength) than the emission of interest so that the centre wavelength will be

843.0 nm for the cone of light incident on the filter and because the peak of the filter will normally drift toward the shorter wavelength with age (Vadnais [1993]) .

5.5 Imaging Photon Detector (IPD)

The images formed by the lenses are collected by an

IPD, which consists of a photocathode, micro channel plate assembly and a resistive anode (Figure 5.4). The IPD used for observing night glow at the Bear Lake Observatory is an

ITT model F4146. It has a Gallium Arsenide (GaAs)

photocathode which can provide a useful sensitivity extending beyond 850.0 nm (Figure 5.5 and Rees et al.

[1990]) .

The quantum efficiency of the IPD is 15.0 % at

843.0 nm and at 630.0 nm it drops a little to 9 % (Figure 5.5). The photocathode is circular with 25 mm diameter. It is mounted on top of micro channel plate (MCP)

intensifiera with a ’z ’ configuration, in which individual photo-electrons are amplified by the factor of 10®. The cascades of electrons are intercepted by the resistive anode, and sampled by four electrodes at the periphery of the anode.

The signal from each anode connection is amplified by a pulse shaping preamplifier^ and is sent into a line driver in which the signal loss is compensated to match with the next stage. After amplification the signals are sent into the signal processing unit. There the x-y position of the electron impact at the photocathode is calculated by the comparison of total charges and each charges from an individual anode connection. The original position of the incident electron is equated with the centroid of the final electron cascade.

The position information is transferred into a PC

photon by photon where the two dimensional images through the integration period are completed. The maximum photon

^It is amplifying charges instead of current. Previous current amplifiers suffered from noises including d.c. offset errors. However the shape of the pulse from a charge in a RC circuit is defined by the amplitude and shaping time constant, so if we measure two parameters we can define the input charges precisely. The amplitude is proportional to the magnitude of the input charge, while the pulse length is

counting rate is dependent on the IPD and system electronics. It is 200,000 counts per second for the Utah system. The two-dimensional image is still too big to be stored in the PC's permanent memory storage for whole nights over several months. It uses 2 bytes for one pixel and a two dimensional image has 65,536 pixels (=256x256 pixels) so this is 131.6k bytes per image including the

record header : 512 byte per each image. To reduce ^ this storage required, a circular integration (section 5.7) is performed to make a one dimensional image consisting of 2

bytes per bin and 256 bins.

The IPD is cooled to -30° C with a combination of a Peltier cooler and a constant-temperature water-antifreeze circulator to reduce the thermionic emission from the photocathode. The thermionic emission of the entire device

(256 X 256 pixels) for Utah FPI at -30° C is approximately

422 counts per second, or 0.006 counts per pixel per second, or 1.6 counts per bin per second (Vadnais [1993]).

There are some ' hot spots' in the IPD images. In principle, the worst of the 'hot spots' for the IPD have been removed from both the images and the thermionic emissions. Most 'hot spots' are reasonably well behaved - i.e. consistent. Some, however, are not, and they can make analysis very difficult.

The positional accuracy of a detector is determined by the probability density function of the displacement of the estimated centroid from the position of arrival of the

original photon, i.e. the Point Spread Function (PSF). This

PSF is the result of the convolution of its several components, such as a photocathode, an MCP and a resistive anode (McWhirter et al. [1982]).

The positional accuracy depends on the photocathode to

MCP gap size and the electric potential (voltage) between them. The accuracy can be increased by increasing the potential and narrowing the gap. However concern about long term field operation means that the choice of gap and the potential should be compromised because the combination of high potential and narrow gap could damage the device, and thus shorten the life time. For the Utah instrument the values are set to 1000 pm and 600 Volts. An Aluminium-Oxide (AlgOg) thin film is deposited on the micro channel plates surface to protect the photocathode from the damage due to the bombardment of heavy ions.

The positional accuracy can be represented by the FWHM

of the PSF of the IPD, this has been estimated by McWhirter et al. [1982] and McWhirter [1993] as around 60 pm.

This FWHM is in the case of the best condition, if we take into account other broadening contributions, e.g. the sampling effect (section 4.5) and the other contribution from the statistical photon shot noise (section 7.2), then the resultant FWHM may become around 150 pm.

The detailed specifications of the instrument, including the étalon are listed in Table 5.1.

Instrument parameters

effective focal l e n g t h(f) f-number

aperture d i a m e t e r(D)

Field of View (FOV)

1580 mm 10.53 150 mm 0.906° Etalon plate separation (d)

optical path length^

(2d • rig • cos

reflectance(R)

free spectral range

finesse

reflective coating resolving power

Interference Filter

diameter

Full Width Half M a x .(FWHM)

peak transmission peak wavelength 20. 49 ± 6.2 X 10""m m 40.98 ± 0.03 m m 0.89 at 843.0 nm 0.88 at 632.8 nm 0.0173 ± 0.00005 nm (=203.5 ± 0 . 6 bins) 16.9 for He-Ne laser of

FWHM 1.2MHz