3.5 Intensified CCD video recordings

The photometer array was bore-sighted with an image intensified (Varo Noctron V) black and white Pulnix video system with a field of view of $\sim$20 $^\circ\times14^\circ$. The video frames were time coded with a True Time GPS timing system and recorded on standard VHS videotape. The bore-sighted video recordings were essential to documenting the location in the sky viewed by the Fly's Eye photometers in all of the observations presented in this dissertation. In this section, we describe essential features of the video recordings.

3.5.1 GPS time stamping

NTSC video cameras can be clocked in ``frame'' or ``field'' mode. The Fly's Eye CCD camera operates in the frame mode. As shown in Figure 3.13, each field, consisting of even or odd interlaced scan lines, is exposed for $\sim$33 ms, and the two fields composing a frame overlap by $\sim$17 ms and together encompass $\sim$51 ms.

Before being recorded to video tape, the video signal is passed through a GPS-based time marking system, which imprints the date and time at the end of the first field's exposure onto both fields of each frame.

Figure 3.13: CCD exposure timing for a frame-mode camera.

3.5.2 High speed video

Data obtained with a variable-speed, triggered, image-intensified video system are presented in Section 5.1. This system was borrowed and operated in 1997 by Mark Stanley of the New Mexico Institute of Mining and Technology. Observations were made using frame rates of up to 4000 s$^{-1}$ in a manually-triggered mode. The camera and an overview of observations are described in Stanley et al. [1999].

3.5.3 Star-field matching

While the Fly's Eye electronically measures and records its pointing elevation, the pointing azimuth was recorded by hand, and any roll in the camera alignment was not recorded during typical observations.

A more direct method of determining the pointing is through post-processing of the video images in conjunction with a star catalogue. A Matlab software package named VIDEOTOOL was written to partially automate the matching of stars in a video sequence taken at a known time and location with stars from an electronic catalogue. The relationship between video pixel coordinates $(x,y)$ and azimuth/elevation coordinates $(\phi, \theta)$ is determined using a number of star positions approximately satisfying

$$\begin{pmatrix}\phi \theta \end{pmatrix} \simeq \begin{pm... ... y \end{pmatrix} + \begin{pmatrix}\phi_o \theta_o \end{pmatrix}.$$ (3.9)

This transformation allows the pointing elevation and azimuth, the video field-of-view height and width, and any axial rotation of the camera to be determined. It does not allow for lens distortions or edge effects, nor a determination of atmospheric refraction as a function of elevation. It addition it should be noted that at high viewing elevations, a rectilinear mapping of image coordinates to azimuth and elevation is inappropriate [Mark Stanley, private communication, 1997].

Figure 3.14: Pointing determination using star fields. (a) A single video field of a sprite sequence. In order to resolve the stars visible at this time, $\sim$10 frames from before the event are averaged to produce (b). A starfield matching program is used to interactively select and fit known stars to the image. In (c), known star positions are overplotted in green, and those used in the least-squares fit are shown in red. (d) The original image with superposed pointing information.

Star positions were obtained from the PPM (positions and proper motion) Star Catalogue, including a 1993 Bright Stars Supplement. This catalogue is available from the Astronomical Data Center and includes 378,000 stars. Star elevations and azimuths from the observation site are calculated for each video event time, assuming a simple form for atmospheric refraction (Section 3.2.3). The VIDEOTOOL program helps the user graphically select a set of matching stars in the the starfield projection and in a video image. Six or more stars are used in a least-squares fit to equation (3.9).

To increase the signal to noise ratio in the video image, at least ten frames not containing sprites or elves are averaged before the fit. Figure 3.14 shows an example of this procedure. Because the Fly's Eye records the pointing elevation of its photometric array, which is not subject to the axial rotation sometimes present in the camera, the measured photometer fields-of-view (Figure 3.11) can be plotted over the video image when the pointing azimuth of the Fly's Eye's mount is also precisely recorded during observations. In addition, if a lightning discharge located by NLDN has been associated with the video event, an altitude scale showing the local vertical directly overlying the lightning is plotted (Figure 3.14d). These altitudes refer only to events directly overlying the cloud-to-ground discharge, whereas sprites are known to occur with horizontal displacements of up to 50 km from temporally proximal lightning [e.g., Lyons, 1996].

Optical calibration of intensified video recordings using crude stellar luminosity data in the PPM catalogue was attempted without success. However, Heavner [2000, p. 93 and <]2286#>private communication# has successfully implemented such a strategy using the stellar databases of Johnson and Mitchell [1975] and Jacoby et al. [1984]. These databases can easily be integrated into existing VIDEOTOOL software to allow post facto video intensity calibration without knowledge of the intensifier high voltage setting. In this dissertation, we extract absolute intensities based on photometer calibration data as described in Section 3.3.