Synchronised High-Cadence Imaging of the Solar Chromosphere: The Rapid Dual Imager (RDI) Abstract A novel, ultra-high-cadence imaging system – the Rapid Dual Imager (RDI), designed to obtain synchronised dual-band images of the solar atmosphere – was recently tested at Big Bear Solar Observatory. The system consists of two identical CCD cameras which can, for the first time, obtain truly simultaneous images at up to 80 frames per second per channel in two wavelength or polarization bands. We present sample observations, made with this system, of a GOES C-class flare, in H and H -0.5 Å. Optical Setup RDI was tested at BBSO’s 65-cm telescope using the optical setup shown in Figure 2. Light from the telescope is first split by a 50/50 cube beam-splitter and then directed to the centre and east benches mounted on the main body of the telescope. Having passed the beamsplitter, the beams then follow almost identical paths along each bench. Each beam is first passed through an infrared-blocking filter and a 30 Åwide H pre-filter, centred on Å, before it then enters a Carl Zeiss Lyot filter to narrow the bandwidth to 0.5 Å (FWHM). The beam is then brought to focus at the CCD via an f = 180-mm lens. For the observations taken during this run, the Camera A (direct path) Lyot filter was tuned to H line centre while the corresponding Camera B (diverted path) filter was centred on H Å. References Phillips et al. (2000): Solar Physics 193 p259 von der Lühe (1993): A&A 268 p374 Vogt & Hénoux (1996): Solar Physics 164 p345 Williams et al. (2004): Solar Physics (in preparation) David.R. Williams 1, R.T. James McAteer 2, Peter T. Gallagher 3, Thanassis C. Katsiyannis 2,4, D. Shaun Bloomfield 2, Mihalis Mathioudakis 2 and Francis P.Keenan 2. 1 Mullard Space Science Lab, UCL, Holmbury St Mary, Surrey, RH5 6NT, UK; 2 Queen’s University Belfast, Northern Ireland, BT7 1NN, UK; 3 NASA Goddard Space Flight Centre, Greenbelt, MD 20850, USA. 4 Royal Observatory Belgium, Ave. Circulaire 3, 1180 Bruxelles, Belgium. Figure 1 (above): Sequence of consecutive, cotemporal pairs of images taken by RDI at BBSO on starting at 18:03: UT. The upper field in each pane is H while the lower is H -0.5 Å. The fields have been aligned to within 1” Figure 2 (above): Position and orientation of the RDI Camera A (H line centre) field on the Sun on at 18:22.18 UT. The field was aligned to SoHO/MDI images, differentially rotated over just under one hour. The RDI plate scale in this image sequence is 0.32” per pixel. Start End Figure 3: Anti-clockwise from top-right: sequence of RDI H images, with intervals of 1300 frames (21.7 seconds) between each image shown. The progression of a flare can be tracked, and the difference in emission morphology can easily be seen by contrasting the start and end images. System advantages Field is common to both cameras with configuration shown (Fig. 4), but can easily be adapted; Long duration (up to 2.2 hours with current disk size); Max frame rate of 80 fps; Gathers data on timescales over five orders of magnitude; System is based on previous proven technology: the Phillips et al. (2000) SECIS system, built to observe eclipses at high cadence. The system is easily portable, with versatile C-mount cameras Figure 4: a) Optical setup used at BBSO, for a common observing field; b) illustration of data flow to, and through, RDI’s control computer. a) b) Applications The choice of filter and optical path for each camera is effectively arbitrary, so polarisers can be used to study rapid variations in polarised emission. Such polarimetric measurements will allow the examination of angle and angular velocity distrubutions of electron beams (Vogt & Hénoux 1996) during the impulsive stage of flares. Similarly, observations in perpendicularly-polarised channels of photospheric or chromospheric magnetically sensitive lines will enable ultra-high time resolution longitudinal magnetic field measurements to be made. This is of particular interest in the study of reconnection in these lower atmosphere regimes. Because of the very short interval between frames, it is possible to apply speckle image reconstruction (von der Lühe, 1993) to achieve near-diffraction limited image quality, thereby revealing information at fine resolution in both the temporal and spatial domains. The analysis of this data will shortly be available as a submitted article (Williams et al. 2004). Control system The 10-bit Basler cameras are controlled by a custom- built PC with ‘grabber’ interface boards. These boards us a loss-less, 150%-compression algorithm to increase the data throughput to the PC. The data are fed onto pre-allocated ranges of contiguous disk addresses on four striped IDE RAID-array disks at a rate of 21 MB s -1. At the maximum frame rate of 80 fps, the camera is windowed to 492x492 pixels, with 9.9 m pixels. The maximum window of 640x492 pixels can sustain 72 fps.