![]() The PC was configured with 1 TB of disk space (two 400-Gb and one 200-GB drives) to record the large data files. The video data were recorded using a Data Translation DT3155 frame grabber mounted in a PC system using a 3-GHz Pentium 4 processor. each frame consists of consecutive scans of odd rows and even rows stitched together). The camera generates video data with interlaced scanning (i.e. The camera produces video output with an effective pixel size of 8.6 × 8.3 μm and a format of 768 × 576 pixels. In all cases, the image scale was chosen to ensure that the pixels provided good sampling of diffraction-limited star images. The camera was either placed directly at the telescope focus or when necessary used with a 2.5 times focal extender. The observations used standard BVRI filters. This camera was chosen because it had adjustable exposure times and adequate sensitivity to observe bright stars. The imager used for MUSIC was a Watec 100N monochrome video camera. The study was carried out as a preliminary stage in the design of a more advanced lucky imaging system that will use an EMCCD camera. The observations were obtained using Macquarie University Selective Imaging Camera (MUSIC) Mk I. The results provide information that can help to optimize the design of future instruments. Unlike most previous studies which have aimed at exploiting excellent seeing conditions, our observations were obtained in a range of seeing conditions from good to poor. We have explored empirically the effects of telescope aperture D, wavelength λ, frame exposure time t and frame selection rate (FSR) on the resulting image quality. In this paper, we present observations that explore a wide range of parameter space. However, previous studies have generally been aimed at obtaining the best possible image resolution and have therefore explored a restricted range of parameters. A number of such systems have recently been demonstrated, for example Luck圜am ( Law et al. 2001), as well as computers with fast processors and large storage capacity. ![]() Interest in the technique is rapidly increasing, in part due to the availability of electron multiplying CCD (EMCCD) technology, which allows rapid readout of CCDs with negligible read noise ( Mackay et al. 2000 Cecil & Rashkeev 2007 Ksanfomality & Sprague 2007) and is now widely used by amateur astronomers for planetary imaging. The technique has been used to image the hemisphere of Mercury that was missed by Mariner 10 ( Dantowitz et al. (2001) demonstrated the ability to obtain diffraction-limited star images at 800-nm wavelength with a 2.5-m telescope. There have been a number of practical demonstrations of this technique variously described as frame selection ( Roggemann & Welsh 1996), lucky imaging ( Law, Mackay & Baldwin 2006) or selective image reconstruction ( Dantowitz, Teare & Kozubal 2000). Since the image quality gain will increase with D/ r 0, this suggests the frame selection technique will work best for D/ r 0∼ 6–7, this being the largest D/ r 0 at which there is a good chance of finding several high-quality images in a typical image sequence of a few thousand frames. For higher D/ r 0, the probability of a sharp image rapidly decreases, being 1 in 3800 for D/ r 0= 8. The probability of such an image is 1 in 9 for D/ r 0= 5 or 1 in 50 for D/ r 0= 6. This suggests that there will be more good quality images available at low D/ r 0. The results are in reasonable agreement with the simulations of Baldwin, Warner & Mackay. We show that Strehl ratios of >0.7 can be achieved in appropriate conditions whereas previous studies have generally shown maximum Strehl ratios of ∼0.3. ![]() Our results are consistent with previous investigations but cover a much wider range of parameter space. The best Strehl improvement is achieved with exposure times of 10 ms or less, but significant improvement is still obtained at exposure times as long as 640 ms. We find that improvement in Strehl ratio by factors of 4–6 can be achieved over a range of D/ r 0 from 3 to 12, with a slight peak at D/ r 0∼ 7. The improvement in Strehl ratio of the stellar images due to aligning frames and selecting the best frames was evaluated as a function of these parameters. The observations were made over a wide range of values of D/ r 0 and exposure time. A high-speed image recording system has been used to observe a number of bright stars. We present an empirical analysis of the effectiveness of frame selection (also known as lucky imaging) techniques for high-resolution imaging.
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