Digital Video Imaging with Small Telescopes – Possible Applications for Research and Education Using the Sofia Upper Deck Research Facility

The amateur astronomy community has a long tradition of performing routine visual observations of the planets with small telescopes, but in the last decade, CCD imaging has largely replaced visual observing as the primary method of recording planetary detail. Low cost 24bit color CCD video conferencing cameras (webcams) coupled to amateur telescopes (typically 0.2-0.3m aperture) have become powerful tools for high resolution planetary imaging. The high sensitivity of webcam CCD chips permits a series of short individual exposures to be obtained as a video stream directly into a personal computer. Software is used to selected out individual frames blurred by poor seeing , sharp frames are then stacked to increase the signal to noise ratio and finally wavelet processing is used to reveal fine detail. Under good seeing conditions resolution of planetary detail approaching the diffraction limit of the telescope (<0.5”) is possible with this technology. Additionally, by aligning and summing relatively short exposures (typically 100ms to 5s) it is possible to image faint objects, such as comets and asteroids, without the need for accurate sidereal tracking. Amateur planetary imaging programs have permitted tracking of small scale atmospheric structure on Jupiter, measurement of drift rates of small mid latitude white ovals on Saturn, monitoring of changes in cloud structure on Venus and observation of the development of small dust storms and water ice clouds on Mars. The optical and imaging equipment routinely used by amateurs could be adapted to exploit the unique observing conditions available on the upper deck of the SOFIA for research as well as education and outreach activities. HIGH RESOLUTION PLANETARY IMAGING USING SMALL TELESCOPES AND DIGITAL VIDEO Planetary observing has traditionally been a major focus of amateur observing programs. Detecting fine planetary detail requires good seeing (low scintillation) and excellent optics. For this reason experienced visual observers are often able to detect and record subtle planetary detail during brief moments of good seeing. In the 1980’s advanced amateurs, began to obtaining high resolution images using fast photographic film and relatively large (~0.4m) amateur telescopes (Eicher and Troiani, 1988). However, routine high resolution planetary imaging using small telescopes only became feasible with the advent of small CCD cameras in the early 1990’s. The high sensitivity of modern CCD based cameras permits very short exposures of planetary images to be obtained through small telescopes (typically 0.2-0.3m aperture). Therefore, by taking a series of CCD images it is possible to capture sharp planetary images in brief periods of steady seeing. Furthermore, imaging is possible in the both the near infra-red and ultraviolet, since many CCD’s have reasonable sensitivity in the 350nm to 1000+nm range. The relatively high cost of dedicated astronomical CCD cameras limited the number of amateur astronomers who could exploit this technology for planetary imaging through most of the 1990’s. However, CCD arrays are often used in consumer electronic imaging devices, such as low light security video cameras and webcams, which have been successfully adapted as low cost imaging systems for amateur telescopes. Ron Dantowitz (1998) demonstrated that it was possible to obtain diffraction limited images of the planets, stars and even orbiting spacecraft using a low light video camera, capturing video DIGITAL VIDEO IMAGING WITH SMALL TELESCOPES 49 at 1/60s exposure, coupled to a small telescope. Dantowitz manually selected the sharpest frames from a video sequence, then aligned and combined them to increase the signal to noise ratio of the final image since individual frames were inherently noisy. Currently the most popular camera systems used for planetary imaging by amateurs are webcams (computer video conferencing cameras). Modern webcams provide high resolution color video input directly into a personal computer, permitting immediate processing of the raw images. Although webcams were first adapted to astroimaging several years ago (Buchanan, 1998), it wasn’t until the recent availability of fast personal computers with large disk storage space that this technology could be fully exploited. Fig. 1. High resolution planetary images obtained using a small Schmitt-Cassegrain telescopes (SCT) and modified webcams. Examples of planetary images that can be obtained with a small telescope and a webcam. All images were obtained by the author using a 235mm schmitt-cassegrain and a Philips ToUcam Pro webcam unless otherwise indicated. (A) Jupiter, imaged under excellent seeing conditions. Note the fine structure in the belts, including small discrete ovals. (B) Saturn. After stacking of raw frames the image was wavelet processed to give a natural image (upper) and a high contrast image (lower). In the A-ring the Encke division is visible near the ansae of the rings. The high contrast processing is useful for revealing fine structures, including a series of mid latitude white ovals which were tracked for several weeks by amateurs in the fall of 2003. (C) Mars just past opposition, the 25” disk reveals a wealth of fine detail. Note also the water ice clouds near the left and lower limbs which are well recorded in the blue channel of the camera (image by Jason P Hatton and Bob Haberman, 250mm SCT and ToUcam Pro). (D) Venus imaged at approximately 24h intervals using a UV filter (SAC-8 CCD camera). Cloud structures are revealed, which show significant changes between each observation. The movement of individual clouds have been tracked by observers at different longitudes imaging at intervals of 2 to 8 hours. A typical webcam, such as the Philips Toucam, consists of a 640x480 pixel array, overlaid with a bayer pattern of red, green and blue filters to give a 24bit color image (8 bits per color channel). Since the individual pixels are relatively small (5.6_m) it is possible to achieve a good image scale with a relatively short focal length for any given telescope, hence ensuring the planet image remains bright on the CCD array. Many amateurs use commercially available Schmitt-Cassegrain telescopes which generally have excellent optics and are well adapted for planetary imaging. To achieve a sufficiently large image scale to satisfy the Nyquist criterior (Grafton, 2004) for image sampling (ie. at least two CCD pixels for minimum angular size theoretically resolvable by the telescope optics) a telenegative lens is used to magnify the image. Good results are generally achieved with a 235mm Schmitt-Cassegrain at between f20 and f35. Video is usually acquired at 5-10 frames per second, with exposures varying from 1/5s to 1/50s. Typically, several hundred to a few thousand video frames are acquired for each final image. Video images of Jupiter were acquired by the author using a 235mm SCT working at ~f35 using a Philips ToUcam Pro. Approx 1200 24bit 640x480 color frames were acquired as video at 10fps, with individual exposures of 100ms. Registax software was used to select the sharpest frames, stack and process the final image. (A) Raw frames reveal some detail, but are relatively noisy and lack contrast. (B) Stacking 1154 of the sharpest frames reduces noise. (C) Wavelet processing was applied to the stacked image to reveal fine detail. Note the detail on Ganymede to the lower right of the planet.