Digital camera astrophotography

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They are unsuitable for planetary photography, or imaging of galaxies other than the very closest, except when mounted to very large telescopes.A digital camera has several practical advantages over specialized astronomical CCD cameras, including price, field of view, and of course terrestrial use. On 15 March 2007, using such a camera for a wide field comet survey, Australian astronomer Terry Lovejoy discovered a new comet.[1]Contents1 Methodology 2 Procedure 3 Camera Settings 4 Vibration Issues 5 Dust Issues 6 Post-processing 7 Alternative Methods 8 Image gallery 9 External links 10 References and footnotes // MethodologyThe most important decision is optical tube selection. Since digital cameras are not cooled, the practical upper exposure limit is in the 2-minute range before thermal noise becomes apparent. Therefore, a "faster" optical tube is desired. The best results seem to be obtained with a focal reducer on a schmidt-cassegrain type tube, or a telephoto lens with maximum aperture of f/2.8 or better.The mid-range consumer cameras are actually the best choice for astrophotography. These have smaller CMOS sensors than the high-end cameras, giving a higher effective apparent magnification. Most recent models offer settings for noise reduction. One manufacturer offers a D-SLR specific to digital astrophotography.[2]The camera must be mounted to a clock-driven equatorial mount, otherwise the images will trail into long streaks due to the motion of the Earth. If the focal length is quite short (less than 250 mm in eq. 24x36), one can use a simple barn door tracker. There are several methods of mounting cameras:Mounting a camera directly at the focal plane of a clock-driven telescope, with an adapter specific to the camera mount. Mounting a camera and (small) lens "piggyback" to a clock-driven telescope, using a bracket made for the purpose. Mating a camera and lens combination to a clock driven equatorial mount, usually by fabricating a mounting plate. Altazimuth mounts, even if motorized or computerized, are unsuitable for photography due to field rotation. On the other hand, it is easily possible to adapt the mid-range equatorial "go-to" mounts to digital camera use.ProcedureAnother important issue is focusing. The camera's ground-glass viewfinder is fine in daylight, but difficult to focus low-light subjects on. A knife-edge device is often used to obtain focus on a reasonably bright star. If the camera is mounted to an autofocus lens, focus can be obtained on a bright planet or even a star. However, it may take several tries to get sharp focus, as lenses intended for terrestrial use are built to focus with great speed, as opposed to great precision.Camera SettingsAn ISO of 1600 and exposure of 30 seconds at f/2.8 to f/4 is a good starting point for most deep sky objects. Most D-SLRs have a 30-second setting in the Manual mode. The camera's white balance should default to a solar or daylight setting in darkness, if the colors look peculiar, set the white balance to solar. If the camera has a Custom Setting for long-exposure noise reduction, enable it. Lossless image formats should be used, as important image information can be lost by jpeg compression.Vibration IssuesAny vibration will ruin a long-exposure photograph. Several methods exist to reduce or eliminate vibration.The "hat trick" - Obstructing the lens aperture for the first few seconds of the exposure with a hat, to allow vibrations to die down A cable release, which reduces the vibration transmitted from pressing the shutter The camera's self-timer feature, which gives a delay of 8-10 seconds from shutter actuation to exposure Infra-red remote to activate the camera's shutter Mirror lock-up to allow vibration from the mirror's movement inside the camera to settle before the exposure is made Computer tethering, or operating the camera from a laptop or even over the Internet. Dust IssuesDigital SLRs mounted directly to optical tubes will eventually encounter problems with dust accumulation on the filter placed in front of the detector. This is especially so when opened directly to the atmosphere for minutes at a time as with some reflecting telescopes. Dust appears as dark spots in the image and...(and so on)


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