Astrophotography
The Astrophotography Section supports the imaging of astronomical objects using film, consumer digital cameras, cooled CCD cameras and webcams or similar. The objective of the section is to provide education, support and encouragement to those interested in this rewarding but potentially demanding pursuit.
An introductory article on astrophotography is provided below. For further information contact the Astrophotography Section Director. Also visit our Members' Gallery to view astrophotographs taken by AAQ members. There are many excellent websites on astrophotography on the internet. Some links to a selection of the external websites are provided below.
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Sections -
Astrophotography
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Written by Max Kilmister
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Astrophotography is a demanding but rewarding pursuit – demanding because to achieve excellence requires a number of factors to all be at the highest level, and rewarding for the satisfaction of having achieved success in a difficult field. It is also rewarding because a permanent record is created which can be shown to others, often of something not directly visible except maybe as a grey smudge.
The entry level into this pursuit need not be high if one is happy to photograph the Moon and the brightest planets with a digital camera - either the cheap fixed lens type or a digital SLR. Such objects should not be decried – there are some renowned amateur astrophotographers who photograph little else, even travelling to another continent to seek excellent “seeing” for high resolution photographs, albeit with specialised equipment. Although not ideal, a Dobsonian telescope can be used for lunar photography, as it is very stable and virtually vibration-free.
Apart from a telescope, a stable mount and a camera, all that is required is a camera adaptor. Both digital SLR and fixed-lens cameras are suitable for lunar and planetary photography. Suitable adaptors to connect the camera to the telescope are available commercially. (Check out advertisements in Sky & Telescope or on the Internet.) Because the CCD or CMOS chips in digital cameras are very efficient at collecting photons, exposures with such cameras will be short – in fact the challenge could well be reducing the brightness.
Camera adaptors are available that allow eyepiece projection – that is, using a normal eyepiece to magnify the image at the focal plane. Without magnification, images of planets are too small to be worthwhile. However, because magnification effectively increases the focal ratio, there are limits to what can be achieved without a driven mount. For example, a photo of Jupiter or a part of the Moon at f64 and with the camera set at ISO 400 would require an exposure of about ¼ of a second, but Saturn about 1 second, possibly resulting in a lack of sharpness due to trailing of the image. A very fast ISO rating (eg, 1600) will allow a reasonably untrailed image to be achieved, but the image could be "noisy".
The correct exposure can be achieved by trial and error or from recommendations in various publications that relate object, focal ratio, ISO rating and exposure time. However, it is not a simple matter to establish the focal ratio of an eyepiece projection setup. There is a formula that can be applied, but this needs knowledge of dimensions of not-easily-established elements of the optical system. Another way is to use photography as follows. First photograph the moon at prime focus. (It is assumed that the focal ratio of the telescope is known or can be readily established.) Next, photograph an area of the moon with eyepiece projection using each eyepiece in turn and print all images at the same scale. By measuring the distance between the same craters on the projected images, the focal ratios of the eyepiece projection setups can be calculated, as image size is proportional to focal ratio. Whether or not one knows the focal ratio for certain, it is still advisable to bracket exposures, as most subjects are not uniformly illuminated.
Another avenue to pursue – one not needing a telescope – is long exposure wide field photography using a tracking platform. The construction of such a device has been well described in numerous publications (eg Astronomy, September 1984) and on Internet sites and will not be attempted here. With careful use, good wide-angle photos of constellations using normal 35 to 105 mm lenses can be achieved with exposures of 5 to 10 minutes if the camera allows such exposures to be set. (This is usually the case with SLR cameras but not with fixed lens types.) |
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Last Updated on Tuesday, 02 February 2010 07:32 |
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Sections -
Astrophotography
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Written by Max Kilmister
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Filters are widely used in digital astrophotography, particularly with monochrome cameras where images taken in turn through red, green and blue filters allow a colour image to be created in a computer using appropriate software. In this case, the filters are mounted in a filter wheel, which is often motorised and controlled via the computer running the imaging software. For photography using a colour camera, a broad band light rejection filter such as Lumicon’s Deep Sky Filter will reduce sky glow from natural and artificial sources. However, most image processing software will allow the subtraction of light pollution effects so that filters of this type are not as helpful as they were in the days of film photography, and even light pollution gradients can be handled with appropriate software.
A side issue here relates to consumer digital cameras. These have a filter in front of the ccd or cmos detector that stops most of the light from H-alpha emission from being collected. It is possible to remove this filter and, at the time of writing, a Canon SLR camera with the filter removed is available from Hutech in the USA. The modification can also be done in Australia. With an appropriate white point setting, a modified camera can also be used as a normal daylight camera.
All digital cameras used in astrophotography need filtering to cut the amount of UV and IR wavelengths reaching the detector. As the human eye does not respond to these wavelengths, but CCD and CMOS chips do, images that include UV and IR do not reproduce what the eye can, or does, see.
The other use of filters is in narrow-band imaging with a monochrome camera, using filters that let through only a narrow range of wavelengths, usually OIII, H-alpha and SII. This type of imaging has become popular with some astrophotographers who live in light polluted areas, where the filters do not allow the light pollution through. This is an advanced topic and will not be covered here. |
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Last Updated on Tuesday, 02 February 2010 07:46 |
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Sections -
Astrophotography
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Written by Max Kilmister
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After experimenting with astrophotography with a fixed camera, either with just the camera itself using a fast ISO setting to allow short exposures, or using the camera body attached to a telescope to take photos of the Moon at prime focus or using eyepiece projection, a sensible progression would be to try piggy-back photography. This is the term applied when an equatorially mounted telescope is used to guide a camera mounted on the same equatorial platform. Camera lenses commonly used vary from 28 mm for wide-angle shots of the Milky Way to 300 mm for telephoto shots of large nebulae such as the North America nebula (NGC 7000) and the Eta Carinae nebula (NGC 3372). The basic requirements for piggy-back photography are: - a digital SLR camera with a bulb (B) setting;
- various lenses to provide for a range of subjects (eg 50, 135, 300 mm) or a zoom lens (zoom lenses are usually of lesser optical quality);
- an equatorial mount with a motor drive on at least the R.A. axis; and
- brackets to rigidly hold the camera and lens.
An autoguider is an optional but very useful addition - it will guide much more accurately than is possible manually and it removes the tedium of guiding. |
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Last Updated on Friday, 10 October 2008 16:12 |
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Sections -
Astrophotography
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Written by Max Kilmister
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In prime focus astrophotography, the telescope is used as a large telephoto lens. Prerequisites for prime-focus astrophotography are:
- a telescope on a motor-driven equatorial mount;
- a guidescope , an off-axis guider, or a self-guiding CCD camera;
- an illuminated reticle guiding eyepiece or an autoguider (unless using a self-guiding camera);
- a single lens reflex camera or a cooled CCD camera; and
- all necessary mounting devices to rigidly connect all of the components together.
If using a digital SLR camera, it is recommended that the camera be powered from a large external battery to avoid the internal batteries dying during an exposure.
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Last Updated on Tuesday, 02 February 2010 08:07 |
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Sections -
Astrophotography
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Written by Brent Joyce
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Introduction
In the last few years imaging planets has become popular using commercially available web cameras. Philips released the popular ToUcam 740k a few years ago. A little over a year ago the ToUcam 840k was released which offered superior low light performance.
Webcam imaging requires the acquisition of a movie sequence of a planet, which may contain hundreds or, thousands of frames. Webcam frames are of short duration and subsequently little detail is acquired in a single frame. A movie sequence is imported into a program such as Registax, which decomposes the movie into its individual frames. These frames are aligned, optimised and stacked by the Registax software. (Other image processing software such as AstroStack may be used.) The stacking process yields a frame which has a vastly improved signal to noise ratio over that of any individual frame in the movie.
With the success of the ToUcam and other similar units in the hands of amateur astronomers, Meade soon launched the LPI (Lunar Planetary Imager) to be followed some months later by Celestron’s NexImage system. This article will describe the use of the Philips’ToUcam and Registax image processing software.
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Last Updated on Wednesday, 11 March 2009 10:06 |
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