Digital camera astrophotography

From Wikipedia, the free encyclopedia

This article has now been successfully imported to Wikibooks. You can find it on Wikibooks under the name Digital camera astrophotography.
If this page can be rewritten into an encyclopedic article, please do so and remove this message and/or add a link to the Wikibook using {{wikibooks}}.

Comet Pojmanski taken with a digital SLR and telephoto lens.

Digital camera astrophotography is astrophotography using common digital cameras mounted on clock-driven telescopes. The cameras are typically medium to high-range D-SLRs with remote control, long-exposure capability and high-ISO settings. Due to the wide field of the cameras' sensors, they are best suited for photography of deep-space objects such as nebulae, or transitory celestial phenomena such as comets or supernovae. 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]

1 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

[edit] Methodology

The 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. 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.

[edit] Procedure

Another 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.

[edit] Camera Settings

An 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.

[edit] Vibration Issues

Any 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.

[edit] Dust Issues

Digital 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 if it is to be removed must be performed carefully to avoid damaging the filter.

[edit] Post-processing

Once the images are stored, any number of post-processing operations may be performed on them. In astrophotography, much liberty is taken with the enhancement of images, including noise reduction (by "stacking" several similar images), and contrast and saturation enhancement. Several software packages exist specifically for this purpose.

[edit] Alternative Methods

The method of taking digital snaps by holding a small camera up to an eyepiece is known as afocal photography. It is possible to take acceptable images of the Moon and brighter planets using this method. Devices exist to clamp the smaller digital cameras to the eyepiece to allow longer exposures in afocal photography.