Digital cameras, digital video cameras, and webcams all use CCD chips (or similar CMOS chips), just like an astronomical CCD camera.  They are also incredibly popular, with millions being sold every year.  So the obvious question is whether these types of imaging devices can be used for astronomical imaging as well.  Fortunately, the answer is yes, although they have certain limitations as well as certain advantages.

Digital Cameras

There are two basic types of digital cameras.  There are the point-and-shoot type, called digicams, and there are the digital single lens reflex type, or DSLR.  The essential difference is that digicams do not have removable lenses while DSLRs do.  This limits the type of astrophotography that can be done with digicams.

Digicams

Above:  Typical digicam, the Canon PowerShot SD850

Since digicams do not have removable lenses, images must be taken afocally, that is, through both the camera lens and the telescope eyepiece.  This increases the focal ratio and reduces the speed of the imaging system.  This is exactly the opposite of what is desired for deep-sky imaging -- the exposures become prohibitively long.  However, it is ideal for planetary imaging, as the planets are small and bright.  This makes digicams well suited to solar system imaging.

Another reason digicams do not work well for deep-sky imaging has to do with the type of CCD chip used in these cameras.  Because of the very small size of digicams, the CCD used is small.  With the large number of pixels used in modern digital cameras, this means that the individual photosites on the CCD are very small.  The smaller the photosite, the more electronic noise is inherent in the chip.  Noisy CCDs are unsuited to imaging faint deep sky objects.  Also, most digicams have limited exposure durations of about 30 seconds, also a bit short for most deep sky applications.

Digital SLRs

Above:  Typical DSLR, the Canon EOS 20D

DSLRs are particularly well suited to deep-sky imaging and have become very popular.  While not nearly as sensitive as a true astronomical CCD, DSLRs have the advantage of cost and versatility -- they can be used equally well for regular photography.  Imaging with a DSLR is very much like film astrophotography, except that the images are digital and immediately available for viewing and processing.  Also, DSLRs tend to be more sensitive than film, although not nearly as sensitive as an astronomical CCD.  Some DSLRs have been produced that are specialized for astrophotography, such as the Canon EOS 20Da.  These cameras are closer to CCDs in sensitivity and provide a large-format sensor for less money than a comparable CCD.

The removable lens of a DSLR makes it ideal for attaching to a telescope for deep-sky imaging.  Fast focal ratios are possible with DSLRs, unlike digicams.  Also, the interchangeable lenses themselves are ideal for imaging as well.  Wide-angle lenses can be used for images of constellations or the Milky Way, and fast telephoto lenses are available for imaging larger celestial targets such as the Andromeda Galaxy or Pleiades star cluster.

Above:  Image of the Andromeda Galaxy with a Canon EOS 20Da.  Three 5-minute exposures through a 180mm f/2.8 astrograph.  Image by James McGaha.

Since a digital camera stores data to a removable memory card, and since the images may be reviewed using the camera itself, it is possible to use the camera without a computer.  Astronomical CCD cameras require the use of a computer to take and store and view the images.  However, it is highly recommended to use a computer to control a DSLR for astronomical imaging, in the same manner that it would be used for a CCD camera.  The primary advantage is that focusing using only the camera can be very difficult; focusing using a computer program such as MaxDSLR or Nebulosity to control the camera is vastly easier and more precise.  Also, automatic sequences of images can be taken using the computer software and then stored directly to the hard drive.

Pros and Cons of DSLRs

Webcams

Webcams are ideal systems for imaging planets.  Why?

The most important factor in imaging the planets is seeing conditions, or the steadiness of the atmosphere.  You can have the best telescope in the world, but if the seeing is poor, all those optics won't do you any good.  However, the atmosphere is constantly changing.  Anyone who has done much planetary viewing at high magnifications knows that you have to watch and wait for moments of steady seeing to catch the finest details. 

The same is true for imaging.  If you try to take a still image (with a CCD or digital camera) of a planet, odds are the atmosphere will blur the picture.  If you take enough pictures, eventually you will end up with a sharp image.  As with most astronomical imaging, stacking multiple frames helps reduce noise and increase detail.  So you might want to stack many planetary images to get the most detail.  Now, if only 1 image in 5 is sufficiently sharp, and you decide you want to stack 100 frames, you will have to take 500 images.  This is a nightmare using a CCD or digital camera.

Enter the webcam.  Webcams capture video clips, which are typically taken at a rate of 10 to 30 frames per second.  This means that in just under a minute you could capture thousands of images.  Then, using software such as Registax, the computer can analyze the images, reject the blurry ones, stack the sharp ones, and enhance the image to give incredible detail.  A $10,000 CCD camera can't give better planetary images than a $150 webcam!

Above:  On the left, a raw frame from a webcam video of Mars.  On the right, the processed image, the result of stacking the best 392 images out of 1016.

Be sure to visit the Webcam page for more details on capturing images with webcams.

Video Cameras

Video systems offer the possibility of real-time viewing of objects on a computer screen or TV monitor.  While most video systems are not anywhere near as sensitive as CCD cameras, some offer features to allow viewing of deep-sky objects.  By hooking a video camera to a computer, images can be captured and stacked as with a CCD camera, allowing detailed still images of deep-sky objects to be captured.

In general, black and white video cameras are more sensitive than color cameras, so for deep-sky viewing or imaging, black and white cameras are by far the most popular.  Low-light security cameras can be used with very fast optical systems (such as SCTs with f/3.3 focal reducers, or even better with the f/1.8 HyperStar system) to view deep-sky objects in real time.  Systems such as the Astrovid StellaCam allow images to be stacked and displayed without the use of a computer.  Using a computer with these systems further enhances their capabilities by allowing images to be combined and enhanced just like a CCD image.

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