Digital cameras have obviously become spectacularly popular in recent years.  One of the most common questions we get from new telescope buyers is whether they will be able to take pictures through a telescope with a digital camera.  Fortunately, the answer is yes, although there are certain limitations depending on the type of digital camera being used.

Types of Digital Cameras

Digital cameras come in two styles, digicams and DSLRs.  Digicams are the typical small point-and-shoot type digital cameras.  Digicams have built-in zoom lenses which are not removable, and have electronic viewfinders (EVFs) and LCD screens for viewing the image before it is taken.  DSLRs are Digital Single Lens Reflex cameras, which are similar to the older 35mm film SLRs in that they take interchangeable lenses and the image is seen in a optical viewfinder (as opposed to an EVF).

Digicams

Above:  A typical digicam, the Canon PowerShot SD850

It often seems that there are as many types of digicams as there are stars in the sky.  It's hard to give specific recommendations on digicams because they change so rapidly, but there are some general considerations.

Digicams have non-interchangeable lenses.  Typically the built-in lens is a zoom, usually described with a magnification range (such as 3x or 8x).  The lens has a focal length range which is usually quoted in terms of 35mm equivalent.  This allows previous users of 35mm film cameras to correlate the magnification range of a digital camera to that of a 35mm SLR.  For example, the camera pictured above has a 7x zoom lens with a focal length range of 28-200mm in terms of 35mm equivalent.  However, the true focal length of the lens is 7.2-50.8mm.  This means that there is a crop factor associated with the camera.  Crop factor is often mistakenly called magnification factor, which is a less-appropriate term (as is discussed in more detail below).

The fact that the chip in the camera is much smaller than the size of a 35mm film frame means that the lens acts like a longer lens (in terms of field of view).  The crop factor of the Canon Pro1 is 3.9x.  The Canon PowerShot Pro1 has a fairly large chip (for a digicam) so its crop factor is lower than that for many cameras.  For example, Canon's PowerShot G6 camera has a crop factor of 4.9x and the Canon PowerShot S1 IS has a crop factor of 6.5x.

The importance of the crop factor for astronomical imaging has to do with the pixel size of the CCD chip in the camera.  As more and more megapixels are being crammed into digital camera chips, the size of the pixels is shrinking.  This has important consequences for astronomical imaging.  All things being equal, smaller pixels are noisier than large pixels.  This means that smaller chips tend to generate more noise than larger ones.  The PowerShot Pro1 has an 8 megapixel sensor.  This means the pixels are 2.8 microns in size.  (Compare this to the DSLRs discussed below.)  This fact makes itself evident in the high noise level that is associated with using a digicam at a high ISO.  ISO is the equivalent of film speed, and increasing the ISO increases the sensitivity of the camera.  For astronomical imaging, using a high ISO is necessary to generate the high sensitivity needed to capture faint objects.  But digicams generate much inherent electronic noise at high ISOs and therefore are not well suited to long-exposure astronomical imaging.

Another limiting factor for digicams is the non-removable lens.  With the lens attached to the camera, the only way to take an image through a telescope is afocally.  This means that the camera must be attached to an eyepiece to record an image.  This has the effect of increasing the magnification of the system and decreasing the field of view.  Since most celestial objects are large, a small field is a limiting factor.  More importantly, the focal ratio is significantly increased.  This makes the optical system very slow, which makes recording faint objects extremely difficult.

For these reasons, digicams are poorly suited to deep sky imaging.  However, they are well-suited to imaging bright, small targets like the planets.

Digital SLRs

Above:  Typical DSLR, the Canon EOS 20D

Digital SLRs use a single-lens reflex system.  This means there is a mirror which diverts the image from the camera lens to an optical viewfinder.  This allows you to see exactly what the camera lens sees.  The mirror flips up and out of the way when an exposure is taken.  DLSRs have two big advantages over digicams when it comes to deep-sky imaging.  First, they use larger CCD (or CMOS) chips, and they have removable lenses.

Most digital SLRs also have a crop factor.  Again this is because they have chips which are smaller than a standard 35mm film frame.  A 35mm frame is 36x24mm.  There are a couple DSLRs with full-frame sensors, notably the Canon 1Ds Mark II.  Full-frame DSLRs have sensors that are 36x24mm in size.  There is no crop factor associated with such a camera.  A 24mm lens on a full-frame DSLR acts just like a 24mm lens would on a 35mm film SLR.  Most consumer DSLRs have a crop factor of 1.5x or 1.6x.  A 1.6x crop factor means that a 24mm lens acts like a 38.4mm lens--in other words, it gives a smaller field of view than the same lens on a 35mm film camera.

Still, a 1.6x crop factor is much less than the 4x or 5x factor associated with most digicams.  Thus, the pixels in a DSLR tend to be much larger than they are in a digicam.  A perfect comparison are the two cameras pictured on this page.  Both have 8 megapixel sensors.  But one has a 3.9x crop factor and the other has a 1.6x crop factor.  The PowerShot Pro1 has 2.8-micron pixels.  The EOS 20D has 6.4-micron pixels.  Therefore the DSLR has much less noise than the digicam.  Also, DSLRs tend to incorporate sophisticated noise-reduction algorithms.  They are much better suited to deep sky imaging for this reason.  They can be used at higher ISOs without significant noise.

The other factor that makes DSLRs so much better suited to deep sky imaging is the fact that they have removable lenses.  For one, this means that the lenses themselves give you much more selection in terms of magnification range and focal ratio (or f-stop as it is more commonly known to photographers).  But more importantly, the lens of the camera can be removed and the body attached directly to the telescope.  This in effect makes the telescope into the camera's lens and is referred to as prime focus imaging.  This results in a wider field of view (if desired) and a much faster focal ratio.

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 DSLR Focus or ImagesPlus 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 Digital Cameras for Astroimaging

Below is a table showing the advantages and disadvantages of using digicams, DSLRs, and astronomical CCD cameras for imaging the heavens.  For more details, see the section on CCD Imaging vs. Digital Camera Astrophotography.

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