Astrophotography with the Nikon D100 Digital Single Lens Reflex Camera
By Tim Hunter
This essay was originally written in November 2004. It examines the Nikon D100 digital single lens reflex (DSLR) camera and its suitability for astrophotography. The D100 was one of the first DSLR cameras that could emulate most of the features of an advanced film single lens reflex camera. It has since been superseded by many digital models from Nikon, Canon, and other manufacturers. The Canon 20Da was expressly designed for astrophotography, and it is being used for wide field photography at the Grasslands and 3towers Observatories.
The Canon 20Da is no longer on the market and has been superseded by the Canon 60Da camera. There are several other advanced DSLR models from Canon and Nikon that have features potentially favorable for astrophotography, mainly large chip size and low noise. It is also to possible to have one of these cameras altered by second party vendors to make them more sensitive to red light. The main limitation of the current everyday DSLR cameras is their relatively poor sensitivity to red light, limiting their usefulness for imaging emission nebulae. This limitation is not a problem for ordinary daytime photography or for low light level photography, such as nighttime sporting events.
I. The Nikon D100 Digital Single Lens Reflex (DSLR) Camera
The D100 digital single lens reflex camera (DSLR, or digital SLR) was introduced by Nikon on February 21, 2002 (Nikon, 2002). It is a compact single lens reflex digital camera designed to resemble the body of a regular Nikon SLR film camera. No lenses are supplied with the camera. It will use any lens with standard Nikon mounts and is fitted with a standard cable shutter-release button. The D100 contains a 6.1 megapixel high definition 3008 x 2000 pixel 23.7 x 15.6 mm RGB CCD chip. Each pixel is approximately 8 microns in size. Even though the CCD chip in the camera is large by amateur astronomy standards, it is smaller than an equivalent 35 mm film frame, and the focal length of any lens attached to the camera is increased by a factor of 1.5. Thus, a 20 mm focal length lens attached to the D100 has a magnification and field of view equivalent to that of a 30 mm focal length lens. The chip is overlaid with a grid of color filters, and it conveniently produces color images in a single shot.
Simple digital cameras have been on the market for many years, and they have gained much popularity due to their easy use, excellent results, and simple operation without the necessity to purchase film or have film developed. Digital SLR cameras are a further extension of the simpler digital cameras that have been marketed by many manufacturers, including Nikon, Canon, Olympus, Fuji, Pentax, Sigma, Sony, Minolta, Kodak, and others.
Single lens reflex cameras use an internal flip mirror to enable the photographer to view and focus on the desired scene through the same lens used to take the photograph or digital image. SLR film cameras gave way to DSLR cameras, because the chips in the digital cameras are considerably more sensitive to light, and they produce digital files instead of slides or prints. These files allow easy computer processing and enhancement.
The D100 contains a multitude of features for the advanced photographer. Most of these features apply to high light level imaging, such as outdoor daylight photography or indoor flashbulb photography. Many of these features will be of no interest for an amateur astronomer using this camera for astrophotography. At least they weren't for me. Of special interest for the astrophotographer is the wide range of exposures and ISO values available with the camera. Its exposures range from 1/4000 sec to 1, 2, …10, 20, 30 seconds, and bulb. Its ISO values can be set from ISO 200 to ISO 6400. It supports a number of file format types including uncompressed TIFF, compressed JPEG, and uncompressed RAW files.
The Nikon D100 camera described herein was purchased in late 2002 and contains a 512 MB CompactFlash (CF) Card for image storage. A second battery was purchased for the camera so that a fresh battery power would always available. The D100 uses a rechargeable Li-ion Battery Pack (7.2V DC) that will give hundreds of exposures under normal daylight working conditions and temperatures. Because much astrophotography is performed at night in cold conditions, the battery may not last an entire evening and has to be replaced after 4-6 hours of continuous use. Fortunately, it can be recharged in only 2 hours. For processing, the camera image data are easily downloaded to a computer via a supplied USB 1.1 compatible connector.
I own two observatories. The 3towers Observatory is my home observatory located on the north side of Tucson, Arizona. My other observatory, the Grasslands Observatory, is located at a very dark site 60 miles southeast of Tucson. It contains a 24-inch f/5 reflector. The Nikon D100 camera images shown in the figures for this essay were mainly obtained at my home in Tucson or at the Grasslands Observatory.
For maximum functionality, a high quality SLR digital camera should be purchased with a backup battery pack and as large an image storage chip as possible. In the case of the Nikon D100, this was all purchased for ~ US $2000. These cameras are very popular with advanced amateur photographers and with professional photographers. While they are still expensive, they are very versatile, and their price is dropping. The Nikon D100 is now a dinosaur and outmoted compared to more modern DSLRs, but it is tough and still works fine. In any event, most of the conclusions reached about the Nikon D100 herein are applicable to more modern digital SLR cameras.
II. DSLR Astrophotography with a Tripod
Astrophotography defies common sense, because for much less money and effort you can enjoy better results in books, popular magazines, and on the web. Astrophotography is also quite challenging. It requires time, effort, and considerable luck to produce a good picture. Black and white astrophotography with a tripod has been performed for the past one hundred fifty years. Color photography is a more recent phenomenon with successful professional color astrophotography being introduced in 1959 (Miller, 1959; Carpenter, 1959).
Since then, tripod based color astrophotography has become very popular. Wally Palcholka, for example, has published multiple tripod color astrophotographs in leading periodicals, and his images of Hale-Bopp and Mars taken with simple tripod SLR techniques have won Picture of the Year Awards from Time and Life (Palcholka, 2004). Fast color films permitted spectacular shots of the night sky with a simple SLR camera, a tripod, and time exposures of seconds to hours. Short exposures produced round, point-like stars, while longer exposures gave star trails.
A one-minute exposure on ISO 400 color film and a wide-angle lens gives an excellent color view of the night sky (figure 1):
Figure 1. January 1984. All the Planets and the Moon were visible in the same quadrant of the sky. Sixty-second exposure with a 17mm f/4 lens on Ektachrome 400 film. T Hunter.
Film astrophotography had several distinct disadvantages compared with digital camera techniques. Film had to be purchased, stored, and then developed after the exposures were made. There was inevitably a long lead time between the exposure and the result.
Film itself was and is expensive, development and printing are expensive, and there is an expense and effort required to convert film images into digital data for computer processing. Many times, I spent all night doing astrophotography only to be greatly disappointed with the results the next day after obtaining prints from the local photo store. The images were out of focus, there was star trailing when none was intended, the sky had an ugly green or reddish glow, or the object of interest was only partially on the frame. I once spent an entire evening trying to photograph Comet Halley near the Pleiades only to find out the next day the film had not wound in the camera (Hunter, 1986).
DSLR 35 mm cameras offer all the advantages of a 35 mm SLR camera with none of the drawbacks of film and many, but not all, of the advantages of digital imaging. Their imaging chips usually have 6 or more megapixels. High resolution images are possible with any medium as long as the size of the resolution elements (pixels in the case of imaging chips and grain in the case of film) are supported by excellent optics, accurate focusing, perfect tracking, freedom from vibration, and excellent seeing (Covington, 1999):
Medium Dots per inch (dpi) Pixels per mm
TV screen 25-75 1-3
Computer Screen 70-100 3-4
Photographic Print 150 6
Sharp Print 300 12
Negative/slide 2500 100
Nikon D100 chip 127
The above table shows the resolution elements of the Nikon D100 digital camera compare favorably with those of film, except the chip is smaller than a 35 mm frame. If we assume that a standard quality color print has a spatial resolution of 300 dots per inch (dpi) or better, then the 3000 x 2000 image scale for the Nikon D100 should be able to produce acceptable prints with an image scale of 10 x 7 inches. In reality, some astronomical images may be enlarged considerably beyond this up to 24 to 30 inches square depending on their composition. If there is a homogenous dark background with foreground stars or nebulosity, considerable image enlargement may be performed with no resulting subjective loss of detail.
Ordinary daylight scenes generally will not stand such enlargement, but it is fair to say that even the early high quality DSLR cameras were widely used for portrait and landscape scene enlargements on the order of 8 x 10 inches and larger (Schedler, 2004).
 Table adapted from Covington (1999, page 219)