F. Miscellaneous Considerations

The Nikon D100 and many other digital cameras allow for video operation.  This is not useful for astrophotography, because inexpensive web cams can perform the same function far better in most cases (Jones, 2004).  However, multiple exposures from a digital camera can be put together to form an animated image.  An example of such an image may be found at  http://www.3towers.com/ObservatoryImages/LeonidsA.gif.  This shows an animated view of Leonid meteors from the 2002 meteor shower.  

While the current generation of digital SLR cameras can not be used for photometry, the Nikon D100 shows a generally linear response with short exposures.  A series of exposures of the region around Altair was taken with a 135 mm f/3.5 lens piggybacked onto a 12 inch Meade LX 200 telescope to prevent star trailing.  The exposures were 1, 2, 4, 8, 15, 20, 30, 60, and 120 seconds, ISO 1000, and they were repeated twice.  The Nikon noise reduction routine was used for each image.  The software program Mira Pro 7 (Axiom Research) was used to analyze each image, and the median pixel count for each image was plotted versus the exposure length (figure 34):

Figure 34.  Plot of median pixel count versus exposure length for a series of 135 mm f/3.5 lens exposures centered on Altair.  The evening temperature was 25⁰ Celsius.  The Nikon D100 camera was piggybacked onto a Meade 12 inch LX 200 telescope to prevent star trailing.     

For exposures somewhat under 60 seconds the median pixel count is linearly related to the exposure length.  Multiple images with the same exposure length showed median variations of 1-2 counts between images, depending on the exposure length.  For all the images, the mean and median pixel counts for the individual exposures were within 1-2 counts of each other.  These results are limited and can not be generalized, because they will vary with changes in ambient air temperature and sky conditions, but they do indicate a general stability and reproducibility of this camera for exposures from 1-60 seconds. 

Special shutter release equipment is available for the Canon and Nikon DSLR cameras for programming a series of multiple exposures (Aguirre, 2001; Hanson, 2004; Nikon, 2002).  This allows for an automatic operation of the camera and for direct connection of the digital camera to a computer.  Computer control of the camera then permits a more remote control of the camera, and it also permits use of software packages that help with camera focusing.  I personally have not had trouble with manual focusing for most images, because the Nikon's view finder gives a sufficiently bright image for focusing.  Tripod mounted shots with a simple camera lens work well if the lens is set at infinity.  Focusing of the camera for objects viewed through a telescope is more difficult, especially if they are faint and do not contain easily recognizable point sources.  Schedler (2004) recommends DSLR Focus for Canon digital cameras.

VII.  Conclusions

Table I – What General DSLR Cameras Can and Can Not Do Well (Astronomically)

Can Do Well

Daylight Photography – observatories, observing sites…

Enlargements – 10 x 12 inches; 14 x 17 inches

Tripod Photography – short exposures up to a few minutes

Piggyback Photography – short exposures up to a few minutes

Lunar, Solar, Planetary Photography – {web cams are probably better}

Prime Focus Telescope Imaging – reasonable substitute for more expensive astronomical CCD cameras 

Can Not Do Well

Enlargements beyond 14 x 17 inches

Repeated exposures in rapid succession – gradual image deterioration

Long exposures in cold temperatures – rapid battery failure

Multi-hour star trail exposures

Prime Focus Telescope Imaging for faint objects – most galaxies and nebulae are beyond easy reach of digital cameras

Can NOT match the speed and efficiency of standard astronomical CCD camera systems

First written 11 November 2004

Edited and Posted 27 September 2009

Further revision 15 June 2014

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References 

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Carpenter EF, Jepperson R. Color photography of faint objects with a special fast film.  AJ 1959; 64: 49-50.

Covington, MA. Astrophotography for the Amateur.  Cambridge University Press, 1999, Cambridge, pages 93, 219.

Hanson M, Greiner RA.  Equipment review.  Canon 10D digital camera.  Astronomy 2004; September: 84-87.

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