Film, CCD and DSLR Cameras

Silicon-based sensors in CCD and CMOS cameras have great advantages over silver-based film emulsions for astrophotography. They are more sensitive, their quantum efficiency is much higher, and their response is linear. They also have a greater dynamic range and higher resolution than film.

Digital cameras are undergoing constant and substantial development and improvement specifically aimed at areas that impact astrophotography, like improved noise performance. Digital is the technology of the future, whereas film products, like Kodak's Technical Pan film, probably the best film ever made for astrophotography, have been discontinued.

Solid state sensors also do not suffer from the reciprocity failure that plagued most films. Reciprocity failure means that the film's sensitivity to light decreases at low light intensities, such as with deep-sky astrophotography. To reduce this problem, film had to be hypersensitized. This was accomplished in a complicated procedure where the film was baked for hours at a high temperature in a hydrogen-nitrogen gas mixture, and stored in a freezer before use.

Another great advantage with digital images is that they can be examined immediately. Problems like focus and tracking can be discovered at the telescope, instead of days later when the film is developed. These problems can then be fixed before the next exposure is taken.

Because short exposures can be combined to equal longer ones, only a small fraction of exposures are lost to problems like airplanes flying through the frame, instead of the entire exposure time of a single long exposure. Short exposures also lessen tracking problems.

The initial cost of a DSLR camera compared to a film camera is more, but there are no film or development costs. For an ardent astrophotographer, over the course of a year or two, these savings will pay for the additional up-front cost of a DSLR.

For long-exposure deep-sky astrophotography, film definitely has had its day, producing wonderful images. But these days film manufacturers have reduced the red sensitivity of most films, making them extremely poor at recording the wonderful red colors of emission nebulae.

DSLR cameras also don't suffer from insidious problems like "film creep", where stars would trail and even become out of focus when the film's emulsion would expand and move during exposures from absorption of moisture under humid conditions. Film also lost sensitivity under these conditions. To solve this problem, serious film astrophotographers had to flow nitrogen into the camera during the exposure, a non-trivial undertaking.

Film, on the other hand, does have some advantages. Film allows a photographer to get into imaging at an extremely low cost, perhaps only a couple of hundred dollars for a manual camera body, and $10 for a roll of film, and $10 for development. The size of the sensor area is larger than most DSLR sensors, allowing a comparatively wider field of view at a given focal length. Once scanned, a 35mm film frame has about as much resolution as a 10 - 15 megapixel digital sensor.

Because digital cameras have higher resolution than film and a wider spectral sensitivity, they are also more critical in revealing errors in things like mount tracking and color and other optical aberrations in telescopes and camera lenses.

DSLR vs. Dedicated Astronomical CCD

DSLR cameras are less expensive for the same sized sensor, and much easier to use than comparable dedicated astronomical CCD cameras.

Monochrome astronomical CCDs on the other hand, are more sensitive with better quantum efficiency than DSLR sensors, but require three sets of exposures through individual red, green and blue filters to create a color image. Astronomical CCD cameras also require a computer, and associated increased power requirements for both the camera and computer for use in the field at a remote observing location.

Astronomical CCD cameras use 16-bit A/D converters and divide the dynamic range into a larger number (65,536) of tonal steps. Older model DSLR cameras use 12-bit A/D converters and divide the dynamic range into 4096 steps. Newer model DSLR cameras use 14-bit A/D converters and divide the dynamic range into 16,384 steps. More steps are usually better for astronomical images, but because of noise, both types of cameras end up producing about the same number of tonal steps.

Dedicated astronomical CCD cameras are actively cooled and produce much less thermal current, another topic we'll discuss in Chapter 3.

Three separate exposures through color filters are required to make a color image with a traditional monochrome astronomical CCD camera, more than tripling the exposure time for a single black and white image. However, longer exposures are also needed in a DSLR camera because of the individual colored filters over each photosite in the Bayer array.

Today, cooled one-shot color CCD and CMOS astronomical cameras are also available. These cameras have very similar specifications to DSLR cameras. Some even use the same digital sensors found in DSLR cameras. The advantage is that they are cooled and have very low thermal current noise. The advantage of a DSLR is that it can be used for daytime photography with a wide-range of system lenses. The latest generation of DSLR cameras also have very low noise, even un-cooled.

DSLR cameras offer a bit more convenience when used in the field at a remote observing site. They don't require a computer to be used, as a dedicated astronomical CCD camera does. Focus can be accomplished without a computer by using a Bahtinov mask or Live View focus. A computer is not required to store downloaded images with a DSLR, they can be stored on removable memory cards used in the camera. Also, an exposure session can be completely automated with an inexpensive accessory like a remote release interval timer without a computer that would be required to automate a CCD imaging session. A remote interval timer allows programming of any number of exposures, with control over the interval between exposures and the length of each individual exposure.

DSLR cameras can be used for normal daytime photography. CCD astro-cameras can also, but only at great inconvenience because they have to be hooked up to a computer. CCD cameras also require a special adapter for normal camera lenses.

CCD astro-cameras do offer significant advantages in sophisticated advanced techniques such as narrow-band hydrogen-alpha imaging, LRGB, and mapped-color images. These techniques, however, are complex, and require quite a bit of expertise.

Most DSLR cameras have low-pass and long-wavelength filters in front of the sensor that perform two functions. One is to apply a low-pass blurring filter to help reduce aliasing and moire effects. The other is to filter out long-wavelengths in the deep red and infra-red portion of the spectrum.

The long-wavelength filtering is done to match recorded colors more closely to normal human color vision, to improve the color reproduction of skin tones, and for scenes with objects with high infra-red reflectivity. Normally this greatly improves the overall color balance for normal daytime photography, however the filter cuts out a lot of the critical hydrogen-alpha wavelength at 656.3 nanometers. This, unfortunately, is the wavelength where red emission nebulae emit their light. Consequently, most stock DSLRs do not record red emission nebulae as well as they could, and these can be some of the most beautiful nebulae in the sky.

DSLR cameras can record the other wavelengths between 450 and 600 nanometers, so they are good at taking pictures of stars, galaxies, planetary nebulae and blue reflection nebulae. They can take pictures of red emission nebulae, but they usually require a special filter and longer exposures under dark skies.

If you are seriously interested in photographing red emission nebulae and want the best results possible with a DSLR camera, you will want to have your DSLR camera modified by removing or replacing the camera manufacturer's stock long-wavelength filter.

Complete removal of the filter can work very well for astrophotography, but makes the camera's autofocus incorrect as well as the viewfinder's visual focus. This type of modification also makes the color balance for normal daytime photography inaccurate in some cases.

Autofocus and viewfinder focus can be corrected by replacing the original filter with a clear glass filter of the same thickness. Color balance for daytime work can be corrected by using a special filter in front of the lens, or inside the camera body and by using a custom white balance. Substituting an ultra-violet / infra-red filter for the original manufacturer's filter can also help with imaging in refracting lens systems that might not be color corrected across the entire spectrum.

Note that these modifications void the camera manufacturer's warranty and can ruin your camera if not done correctly.

Several third-party vendors, such as Gary Honis, Hap Griffin, LifePixel, and MaxMax provide modification services for astrophotographers. They charge a fee in the range of $300 to $700, depending on the camera, to remove or replace the camera's original long-wavelength filter.

Film vs DSLR vs CCD: Specifications

*Prices, availability, and specifications of these cameras are subject to change without notice by the vendors and manufacturers.

Canon does not supply data on technical specifications such as read noise, full-well capacity and dark noise. Amateur astrophotographers, however, have, in some cases, determined these numbers by tests.

Notes

1. Quantum efficiency for the 60D is from www.sensorgen.info.

2. Read noise, full-well capacity and gain for the 60D is from Roger N. Clark's Digital Camera Sensor Performance Summary.

3. This figure is actually the dark current for the 1000D (Digital Rebel XS) at 20C from Fred76 in a post on Cloudy Nights. The 60D is better than this but no hard numbers are available.

Cyril Cavadore has published the technical data information for the Canon 20Da. He found a dark current of 0.25 electrons per second at +20C. Again, Canon has improved these numbers with subsequent models.

4. SBIG ST-8300C technical data from SBIG.