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Cameras
I have used several digital SLR cameras for astrophotography. I first started using a Canon D60 which I purchased in 2002. Although this was one of the first "affordable" DSLRs on the market - I paid nearly $2400.00 USD for it - it suffered from a large amount of noise and amplifier glow, especially when used at the higher ISO settings. In 2004, I purchased a Canon 300D Rebel which I soon modified for astronomical use by removing the internal infrared (IR) filter, thus allowing the camera to capture the most important H-Alpha light which is centered at 656nm. In 2005, I purchased the Canon 20Da, which is the camera I am presently using as my primary imaging device.
Canon 20Da DSLR
The Canon 20Da is the first DSLR on the market that was designed to be used for both standard photography as well as having functions that enhanced its use for long-exposure astronomical photography. It was originally marketed only to Japanese customers, but due to high demand, Canon Japan decided to release it to the US market with much success. However, despite the success and accolades among astrophotographers, Canon discontinued the 20Da, and there are presently no plans for a replacement model in the near future. That being said, one can still pick up a used 20Da on Astromart, although they are fairly expensive costing around $2000.00 to $2500.00 USD typically.
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Original Japanese Canon 20Da Ad photo
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The Canon 20Da is basically the standard Canon 20D body with three major modifications which specialize it for deep sky photography.
The first and most important feature, in my opinion, is that it has a real-time focusing mode. Anyone who has done astrophotography knows that focusing the camera can be a very difficult and time consuming task. This new real-time focusing mode solves that problem by allowing the user to preview a small portion of what the sensor is reading on the rear LCD screen of the camera, thus allowing focusing to be done in real time complete with a live preview. There are basically two modes which are accessed via the standard shutter speed settings, FC1 and FC2. When set to FC1, the center 20% of the sensor is displayed on the rear LCD screen of the camera. When set to FC2, the center 5% of the sensor is displayed. I have found that even when using a narrowband H-Alpha filter which is quite opaque to the visible light spectrum, that this live preview mode is sensitive enough to pick up some of the brighter stars in the sky.
 Second, the internal infrared (IR) cut filter, which is standard in all consumer digital cameras, has been replaced with a modified one that allows transmission of H-Alpha light to reach the sensor. H-Alpha light is centered at 656nm and is the prominent wavelength of light given off by emission nebulae such as the California nebula. With the standard IR filter, the 20D would not be very sensitive to light at this wavelength. Canon claims the 20Da is approximately 2.5 times more sensitive in H-Alpha transmission that the stock 20D. It is important to note however, that although the filter is modified and allows transmission of H-Alpha light, it still attenuates most IR light, including the wavelengths used for near IR photography. Because of this, the 20Da can still be used perfectly fine without any additional filters or processing for standard daytime photography. However, it cannot be used to capture near IR images.
The third and final modification was that the noise reduction function of the camera has been modified to do a much better job at reducing noise when taking long exposures. During long exposures, the image circuitry is placed in a more thermally passive electronic state during the long exposure "B" mode. As a consequence, hot pixels are noticeably reduced especially at temperatures below 40degF. This modification also reduces the donut shaped amplifier glow signature which is located in the lower right corner of the image frames.
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