This is a comparison test between a Canon EOS stock 60D, a stock 60Da, and a modified T2i (550D).
Background Most normal DSLR cameras have a long-wavelength filter built into them in front of the sensor that filters out most of the red wavelengths. This is done to make the camera's color response match human color vision. Unfortunately, this also filters out a lot of the red color that comes from hydrogen-alpha emissions in red nebulae. Humans have a little bit of visual sensitivity at the hydrogen-alpha emission line of 656.28 nanometers, but DSLR sensors have a lot more. This extra sensitivity gives images more of a red color, especially in skin tones. So manufacturers filter most of it out. This is good for normal daytime photography, but not so great for astrophotos of red emission nebulae. Amateur astrophotographers have been dealing with this problem by modifying the long-wavelength filter. They either remove it completely, or substitute another filter that passes almost all of these hydrogen-alpha wavelengths. This is good for astrophotography, but slightly skews normal daytime colors. This can easily be compensated for by using a replacement filter such as the Xnite CC1 Color Correcting filter and by using a custom daylight white balance. Canon's Astrophotographic Cameras In 2005 Canon came out with the EOS 20Da to address this problem in a stock camera specifically designed for astrophotography, but one that would also give good color for normal daytime photography. The long wavelength filter in the 20Da transmitted more of the hydrogen-alpha wavelength, but still not as much as a modified DSLR. In 2012 Canon came out with the EOS 60Da, a replacement for the 20Da. The long-wavelength filter in the 60Da transmits 1.5 times more hydrogen-alpha light than a 20Da, and 3 times more than a stock 60D. There is a lot of interest in Canon's 60Da astrophotographic camera and questions about how it performs for deep-sky astrophotography of red emission nebula compared to a non-modified camera, as well as to a stock camera that has been modified. I was lucky that Canon asked me to write an article on Astrophotography Techniques with the Canon EOS System for the Canon Digital Learning Center on the web. To write the article, Canon was kind enough to loan me a Canon EOS 60D and a Canon EOS 60Da for testing, as well as several lenses. Several different targets were shot in a couple of different ways to compare the performance of these cameras for long-exposure deep-sky astrophotography. Photographic Test Results In Figure 1 at the top of the page, we can see the results of imaging M27, the Dumbbell Nebula. As a planetary nebula it is an interesting test object because it has a lot of cyan (blue-green) oxygen III emission in it as well as some red hydrogen-alpha emission nebulosity. The results in Figure 1 are pretty much what we would expect. The stock 60D records a lot of blue-green oxygen III because that is around the wavelengths that human vision is most sensitive to, and so the camera is designed to approximate this as closely as possible. It only records a little bit of red hydrogen-alpha compared to the 60Da and the modified T2i (550D). In addition to the same amount of oxygen III recorded, the 60Da records a lot more red from the hydrogen-alpha emission. The modified T2i (550D) records the same amount of oxygen III, and a little bit more hydrogen-alpha than the 60Da. This is also expected. The replacement filter passes almost all of the hydrogen-alpha wavelengths, whereas the 60Da is still filtering some out. It is also interesting to compare the signal-to-noise ratio in the images produced by these three cameras as we can see in the chart below.
The same small area of background sky, containing no stars or nebulosity, was measured in Photoshop. The signal-to-noise ratio was computed by dividing the mean pixel value in this area by the standard deviation. As expected, the modified T2i (550D) has a slightly higher signal-to-noise ratio than the 60Da. Both the modified T2i (550D) and 60Da have a better signal-to-noise ratio than the stock 60D. This is because the T2i (550D) and 60Da transmit more light at the longer wavelengths. These wavelengths produce photons which pass the filter and are detected by the sensor, and turned into signal. The signal-to-noise ratio is higher in the T2i (550D) and 60Da simply because there is more signal present. As seen in the comparison images, the T2i records more hydrogen-alpha emissions because its replacement filter passes more of this wavelength than the 60Da filter. This gives the T2i (550D) a bit of an edge in the signal-to-noise ratio also over the 60Da for the same reason. It records more signal because the filter lets more photons through. Head-to-Head Tests, M27 - The Dumbbell Nebula
In Figure 2 above we compare the response of the 60D to the 60Da. These are the same images as in Figure 1, but isolated in a head-to-head death-cage-match-shootout comparison mouse-over so we can blink them a little bit easier.
In Figure 3 we compare the response of the modified T2i (550D) to the 60Da. Again, these are the same images as in Figure 1, but isolated in another head-to-head death-cage-match-shootout comparison mouse-over so we can blink them a little bit easier. Head-to-Head Tests - M8, The Lagoon Nebula
In Figure 4 above, we can see a direct comparison between the red hydrogen-alpha sensitivity of the Canon 60D to the 60Da on M8, a large hydrogen-alpha emission nebula. The 60D has picked up an approximately equal mixture of blue-green oxygen III emissions as well as red hydrogen-alpha emission. The 60Da on the other hand, is dominated by its sensitivity to the red hydrogen-alpha wavelengths. Fainter red nebulosity, in the outlying part of the nebula, is recorded by the 60Da due to its increased response to the hydrogen-alpha emission wavelengths. Using a Stock Camera for Red Emission Nebula I don't want to give the impression that a stock, unmodified camera, can not be used for red hydrogen-alpha emission nebula. This would not be correct. Stock cameras do record some hydrogen-alpha, even if most of it is filtered out. I've covered this before in my previous article on DSLR Cameras and H-a Emission Nebulas, but it's worth repeating here with an example.
You can shoot red hydrogen-alpha emission nebulae with a stock camera. The secrets are to shoot from a dark-sky observing location, and to use a light-pollution filter like the Astronomik CLS or Hutech IDAS LPS filter. Even from a dark-sky site these filters help tremendously because they filter out natural airglow which is present everywhere all the time. The North America Nebula in Figure 5 was shot with a stock, unmodified Canon EOS 60D. The unfiltered image was a stack of 6 one-minute exposures at ISO 1600 taken with a Canon 200 mm f/2.8 L lens. The filtered image used an Astronomik CLS filter and was taken from a stack of 2 three-minute exposures at ISO 1600. The total exposure for each image is exactly the same to provide a valid comparison. Longer sub-frames were allowed by the filter to reach the same sky-limited exposure. Daytime Color Balance
The two images in the Figure 6 mouse-over comparison above show a standard Macbeth color chart. Both were exposed under the exact same daylight lighting conditions in direct sunshine with the Sun high overhead. We can see that the Canon engineers have done a good job in the 60Da of fairly closely replicating the color of the 60D. Red colors in the 60Da image are more saturated and there is some slight red contamination in some of the other colors, but overall, this is not really something that you are going to notice in normal daytime snapshots of the kids playing ball. If you are a professional photographer working in a field where color is critical, say in food, product, or fashion photography, then the color accuracy of a 60Da is not going to meet your needs and you should not be using an astrophotographic camera for this type of work. Canon even goes so far as to not recommend the 60Da for daytime photography at all, but I think this is a bit excessive. It can easily be used for normal daytime snapshots. If normal daytime color is critical with the 60Da, you can always use an Xnite CC1 color correcting filter and a custom white balance. Dark Signal The following comparison looks at the dark signal in the 60Da compared to the modified T2i (550D).
Visually, the two frames were similar, but the T2i (550D) image seemed to have a bit more "noise".
The numbers above give the same results as the visual impression. The internal temperature in the camera for both cameras was 31C (88F). Both dark frames were processed exactly the same. The same non-linear DDP curve was applied to both. A small section of the image was enlarged 400 percent to make the thermal signal more visible here. Summary The Canon EOS 60Da is a very good camera for long-exposure deep-sky photography of red hydrogen-alpha emission nebulae. It's sensitivity to these wavelengths is almost, but not quite, as good as that of the modified T2i (550D). This is simply because the replacement filter in the T2i (550D) transmits more of the hydrogen-alpha wavelength than the filter in the 60Da. The 60Da and modified T2i (550D) can definitely be used for normal daytime snapshots without any special filters if you are not incredibly picky about the colors. Most people will not notice the difference. You will need to set a custom white balance with the modified T2i (550D). The 60Da has a little bit less noise from the thermal signal compared to the T2i (550D). Another nice thing about both the 60Da and the T2i (550D) is that in addition to being able to use them for long-exposure deep-sky astrophotography, they can also be used in 640 x 480 Movie Crop Mode to shoot high-resolution planetary photographs with Lucky Imaging. Both the 60Da and 60D and T2i (550D) use the same CMOS sensor and DIGIC 4 processor. Bottom line 60Da
Bottom line T2i (550D)
Conclusions
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