The recent availability of sub-$1000 digital SLRs has brought about quite a bit of excitement in astrophotography circles. The ability to remove the camera lens, like a traditional 35mm SLR, and hook up the camera directly to the prime focus of a telescope, like a traditional 35mm SLR, is something that astroimagers have been interested in for several years now ever since larger resolution digital SLRs started making their way into the public's hands.  Until recently, such cameras have been extremely pricey as far as SLRs are concerned.  Not so now.  People are seeing NOW as an opportunity to get full-scale compositions and wide fields with resolutions similar in size to the big astronomical CCD arrays at a tenth of the price.  Who could blame astrophotographers for being excited?  They require one shot only, not the three separate exposures required by astronomical CCDs.  Recent advancements have put most of the features we need into a single package, including good noise performance, longer shutter speeds, and film speed emulation.  

As the owner of both a digital SLR (the Canon Digital Rebel) and an Santa Barbara Instruments Group ST-7E, I have the chance to field test both types of cameras, showing comparative results.  For the purpose of this comparision, because my  SBIG ST-7E is a small array in comparison to the DIgital Rebel, I will use it's bigger brother, the SBIG ST-10xme.  I've had the opportunity to borrow such an instrument, which is considered very near top-of-the-line in large format astronomical CCDs.  Though it still falls some 50% short of a full-size 35mm camera frame, it is large enough for an overwhelming majority of sky objects.  

So, the purpose of this comparison will be to judge the suitability of digital SLRs as a replacement for both more expensive astronomical CCDs and traditional film cameras.

The best way to do this is to look at similar images of the same object taken with each camera.  For this comparison, it's hard to think of a better object than the M42 region!  

The image on the left was taken with an astronomical CCD camera for a total exposure length of 60 minutes.  Because these cameras require tri-color imaging techniques - filtered exposures for each of the red, green and blue channels - the image uses 20 minutes of information per channel; thus, 20 minutes with a red filter, 20 minutes with a green filter, and 20 minutes with a blue filter.  While this may seem very difficult to do, the integrated color filter wheel and acquisition software makes the process somewhat simple.  Because the camera works on a 1:1:1 ratio, meaning that equal exposures of the channels provide the best color balance, the colors shown in this photograph are quite representative of the actual colors inherent in the nebula.  In other words, the shot on the left is very close to the way the nebula SHOULD appear in photographs from the standpoint of color.

The image on the right was taken with the Canon Digital Rebel, also known in Europe as the 300D.  Because such cameras use tri-color pixels, a SINGLE shot from these cameras will produce a color image, unlike an astronomical CCD camera that requires individual exposures for each channel. This particular image contains 90 minutes of total exposure time (18 separate exposures of 5 minutes each).

This most obvious difference in the two images is the color.  Blue and purple dominate the scene of the digital SLR image.   Secondly, the amount of detail in the first image is much greater than in the second.  Despite the ability of the digital SLR to take single RGB exposures, these cameras lag behind the astronomical CCDs in their ability to achieve the same level of detail given equal, total exposure lengths.  In fact, the decrepancy is much worse than that when you consider that the image on the right is some 50% LONGER than the one on the left.

Even so, why is the color balance so strikingly different in the exposure with the Canon digital SLR?

Looking at the individual red color channels can show us why the second image is so dominantly blue and purple:

Ignoring the blooming (vertical streaks) in the red channel of the CCD exposure, the images are quite similar in all other regards, including exposure length and processing.  Quite simply, the digital SLR fails to capture the red parts of the nebula.  Interestingly, the blue and green color channels are much less effected.  In fact, if we viewed all three color channels of the digital SLR image we would see that the blue and green channels are quite stronger than the red channel.  This is not the case with the CCD shots, where the red channel dominates the other two, something attributed to the fact that there is naturally more red information (reddish hydrogen gases) in the nebula itself.   

The lack of red sensitivity in the digital SLR image occurs because the camera has an IR blocking filter over the imaging chip.  This filter exists in digital cameras because infrared light ruins the color balance in a color image.  However, the filter is so aggressive in the way it blocks the infrared part of the spectrum that it cuts too deeply into the upper end of the red portion of the spectrum, which so happens to be where the most dominant forms of these hydrogen gases shine (around the 656nm spectral line).  Put another way, only a small portion of the red information gets through to the chip while the blue and green information is unaffected.  Therefore, when one processes such an image, attempts to bring out the faint details in the nebula, which are red, does nothing but to increase the level of the green and blue channels.  Unfortunately, as in this above digital SLR color image, pushing the mix too far makes for a purple and blue mess.   In order to maintain the proper color balance, I should have cut back on the amount of detail I was trying to achieve in the final composite.  That would have given me a better color balance, but then YOU would be denied the opportunity to see some of the fainter details, details which were recorded not on the red channel as they should have been, but rather on the green and blue channels.  

When taking shots with the Canon Digital Rebel, or similar digital SLRs, you have a few options to give you a better balance of colors. First, you can figure out a way to remove the IR blocking filter from the surface of the chip.   Hutech Co. distributes a camera with this modificaton already done, though it costs around 70% more than the regular version of the camera.   Or, some people perform the modifications themselves, but this isn't an easy procedure and it definitely voids the warranty for the camera.  Plus, removal of the filter, while substantially improving your astrophotos, would hinder its use for daytime photography since the autofocusing routine is greatly affected.   I suspose you could purchase TWO digital SLRs, one for astrophotos and one for daytime use, but that option starts at a rather hefty price tag, which somewhat negates the appeal of such digital SLRs in the first place.  

Another way to improve the color balance of your astrophotos with a digital SLR would be to simply take longer exposures.  After more total exposure time, you could certainly generate the red information you need to give you the color balance you seek while maintaining a nice level of detail.  However, you would just have to refrain from the temptation of bringing out too much of the information in the green and blue channels.  Otherwise, you would run the risk of turning your image into another blue and purple mess.  The balance between detail and color is fully subjective, though you should work hard to be honest with the information you have.  If the red channel is weak, don't try do make too much out of it.  If it's not there, it's just NOT THERE...and it isn't going to be there regardless of how much processing you do.

Another way to improve the color balance in your digital SLR images is to take a luminance image with an h-alpha or red filter and take separate exposures with the camera set in grayscale mode.  Once you've gotten the proper amount of signal you are looking for, you can then take this image and place it into the red channel of the final RGB image.   In other words, since you don't have the red information in the original image, you simply replace this red channel with one that has what you need.  This technique works effectively; however, the length of exposures needed increase substantially when using the such filters.  Some people work around this problem by using the red information from a separate astronomical CCD image, but if you could do that, then do you really have the need for a digital SLR in the first place?  It makes more sense to get more total exposure time to begin with rather than to try these more difficult techniques.  

Finally, as I have already implied, you can get better color balance in ANY color digital SLR image by simply resisting the temptation to raise the blue and green channels in an effort to get more detail.  Just be true to the image.  Work your image while processing it, but don't make more out of it than it can give you.  

Therefore, to answer our original question - can digital SLRs replace dedicated astronomical CCD cameras? - the answer is a resounding "NO."   Not even close.  But they do share in some of the advantages of such CCD cameras.  They continue acquiring signal in a linear fashion throughout an exposure (unlike film) so digital SLRs can be used in less-than-optimal sky conditions, such as in suburban neighborhoods.  They also provide a much wider field than most astronomical CCDs unless you are prepared to spend over 10 times the price.  Thus, they become a suitable replacement for film cameras for the majority of the images in your astrophotography program.  The exception is for those types of shots that require longer, single exposure shots, like for star trails.  This is because digital SLRs are limited to shorter exposures due to the noise they generate, especially when compared to super-cooled astronomical CCDs like the SBIG cameras.  But with the current trend of "stacking" multiple images to increase the TOTAL exposure time and the ability to subtract dark frames (repeatable images of the noise itself), this is no big loss...merely a minor inconvenience.  Plus, shorter exposures allow for astrophotography with even less expensive, less solid mounts.  

The price of digital SLRs has dropped to the point where they are a very serious threat to any film platform, especially for those who can't get to a truly dark sky very often (the domain required for film).  But the best amateur astrophotos in the world are currently produced by people equipped with expensive astronomical CCDs, like the SBIG ST-10xme used in this comparison test.  That fact will not change in the foreseeable future.  

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