The differences between CCD and digital SLR cameras

It will be a year next month since I started using a CCD sensor… I know the number of CCD pictures taken might say otherwise, but no, it’s only been a year, give or take… Prior to that, I was using a stock, then baader modified, Canon 350D SLR mounted behind a C11, or with my Astrorubinar piggybacked on the C11. When using the main scope, I was guiding by hand using an off-axis guider and a 12.5 reticule eyepiece, but, knowing what I do now, the setup was much too inaccurate  to guide effectively. Using the 500mm/5.6 Rubinar piggybacked and guiding manually with the C11 did, on the other hand lead to good results. This is why I did keep some of these pictures posted in the Rubinar gallery. It is quite fun to retake nebulas I covered with the Canon with the Atik11000-CM CCD and compare the differences. A prime example of this are “The veils” in Cygnus. I had originally taken the eastern veil because I could not fit the whole thing on the Canon sensor which is not fullsized. So, last night, I started centering the CCD equiped Rubinar in order to get the same shot. Only that I forgot that the Atik sensor was much larger that the canon’s and now, I could get the entire nebula to fit on the same picture. Here are the two shots, the “original” (13×6 min exposures processed unde Iris software):

NGC6992

and the newer one (9×10 min exposures processed under MaximDL software):

Dentellesraw1s

The difference in sensor size really is apparent isn’t it? Also, while the Canon image is quite good by my standards, the CCD picture is alot crispier thanks to a better focus as the 350D lacked any tools (FWHM) to achieve perfect focus. Also, the colors of the CCD seems more “true life”, if there is such a thing. Click on the pictures above to see them full size.

Another example for my American friends is “North America” Nebula taken first taken with a 200 mm Sigma telelens (as it would not fit at all on the canon sensor with the Rubinar):

ngc7000

it is the result of 13x6min exposures on 350D as well. Below, the result from 9x10min CCD exposures:

Americaddp2

Lucky are you to have a dedicated Nebula for your country (I’ve yet to look for an hexagon shaped nebula)! Anyhow, here, even though the field of the Canon shot is much larger than that of the CCD (thanks to the short 200mm telelens focal length), there just is no contest with the CCD camera. In conclusion, it is much easier to get good result with a CCD than with a SLR CMOS sensor for obvious reasons:

  • Signal to noise ratio is much better with the CCD since it is cooled to -20°C or so. This is the reason why 6 minutes exposures at 400 ISO (or whatever settings since ISO sensitivities are completely artificial in a SLR; the sensing chip has ONE sensitivity full stop…) is the maximum for a 350D at 20°C.
  • Quantum efficiency is much better as well for a CCD compared to a CMOS chip, so it takes a shorter exposure to record the same amount of photons.
  • Even though that has changed with the newer Canon 450D and the Liveview software, there was no way to achieve perfect focus with a 350D apart from trial and error.
  • It is much trickier to capture proper colors as the original Infrared Canon sensor filter has to be removed or replaced with a third party in order to record Halpha wavelength (pretty obvious on “America” above which has a strong red tint to it.

But, to be fair, the internet is full of examples of what can be achieved with a well used SLR. Some of them are indeed impressive and do rival with what CCDs do. Also, one item where an SLR cannot be beaten  is price. It is true that an SLR, even modified, will cost a few hundred Euros/Dollars whereas an CCD costs a few thousands. Also, you can’t take pictures of your familly with a CCD! For these reasons, I think SLRs are the best way to start astroimaging, especially is you are not sure you will last long in the hobby. When the urge to photograph the sky is over you can still carry on snapping the wife and kids! But then again, more often than not, after that first picture, you’ll be completely and hopelessely hooked…

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