What is the difference between imaging with filters and imaging with a single shot color camera?

        CCD cameras are able to produce color images by using a variety of color filters in the RGB format.  All CCD Astrophotography cameras are monochromatic (black and white) with filters is placed in front of the CCD sensor, but there are different manners in which these filters are placed.  Typically, filters are assembled on a filter wheel, usually five filters in the assembly.  In most setups that I have seen there is a luminance filter (IR or UV blocking), red, green, and blue filter; in some cases the fifth position on the filter wheel will have a hydrogen alpha or other specialized filter.    You have probably seen the acronym RGB which of course stands for red, green, and blue referring to the filters required to create a color image with a monochrome camera.  Each filter is specifically designed to pass along their corresponding filter color of the spectrum and later combined by a computer to assemble a full-color image. 

      Chances are you already do own a Bayer camera and may not even realize it!  If you have a Digital camera, you have a Bayer camera!  All current digital cameras are Bayer cameras, the CCD chip of your camera has a thin Micro-Lens assembly slightly above the chip. 

     I have to admit that the first time I imaged with color filters I was very disappointed by the results, I remember imaging with my red filter thinking that the image I would download on my from the camera would be red, it was just plain black-and-white.  Frustrated, I moved on to the green filter and although I received to the same black-and-white image I did notice that the nebula I was imaging looked a little different, there was more detail than it was in the red image.  I completed my cycle by imaging with the blue filter page and noticed that it was quite different in the nebula had even more detail than either of the previous filters.  When I completed the cycle by using the luminance filter and noticed that the image was even brighter than all of the filters I had previously used, hence the name luminance filter.  At the time I began imaging had no idea that there were differences in the amount of time that you should place on each individual filter.  As an example, most CCD imagers often use their luminance filters much longer than the red, green, or blue filters because the luminance filter provides a greater amount of detail while blocking certain spectrums of light.  If you have ever seen the reader's gallery in Sky and Telescope or in Astronomy Magazine, you may have noticed that the images might have details such as, 'taken with LRGB with filters at 120, 90, 80, and 90 minutes respectively.'  It is often very important to have a much higher exposure time with the luminance filter to bring out the details that are lost with imaging through RGB filters.

So what is the difference between a camera using a filter wheel and a single shot color CCD camera (also known as a Bayer camera)?  To be honest, there is very little difference between these two cameras; they are both monochromatic cameras; however, the single shot color camera has a specialized micro lens filter placed slightly above the CCD chip.  Remember that when a CCD camera creates a color image it utilizes color filters in the RGB format.  Let's use a Santa Barbara Instruments STL-11000M monochromatic camera as an example.  The most popular version of this camera contains an internal filter wheel which allows the filters to be rotated in front of the CCD chip automatically and achieve color imaging by exposing each of these filters in sequence for a certain amount of desired exposure and then combined on a computer to create your color image.  Remove the filter wheel and you have a black and white camera with incredible detail (11 megapixels).  One of the principle advantages of the monochromatic camera with a filter wheel is that they offer much better detail than a single shot color camera because the entire CCD chip is exposed to each filter separately and then combined later to create the color image.

For the single shot color camera the principle is basically the same in that the camera is monochromatic but has a thin micro lens (called a Bayer filter) containing a red, two green, and a blue filters placed slightly above the CCD chip. A single complex of a Bayer filter is divided into four sections with a Green lens in the upper left, Blue in the upper right, Red at lower left, and Green at the lower right.  The image on the right shows a make up of a Bayer filter over a CCD chip  When the user exposes the camera the entire chip is exposed to each color simultaneously and the camera uses a computer process to extrapolate color data from each adjacent pixel thereby creating a single shot color image.  When you compare to two images, they will contain the exact same number of pixels; however, the camera using the filter wheel will reveal smaller details not able to be captured with the single shot camera due to the extrapolation process of the micro lens filter to create the single shot camera’s color image.  To be fair, it should be noted that many cases you can use a single shot camera and specifically tell the camera to expose only the red, green, or blue filter for each shot; however, the details will still be less visible then the camera with a filter wheel because it's simply not possible for this single shot color camera to expose its red filter or any other filter for that matter to the entire chip because of the design of the micro lens.

So which camera should I choose?

The answer really depends on your experience level, what you want to image and what you are willing to spend on your new camera.  In the astronomy community there are strong feelings and good reasons to choose both cameras, each has advantages and disadvantages.  For example, if your camera uses a color filter wheel, you spend much more time taking exposures because you have to individually capture each color.  Ideally this will produce sharper results than a single shot color camera but there is a downside, time.  Once when imaging I had great skies when I began imaging with my Red and Green filters but when it came time to shoot with the Blue filter, the skies clouded up and I was unable to image.  Not a real problem since the next day I completed the Blue images but it was a hassle. 

If you are constrained by time (who isn't) consider how this could impact your imaging.  Time considerations alone should not sway you to buy a Bayer camera but it should be a factor.  My advice, if you are new to imaging, buy a Bayer camera to start.  When you gain experience and want to upgrade, list your Bayer Camera on AstroMart or eBay and you can easily sell your camera.  Bayer cameras offer simple operation, allow for shorter imaging times and are frequently less expensive than Monochrome cameras (color imaging filters and the filter wheels  cost extra).  Since you may be new to imaging, using a single shot camera will offer a shorter, albeit tall learning curve and fast results which will likely hook your interest for LIFE!!

Also, invest in the future and buy a camera larger than you think you will need.  I have bought and sold cameras since I began and every time had Pixel envy!  Buy a dual chip camera so your guider and imager are in the same camera, I'm not knocking a separate guide camera but if your guide scope and imaging scope are not orthogonal guiding could be difficult and produce poor images!  SPEND THE MONEY on the dual chip camera, you will not regret it!!  Having a dual chip camera has made my imaging much more enjoyable and productive.  My current camera, an SBIG ST-2000XCM, works flawlessly with Maxim DL and allows me near total automation of an imaging session, from acquisition of guide stars to auto-saving and even focusing with Robofocus!  One exception to my feelings on separate guiders would be the SBIG SG-4 guider it is a great autoguider (especially for those shooting DSLR) that does not require a computer for operation! 

Bottom Line:  Choose a camera to match your needs!  Monochrome imagers are great but require more time to image, cost more and filters (top of the line 50mm Astrodon LRGB filters cost $825 for the set) and filter wheels have to be purchased separately.  However, monochrome imagers produce more detail than Bayer cameras and are the accepted standard for nearly all serious amateur astronomers (check out the SBIG ST-11000CM single shot color camera which is quickly gaining popularity).

Bayer cameras offer single shot images, saving time and generally cost much less than monochrome cameras.  On the downside, images are slightly not as sharp as they are with  monochrome imagers.  If you are just starting out and looking for quick results to build confidence with your imaging, buy a single shot Bayer camera, specifically an SBIG single shot color camera.  It has been my experience that SBIG cameras are easy to use, are offered as dual chip imagers, come with great starter imaging software and are built with superior quality.  Take a look at Astronomy and Sky and Telescope magazine's Readers Gallery and note the number of SBIG cameras are used to take those images.   Both versions of cameras are capable of dual chip guiding if so equipped so that aspect is a wash.  If you are on the fence on imaging, take the plunge now, you won't be sorry!

 

 

        The Lagoon Nebula

Single Shot Color Bayer Camera

SBIG ST-2000XCM Camera

www.sbig.com

 

 

 

 

The Horsehead Nebula

Single Shot Color Camera