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Tony Hallas' Cosmic Imaging: Getting clean images

September 2010: High-quality astroimaging involves using correct post-processing techniques.
Tony Hallas
By now, the charge-coupled device (CCD) camera is well-known to many of Astronomy's readers who take celestial images. But there are still holdouts who prefer the DSLR for its low cost and ease of use. Also, some new imagers aren't sure if a CCD camera is the right choice. To help you understand what's involved if you choose a CCD camera, and to lay the groundwork for future columns that deal with processing CCD data into stunning images, I dedicate this column to CCD basics.

Make no mistake: A CCD camera, even the smallest one, can run circles around a DSLR when it comes to faint detail. For this reason, and because a CCD camera has the ability to take data through discrete filters with ease, the CCD camera is my choice for astrophotography. There are a few things you should know, however, about the common properties of these cameras.

First, CCD chips perform linearly. You can plot the data collection as a straight line. CCD cameras are not perfect, however. As an exposure progresses, current builds up in the chip's individual pixels. This excess — called "dark current" — shows up as bright dots on an otherwise black background. So even in total darkness, a CCD chip produces an image.
September 2010 cosmic rays
Cosmic rays strike CCD chips during exposures, and most chips are sensitive enough to record the hit as optical “noise” (circled). Most astroimaging software offers an algorithm that will let you easily remove all traces of cosmic-ray hits.
Tony Hallas
All is not lost, however, because you can re-create dark current and subtract it from the data you collected. Imagers refer to this process as "dark subtraction" because you record a "dark frame" in total darkness (with the CCD chip covered) so you capture only the dark current. Once you do this, what's left is the real data — your image.

CCD cameras are so sensitive that they actually record cosmic-ray hits! These show up as optical "debris" scattered throughout your image. Because this pattern is random, it will be different on each of your frames. (Imagers usually combine a number of exposures called "frames" into a final image.) This randomness allows us to eliminate the cosmic-ray noise by using a software algorithm that compares all the frames. Anything constant to all frames remains, and anything random is rejected. The "median combine" and the "standard deviation sigma combine" are examples of two algorithms imagers use for this purpose.

A third noise is optical. Any dust present on a filter or on the CCD camera's window (if it has one) appears on the image as a donut or a black blob. Your optical system also might create vignetting, which causes the corners of the image to darken. You can fix both of these problems by shooting a "flat field" frame, which consists of an image of a (usually) white surface. Software then divides each pixel of the image by the corresponding amount of light collected on the blank image. When you apply this correction, the shadows largely disappear.
September 2010 dark subtraction
Dark subtraction is a process astroimagers use to remove bright dots resulting from excess current that builds up in the pixels of a CCD chip. To perform a dark subtraction, take an image (called a dark frame) with the CCD camera covered. Then, using software, subtract that image from your original.
All images: Tony Hallas
So, to summarize, take three steps to achieve clean images with a CCD camera. First, shoot dark frames at the same time and temperature as the data frames. Then apply them to the image to remove dark current. Second, shoot flat field frames to remove optical noise.

Finally, combine several frames of your image together with a median or standard deviation sigma combine algorithm to reject cosmic-ray hits. Note that your dark frames also get hit by cosmic rays, so you'll have to perform a combine on them as well.

Use these techniques and you'll get the most out of the CCD images you capture through your telescope. Next month, I'll talk about combining images and managing the signal-to-noise ratio.

And don't forget to send your best image to Astronomy's 2010 Astroimaging Contest. You have until October 1. For more information, read David J. Eicher's editorial on page 8 of this issue.
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