This page explains what Image Processing is all about.

We have Photoshop as well as GIMP installed on the Goodsell 104 machines. Gimp (http://www.gimp.org/) is a free, open source graphics creation and manipulation application very similar to Adobe Photoshop.

Table of Contents:

 

Introduction

Do astronomical images document physical reality? Or are we looking at artistic creations of digital manipulation? We always see beautiful and often strange color pictures of galaxies, planets, and nebulae. Do the pictures really reflect the colors these objects would have if we visited them in a spacecraft? Astronomy is much more than just taking a picture with a telescope. Frequently, the picture needs to be processed to bring out details, compared to other pictures, and adjusted to make color images. 

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What is an Image?

To take pictures of stars or any object in the sky we don’t use color film anymore; in fact, we hardly ever use film. Rather, the cameras record light from the universe with special electronic detectors. These detectors produce images of the cosmos not in color, but in shades of black and white. Most modern optical telescopes use a CCD (charged coupled device) camera to record images of the sky. A CCD is a device that turns light into electronic signals. When light hits a CCD camera, the CCD records a signal; the brighter the light, the stronger the signal.

The electronic image that the CCD records consists of a grid of these pixels. Each entry in the grid includes the pixel’s x-coordinate, its y-coordinate, and the number of photons, or “counts,” that hit that pixel during the camera’s exposure time. Image processing software reads the data and assigns each pixel a shade of gray (or a color) depending on the number of counts. Here is an example of how the data might look for part of an image from one of Goodsell’s CCDs:

Numbers 1-9: range of the intensity of light

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Think about how hard it is to visualize an image from the intensity numbers only. That’s why we have an image processing program that assigns a color to each value in the grid. For example, 0 might be white. 1 and 2 can be shades of gray, 3 and 4 might be shades of red, etc. The program could also display each integer as a different color. A real image would have many more pixels and would use substantially more than 10 colors. The way that image processing programs display the image is the same.

 

Coloring Modes

During image processing, we often use color as a tool, whether it is to enhance an object’s detail or to visualize what ordinarily could never be seen by the human eye. We can process images in three ways:

Natural color: the colors simulate closely what the object would look like if we visited them.

Representative color: helps scientists to visualize the composition of an object.

Enhanced color: helps scientists visualize the important elements present in an object.

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Filters

Finished color images are actually combinations of two or more black-and-white exposures to which color has been added during image processing. Light from astronomical objects comes in a wide range of colors, each corresponding to a particular kind of electromagnetic wave. Our telescopes can detect all the visible wavelenghts of light plus many more that are invisible to human eyes, such as near ultraviolet. Astronomical objects often look different in these different wavelengths of light. To record what an object looks like at a certain wavelength, the Meade CCD cameras use special filters that allow only a certain range of light wavelengths through. Once the unwanted light has been filtered out, the remaining light is recorded.

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The Meade telescopes have a set of 3 filters (red, green, blue) that are used to approximate the same wavelength range of the EM spectrum as covered by the human eye.

 

Chromatic and Composite Order

Chromatic and composite order: If we retain the wavelength direction then the ordering of filters is referred to as chromatic. If we select other orderings, also in the figure, the order is referred to as composite.

This is an example of composite order assignment: the blue filtered light is assigned green, the green filtered light is assigned red and the red filtered light is assigned blue.

This is because of the “additive” color effect: when colored lights are projected into a screen, such as in a color television, an additive effect is seen, i.e. the individual colors add together to make the color seen. Hence red light added to green light produce yellow light. This is the principle on which the color television works – all colors can be produced from an additive mixture of red, blue and green light.

Hue is a tool to color in image processing. When changing the Hue values we are selecting degrees in the color wheel: 0 for red, 120 for blue and 240 for green. The RGB wheel uses red-green-blue as its primary colors. Colors in RGB are based on the idea of transmitted light achieving the additive color effect described earlier. That is how the different colors are made by mixing frequencies of light. RGB can produce 16.8 million colors.

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Assigning RGB Color to an Image and the Result

In the final processing of these images we can reveal detail usually hidden to the human eye. To do so we can use methods that are either similar to astronomical techniques for studying observations or those that are standard in photography (if you are familiar with photographic techniques, we can do the digital equivalent of dodging and burning). When using these techniques we do our best to avoid generating features in the image that were not originally in the data.

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Each of these images is a grayscale image. To make a color image, you must combine three of these grayscale images. A free software package called GIMP will help you make tri-color images. 

 

File Standards: FITS and TIFF

Astronomical images are usually saved in a format called the Flexible Image Transport System, or FITS. Unlike the GIF or JPEG images you have probably viewed before, FITS images can’t be viewed in web browsers. You need a special viewer to see them – like FITS liberator, a Photoshop plugin. The images from Goodsell CCD cameras can be saved in FITS files and TIFF files. In fact, to work with FITS files we can use free software, called Iris. Today we will work with TIFF files though, and we will retrieve them using GIMP, an analog software to Photoshop, also free.