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a particular object can check the data at the Infrared Processing and Analysis Center (IPAC), at Caltech, which is not too far from the Jet Propulsion Laboratory (JPL). IPAC has also become the curator of other infrared data.
More recently, astronomers have been able to utilize the Infrared Space Observatory (ISO), Fig. 4.24(a), a project of the European Space Agency. ISO provides a wavelength coverage extending farther into the infrared, a larger telescope, arrays of more sensitive detectors for good imaging, and for the first time the ability to make high quality spectra. During the HST servicing mission, NASA added an infrared camera and spectrometer (NICMOS). NASA is now making plans for the Space Infrared Telescope Facility (SIRTF), Fig. 4.24(b).
When we look at a normal optical photograph of some astronomical object, we have a sense of how our brains should interpret that image. In a sense it is how the object would look if we could view it through a large telescope, or if we could somehow be transported close enough to the object so we could see it with this detail with the unaided eye. However; what does it mean when we display a radio image like that in Fig. 4.25?
There is even a terrestrial analogy to this question. You have seen 'night vision glasses', which allow you to 'see' even with no illuminating light. Remember; we normally see earthbound objects as they reflect sunlight or roomlight, our eyes being sensitive to the range of wavelengths at which the Sun's emission is strongest.The night vision glasses work differently: they actually detect the infrared radiation given off by objects (most of which are usually close to 300 K). So, the night vision glasses have infrared detectors, but our eyes are not sensitive to that infrared radiation.Therefore the glasses also convert that infrared image into an optical image, usually with the brightest part of the optical image corresponding to the strongest infrared emission. So, the image you see is an optical representation of the infrared image.
We can do the same thing with astronomical infrared (or ultraviolet, radio, etc.) images. We can make a false gray-scale image, by creating an optical image where brighter regions correspond to stronger infrared emission. It is important to remember that, while such images are often constructed to have a true-looking appearance, they are just a particular representation for that image. Sometimes our eyes are better at picking information out of a color image than a black and white (or gray) image. It is therefore sometimes useful to make false color images. In this case, we arbitrarily assign a color to each level of infrared emission. Often the colors will run through the spectrum from red to blue (or the other way). Often, a sample bar will be placed next to the image, showing what intensity level each color represents.
So far, we have been talking about what we do when we have one piece of data at each location, say the average intensity in some particular wavelength band. Suppose we have observed in more than one band (for example, IRAS observed in four infrared bands).We could certainly make a separate false gray or color image of each band (and we often do this), but what if we want to compare the bands, or simply display all the information together?
Various representations of a radio image. In this case it is a 6 cm wavelength image of the Orion Nebula (which we will discuss in Chapter 15), made with the Byrd GBT (which we will discuss in the Section 4.8). (a) Contour map. (b) Gray-scale map. (c) False color image. (d) Color contours, in which colors change where there would be a contour line. [D. Shephard, R. Maddalena,J. McMullin, NRAO/AUI/NSF]
We then make a false gray-scale image of each band. We then tint each band a different color generally making the longest wavelength band red and the shortest wavelength
band blue (mimicking what happens in the visible part of the spectrum).We then have a false color image in which the color has some intrinsic meaning (in that hotter objects will appear bluer).
We should point out that this technique can also be used to make a true color visible image.You might say that if you want a color image you simply use color film and take a picture. However no two types of color film are the same. Some are meant to enhance skin tones and are set to emphasize reds, for example. Therefore the way to make a color photograph that looks like what you would see with your eyes, we take a series of black and white images, through red, green and blue filters.We then combine the images, utilizing the various wavelength ranges in the same proportion as the eye uses them.This technique is well suited to making 'true color' images with CCDs.
There is another method of displaying two-dimensional images, contour maps. You should be familiar with topological maps on Earth, which are normally displayed as contour maps.All points within a given contour level have a value (e.g. average intensity in a particular wavelength range) greater than the value assigned to that level. Contour maps give a good feel for how the quantity you are displaying changes over some region. The closer together the contours, the more rapid the variation in the plotted quantity.
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