In this tutorial you will load, display, and save images using AIP for Windows. You will learn these specific skills...
Pick out your favorite CCD image (or at least an image you can find on your hard drive) and follow along with the steps outlined below.
Step 1: Starting the program
Start AIP for Windows. The program will load and display its introduction panel.
Step 2: Loading an image
Most image formats circa 2000 are supported. There are about 40 standard formats available. In particular, most CCD camera formats are supported, plus the astronomical standard FITS is supported. To open an image follow the following path through the menus.
File | Open Image...
A dialog box will open. Use this box to navigate to the image you wish to load. Then click Open. The image should be displayed along with the Display Control window.
Step 3: Auto-Stretch
If the image appears all black, click the Auto-Stretch checkbox on the Display Control and click the Stretch button. The image should now display in a continuous range of gray tones. The auto-stretch feature automatically determines the range of gray levels best sutied to display an image by examining the histogram of the image. It then stretches the image linearly to display the greatest contrast. To "unstretch" the image, click UnStretch on the Display Control. The image should now appear all black. Click Stretch again and the image will reappear. By default, images are stretched when they are loaded to make them visible. The Auto-Stretch checkbox can turn the automatic stretching feature on and off. You can also change the default using the Preference dialog. It is important to note that none of the controls on the Display Control window affect the original image. The controls only affect the way the images appear
Step 4: Display Controls
Thje display control window contains three controls which give you complete flexibility as to how the image is displayed. The Min Pixel Value control determines the lower cutoff value. Below this value any pixel is displayed as black. The Max Pixel Value determines the upper cutoff value. Above this value any pixel is displayed as white. Between these two values any pixels will be linearily stretched to produce gray scale values between black and white.
The brightness and contrast of the image are controlled using these two controls. Try changing these controls and watch what happens to the displayed image. What values provide the best display? Does the Auto-Stretch feature provide the most reasonable results? When you are finished experimenting, click the Stretch button.
Step 5: The Gamma Control
Try moving the Gamma slider. The initial default value is 1.0, but a value of 1.8 is generally prefered for most computer monitors. The gamma value controls excatly how the pixel values between the Min and the Max values are mapped to the monitor gray scale values.
The Gamma control can make the stretch between the minimum and maximum pixel values nonlinear. The gamma value can either boost or surpress the lower values using power law scaling. This control can be useful for searching for faint objects in an image. Try changing the Gamma value for your image.
You can set a default value for gamma by using the following menus.
Preferences | CustomizeSome Background Note that the human eye can detect at most about 100 gray levels. Contemporary computer monitors can normally display 256 gray levels. That is, the computer is capable of displaying more information than your eye can utilize. Note also the a 16-bit digital image can essentially contain up to 65000 data numbers or gray levels so that a normal astronomical image can contain far more information than can be displayed by a computer monitor or actually seen by the human eye.
You can set the minimum and maximum data values to be displayed by using the Stretch controls. This essentially determines the brightness and contrast of the image. However many data values are specified by means of the Stretch controls, these must be converted into the 256 values that can be displayed by your monitor. This conversion is specified by the image map. The specifics of the image map can be controled by the specified Gamma value.
Step 6: The Zoom Control
Try clicking the Zoom control in the display control window. The image can be displayed as larger or smaller. You can also type in a desired zoom value. The available range for the zoom control is 25% to 400%. Try changing the Zoom for your image.
Zooming up large images can take time as the image is resampled. Zooming up will also use more memory. A default value can be set in the Preferences dialog.
Step 7: The Preferences Dialog
If necessary, reset the zoom to 100% and click the Stretch button to return the image to its original state. Then select these menus.
Preferences | Customize...
This will display the preferences dialog window.
This dialog will display a series of tabs across the top. For each of these tabs you will find sliders, checkboxes, and textboxes. We will mention here a few of the more important settings. For more details, press the F1 key while the mouse is in the preferences window. This will bring up the online help system. Help is context sensitive so the details of the help you get will depend on which window is active at the time you press F1.
Monitor Gamma You can set the gamma to match your monitor. Values around 1.8 seem to work best. Adjust this value while looking at a displayed image until the gray scale values appear evenly displayed. To make this easier you can display a test pattern. To load the test pattern use...File | New...When the Create New Image window appears, click the Grayscale Test button.
Auto-Stretch when Image Changes This is useful in that it automatically optimizes the image display whenever a new image is displayed.
Click the Save Preferences button in the preferences dialog window when you are finished setting preferences.
Step 8: Importing Images.
AIP is primarily an image processing and analysis system for astronomical images obtained from CCD cameras. The "native" format for AIP images is the raw CCD camera format for SBIG cameras (STx), Cookbook cameras, and FITS format images. AIP can read and open images in these formats. These are monochrome images that can be 16 or 32 bits deep. In other words, the images have only one channel of information... the pixel values.
However, AIP can also open and display color images which use formats such as JPEG and GIF. To display such images, the files must be imported.
File | Import...
When you import a color image the icon in the upper left of the screen is a color galaxy. When you import a monochrome image or read a raw CCD image or FITS image the icon in the upper left of the screen will be a grayscale galaxy. Many of the AIP tools will be disabled when a color image is imported. Before such an image may be processed using AIP it must be reduced to two or three monochrome images. This process is discussed in a separate tutorial.
Step 9: Saving Images
It appears that AIP can only save CCD images in FITS format. FITS is the standard image format in astronomy. If the image is changed in any way and the adjusted image is needed for subsequent analysis, the adjusted image must be saved in FITS format. For example, if flat field corrections are made, the flatted image must be saved as a FITS format image.
File | Save as FITS...
Once a destination directory and a filename have been selected the FITS Header window will be displayed. The standard extension for FITS files can be either .fit or .fts. Appropriate header information should be included with all images.
If a version of an image is required for display or for processing by non-scientific image processing packages such as PhotoShop, or PaintShop Pro, or NIH Image, the image should be exported. A standard format for such processing is TIFF. For web publishing, images can also be exported as JPEG or GIF format.
File | Export
The compression and quality for JPEG images can be adjusted from the Preferences dialog.
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