In this tutorial you will learn how to use the AIP measurement tools for evaluating images. Tools are available to perform these functions...
Results for all measurement functions are automatically written to a Data Log. This log can be reviewed during a measurement session and saved to a text file.
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: Reading individual pixel values
Open an image. After the image has been displayed move the cursor over the image to a point you wish to measure. Press and hold the left mouse button. The cursor will change to a cross. For the pixel where the cursor is positioned, the X and Y pixel positions and the pixel value will be displayed in the Display Control window. You can move the cursor and the readouts will update dynamically. When you release the mouse button the last values displayed will remain on the Display Control window. For color images the R, G, and B values will be displayed.
Step 2: Image Statistics
To obtain statistics for the entire image follow the following menu selections...
Measure | Statistics
The Info window is displayed. The information shown in this window includes...
AIP infers the pixel sizes in terms of the image format. AIP knows the chip characteristics for most of the contemporary CCD cameras used in astronomy. If the pixel sizes are incorrect they may be changed by using the Set Pixel Size button in the Display Control window.
The information that appears in the Info window is also stored in the Data Log. All image measurements are written to the Data Log. The Data Log is always running in the background and may be viewed (and edited) at any time.
Window | Data Log
The Data Log may also be cleared and may be savd to a file. The contents of the Info window and the Data Log may also be copied and pasted into a spreadsheet program for subsequent analysis.
Step 3: Image Histogram
The image histogram is the distribution of the pixel values in an image. To derive histogram data one merely counts up the number of times a particular pixel value occurs in an image for all possible pixel values. To view the histogram for your image follow the following menu path...
Measure | Histogram
This will generate a plot of the histogram for your image. What is plotted is the number of pixels for each possible pixel value. For reference the histogram window also displays the minimum and maximum pixel values in the image.
Histograms for astronomical images normally show a strong peak at the lowest values followed by a rapid decrease. There is generally a second much lower peak near the highest values. The large peak corresponds to the sky background and the smaller peak at higher values corresponds to the recorded stars. Any nebulosity in the image will make the large background peak highly asymmetric toward higher pixel values. Can you see these features in the histogram of your image?
Step 4: The Profile Tool
The Profile Tool will generate a plot of the pixel values for an entire row or an entire column. This can be useful for exposing detail that might otherwise be hidden in very bright or very dark areas of an image. This can also show the structure of an extended object, and can help distinguish between nebulous objects and faint stars.
Measure | Profile Tool
By default the Profile will display the row that passes through the center of the image. Use the slider to change the row that is displayed, or type in the desired row. Radio buttons are available to choose to display either rows or columns. Use the Profile Tool to explore your image.
Step 5: The Centroid Tool
This tool can determine the precise pixel position for an object. This tool can determine the centroid position for a star to within 0.01 pixels. This tool is the basis for all astrometric measurements.
Measure | Centroid Tool
The centroid Tool contains a slider and a text box for setting the size of the region used for computing the centroid positions. This size is the radius. This setting should be large enough to encompass all of the light from the object of interest. For an optimized CCD with good sampling, a value of 6 pixels is generally a good choice.
Move the cursor onto your image. Press and hold the left mouse button and the cursor will change to a circle with a cross. Drag this circle to the vicinity of a star and release the mouse button. The cursor will change back to the default cursor and the circle will remain on the image. Click the OK button and the X, Y centroid will be computed and displayed. If the circle was not centered on the stellar position you will see the circle jump to the centroid position. The centroid coordinates are written to the Data Log.
- Try selecting a series of stars one at a time for centroid measurements.
- Try changing the radius and see how this affects the results.
- Try making the radius large enough to encompass several stars and see what happens. (You will compute the "center of mass" for the star group.)
Customizing Tool PositionsNote that you can customize the default position where any of the tools appear. Once a tool has appeared, you can customize its default location by dragging the tool to where you would like it to appear. Then use...
Window | Save Current Tool Window Positions
The next time you access the tool it will appear at your prefered location.
Step 6: The Distance Tool
The Distance Tool uses the capabilities of the Centroid Tool to measure distances and angles between any two stellar centroids. Distances are determined in pixels in the image (also in microns and arcseconds) and angles are measured from the north direction on the sky.
Measure | Distance Tool
To insure that the computed results are reliable in units other than pixels you must make certain to specify two items. The Distance Tool provides text boxes for these two items.
Telescope Focal Length The telescope focal length must be specified in mm.Parallactic Angle This angle is used to specify the direction toward north on the sky in your images. This angle specifies the angular deviation from true north for a column on the CCD image. This angle must be specified as measured clockwise from the "top" of your image. If north is toward the right in your image, you would specify 90o. If you have carefully alligned your camera so north-south motion moves an object exactly along a column and east-west motion moves an object excatly along a row, then this angle must either be 0o or 180o.
Select a pair of stars using the cursor. When you press the left mouse button while the cursor is in an image the cursor will change to a circle just like the Centroid Tool. Keep the mouse button depressed and drag the circle to the first star of the selected pair. Release the mouse button when the circle is nearly centered on the star. Then click the Star 1 button. Move back to the image and depress the left mouse button again. Now drag the circle to the second star and release the mouse button. Now click the Star 2 button. The distance between the two stars will be displayed in pixels, in microns, and in arcseconds. The position angle between the two stars will be displayed in degrees.
The separation distance in pixels is always reliable. The separation in microns depends upon the presumed size of the pixels. The separation in arcseconds depends on the presumed size of the pixels and the presumed focal length of the telescope.
- Try changing the Parallactic Angle and see how this changes the position angle.
- Try changing the measurement order for the stars in a pair and see how this changes the position angle.
- Try selecting several other pairs of stars. Click the New Stars button to reinitialize the measurements.
All data accumulated by the Distance Tool is written to the Data Log.
Step 7: The Pixel Tool
This tool provides a means of gathering statistics for a region of interest (ROI) in an image. This tool is the basis for all photometric measurements.
Measure | Pixel Tool
The Pixel Tool can specify an inner radius and an outer radius in pixels. The ROI is specified by the region between the inner and outer radii. By default the inner radius is set to 0 and the outer radius is set to 6. Move the cursor to a region of blank sky without any stars or image defects. Click the Get Statistics button. The tool will display the mean pixel value and the standard deviation for the ROI. Since the signal to noise ratio (SNR) is defined as the ratio of the mean to the standard deviation, the SNR may be determined using these displayed values. Move the cursor around the image and sample these statistics at other locations.
Use the sliders to change the outside radius to 12 and the inside radius to 6. The cursor should now appear like a bullseye. The ROI is now the annulus between the inner and outer radius. This geometry is useful for obtaining information about the sky background immediately surrounding a star. Center this cursor on a star image that is not over exposed and does not extend beyond the inner radius. Click the Get Statistics button. Notice that measurements using the Pixel Tool do not automatically snap to a centroid star position. The measurements you get depend only on where you place the cursor. Determine the statistics for the sky background for several star images. How do these statistics compare with the statistics for the simple circular ROIs that were not in the vicinity of any stars?
Note that the shape of the ROI can be changed from a circle to a square. This could be useful for certain types of measurements. All data accumulated by the Pixel Tool is written to the Data Log.
Step 8: The Data Log
Examine the contents of the Data Log window.
Window | Data Log
Since all measurements are written to this log you should see the results from all the measurements you have just made. If you wish to export this information click the Save to File item. A dialog box will appear asking for for a file name and allowing you to select a location where the file will be saved. These files are saved as plain ASCII text and can be edited with any text editor.
To import this data into a spreadsheet such as Excel simply open the file as text with fixed-width columns. Data labels should appear in one column with the data in the adjacent column.
If you will be generating different types of data, you might want to save the Data Log after each type of measurement and then clear the Data Log before adding new data of a different type.
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