Introduction to Image Processing and Aperture Photometry
In this lab exercise you will learn to do some basic image processing. This will include becoming familiar with the user interface to the Mira 6 software, loading and manipulating images, and measuring star brightnesses (Aaperture photometry@) on the image. As an example, you will determine the magnitude of a supernova that occurred in 1998, in the galaxy NGC 3877. You will use an image acquired at Appalachian State University=s Dark Sky Observatory, when the supernova was very near its peak brightness. You will then use the known absolute magnitude of such supernovae to calculate the distance to it and its parent galaxy.
In a later lab you will use the software to measure the brightness of some variable stars, and use the result to compute the distance to the globular cluster containing the variables.
1. Opening the image: To view an image you left-click on the open folder icon on the main Menu bar at the top of the screen, above the main tool bar. Click on the Images folder under Mira and then double-click on ASN1998SV@ to load the image, an image taken with the 32-inch telescope at ASU=s Dark Sky Observatory.
The image will appear in a main window and at the top of that window will be two small images and a small toolbar. The left image is a magnified view of the region outlined by the rectangular cursor on the main image. The right one is a small >thumbnail= of the whole image showing the current boundaries of your main window=s view. Also, a Magnifier Toolbar appears beside them, that has some buttons we will use.
2. Cursor and magnifier: To see parts of the image outside the window, click and drag the buttons on the slider bars at the bottom and right edges of the image window. You can re-center the magnified view on any particular point by clicking on the button at top left that shows two black, crossed arrows another and then left-clicking on the point of interest in the viewBtry it. To see the whole image, left-click on the little magnifier glass icon and then right-click somewhere on the image and it will de-magnify (a left click would further magnify it.) The supernova is the brighter star in the galaxy visible on the left side of the image. Experiment with the magnifier and the minimize/maximize buttons on both the image and Mira title bars to see how to get different size displays.
A variety of other image processing and display functions are available by clicking on one of the buttons at the top left of the image window. Note that by moving the cursor to any of the buttons and holding still there for a second brings up a small window that tells you what that button does. Try some of these.
3. Changing the appearance: the Transfer Function
. Click on V
iew on the menu bar and then click on T
ransfer Function. Another little window will appearByou will likely want to move it to one side by clicking on its title bar and dragging it away from the image. Slide the controls on the brightness, contrast and Gamma (γ) to see their effects on the image. You can also grab the little blue sliders and move the limits on the displayed brightness to change the appearance of the image. Try some different settings to see the effectBthis is like the >stretch= function in the CCD camera control software. You can also experiment with changing the actual mathematical >transfer function=Bthe relation between the brightnesses in the image data and how they are displayed, by clicking on the AGamma Power@ menu item displayed and choosing another function to try.
Note that the >Reset= button will bring you back to the basic display you started with!
4. The Palette.
Click on the artist=s palette button (or click View/Palette Manager), to bring up the Palette manager window. Drag it aside so it is off the image. Experiment with different >false color= palettes by double-clicking on their names (>colorful=, etc.) and adjusting the brightness
, contrast and gamma. Note how the eye can detect subtle variations in the image more easily with color changes than just changes in monochrome contrast and brightnessBthis is the utility of so-called false color
When you are done experimenting, return the image to Grayscale and close the Palette Manager and Transfer Function windows the way you close any window
, by pressing the in the upper right corner of the title bar for that window.
5. Cursor statistics
Move your cursor back onto the image. If it is (still) a magnifier then click on the little red arrow on the Image Toolbar to go back to the cursor mode. Note the square cursor box in the centerBthis is moved by dragging it with the left button on the mouse. Grab it and move it around to see how this works. Drag it to a star and release it there. Then, click the Cursor stats button (Σ) to see what is displayed (you will have to click on that image to bring it completely visible). These are the statistics on the values of the light at pixels within the cursor box (average, minimum, max, and their locations). Close that window.
Drag the cursor box to a star (not the one in the galaxy, but another) but leave the star a bit off-center. Then click on the button that looks like a bull=s eye ( ) and note what happens to the cursor box. Try it again. It should centroid
the starBcenter it in the cursor box (It may not appear exactly centered since it centers on the maximum brightness.) A window will appear with some information on the centroid resultBthe location in pixel coordinates and the peak brightness used to center.
Let=s see what a star image >looks like= by plotting its profile. If the cursor is not still on a star
, then drag the cursor box to a star and centroid it. Then change the cursor from the rectangular box to a crosshair by selecting Crosshair from the Cu
rsor menu bar entry. Then click the Horizontal Cut button (with the red, horizontal, zig-zag graph.) A plot appears showing the bell-shaped profile of a stellar imageBthis is a plot of the intensity of the image from the left edge of the cursor to the right edge.
Close the Horizontal Cut window and click on the FWHM buttonBthe right-most on the main toolbar at the top of the page (looks like a red bell curve.) A new window will appear giving the FWHMBthis >Full Width at Half Maximum= is a measure of how good the seeing was that night. The FWHM is the full width of the bell curve half way down from its peak. To see how Mira fit a curve to the profile of the stellar image in order to figure this out
, click on the button showing a small graph with a red curve (the button just left of the artist=s palette).
One unit in pixels is about half an arc-secondBwhat was the FWHM in arc-seconds for this image? Record that value on your Data Sheet. Close the FWHM dialogue box.
9. Aperture Photometry:
Measuring star brightnesses. To measure the brightness of star images in MIRA click on Measu
re on the menu bar and then click on Aperture P
hotometry. A new Aperture Photometry toolbar will appear on the left of the image. In order to measure the magnitude of the supernova we need to first identify some standard stars for comparison
, and tell Mira what their magnitudes are. We do this by clicking the AP*
@ (top) button on the Aperture Photometry toolbar. Then, one at a time, click on the comparison stars on the image
, as identified on the Finder Chart (on the next page), and enter their magnitude in the AStd.Magnitude@ part of the small Standard Data dialogue box that opens each time (click Ok after entering the magnitude.) Mira will auto-centroid each time you click on a star. Note that an AAperture Photometry@ window will open and add info as you add each star.
When you are done entering the standard stars, click on the AP@ (without the asterisk) to add the unknown Object (as object 5 in this case, where objects 0-4 are the Standards), and add the supernova. Then click on the little calculator button on the toolbar to make Mira use the standards and compute the magnitude of the supernova. It will display a small Photometric Calibration windowBjust click Ok. Note that it changes the magnitudes of the standards
, too, since it fits a line/curve to the standard data and (like you may recall from the Solar Rotation lab!), no individual point will likely lie exactly on that line. Record the magnitude of the supernova on the data sheet.
10. To calculate the distance (d, in parsecs), we can use the equation
m = M + 5 log (d/10)
where m = your measured apparent magnitude
M = absolute magnitude = -17.9 for this supernova
To use the equation, rearrange it as follows:
m - M = 5 log (d/10)
log (d/10) = (m - M) / 5
d/10 = 10 (m - M) / 5
Use your value of m, the M = -17.9, and solve for d/10 . Then multiply by 10 to get d. You can change this to light years by multiplying by 3.26ly/pc .
Finder Chart with Standard Magnitudes
AST 1001/2 Name
Intro to Image Processing
FWHM for the SN1998S image:
magnitude for SN1998S m =
Calculation of distance of SN1998S:
Distance in light years: