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Differential Magnitudes

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Figure C.7 The Multi-Image Photometry tool can perform differential photometry on hundreds of images, one right after another, automatically. With this tool, amateurs have detected planets orbiting other stars by measuring the slight dip in light when the exoplanet passes in front of its star.

tool will attempt to track these stars from image to image. As there is the possibility that the field will shift between exposures, it is necessary to identify a radius, centered around the C1 star, that will be searched to find the star centroid on each subsequent image in the set. This is done using the Track Search Radius control on the Tracking tab. For this image set, a value of 20 pixels accommodates the rather large drift between exposures without locking onto a neighboring star by mistake.

Step 6: Select the C and V Stars. Now you can select your V and C stars. For this tutorial you need to select the star at X = 215, Y = 102 (Z Draconis) for the V star, X = 112,Y = 65 for the first comparison star, C1, and the one at X = 335,Y = 98 for a second comparison star, C2. When selecting stars, the first one you select is always the variable, and the rest are numbered comparison stars. When you select each one, a bullseye is drawn showing the inner, middle and out er radii of the photometer aperture and a label, either V or Cn (where n is the number of the comparison star).

You can delete a comparison star by clicking on it a second time. The star will be removed from the list, and the other comparison stars will be renumbered. However, if you click the variable star a second time, the entire list will be cleared. You can also clear the list by clicking the Clear All Stars button on the Setup tab.

AIP4Win allows you to select up to 32 comparison stars, and will track each one. If a comparison star drifts out of the image, an error message is displayed at the conclusion of the measurement run.

Step 7: Set the Tracking Mode. The Tracking Mode radio buttons on the Tracking tab allow you to select the tracking mode. If the images are well tracked, select Automatic tracking. As the stars drift from one image to the next, AIP4Win finds them by searching one target search radius in all directions from the star's location in the previous image.

If the tracking error between images is greater than the tracking radius, select the Manual tracking mode. Manual tracking is cumbersome—it really pays to have a telescope with a good clock drive—but it allows you to do good photometry even though your telescope has poor tracking.

Although the star images wander quite a bit, automatic tracking will follow the stars in this data set, so click the Automatic radio button.

Step 8: Set the Target Tracking Mode. The Target Tracking Mode radio buttons control whether the V and C stars are tracked individually, or whether the CI star only is tracked, using a fixed offset to identify the V and remaining C stars. The Track C1 mode is for fixed targets such as variable stars. The Track C1 & V mode is for photometry of moving objects such as comets and asteroids. The Independent mode tracks every star individually, should that ever be necessary.

For this tutorial we are following the light curve of an eclipsing binary star, so the C Track mode should be selected.

Step 9: Start the Photometry Operation. Start the Multiple Image Photometry operation by clicking the OK button. You will be prompted for a location to save the photometry data that will be generated. Choose a directory and file name on your hard drive (not on the CD-ROM) for this purpose.

Each successive image in the set will now appear on the screen, the V and Cn stars will be found and marked, and their relative magnitudes logged. When the operation has finished, a graph is displayed showing data plots for the differential magnitude (V - CI) and for the difference between the first two comparison stars (C2 - CI), to provide a quick check of your data.

In thius exercise, you have measured the difference between the variable and comparison stars to find the moment when the variable was in maximum eclipse.

Step 10: Import the Data into a Spreadsheet. When the tool has finished, you will be left with a report file in the Data Log that, like the output of other tools, can be imported into most of the popular spreadsheet programs.

Chapter 10 explains how astronomers use standardized color filters and care-

Astronomical Image Processing for Windows

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