Aperture Tool

The Aperture Tool is a small floating Tool Window used to adjust the shape of the markers used in the Aperture Photometry package.

The Aperture Tool Window makes it easy to change the marker parameters. Doing so changes the amount of signal being measured through the inner aperture ad affects the measurement of the local sky background between the 2 outer apertures. In the description below, the term "marker" refers to the set of 3 apertures used to mark and measure each object.

Using the Controls

Aperture Size Mode

The top-most control is the Mode selection box. Mira provides two ways to express the size of the measuring apertures: directly in radius or as a scale factor that multiplies the FWHM of the image. Select Radius or Scale, and the values in the aperture size fields will change to the appropriate values.

Aperture Size

The upper 3 edit fields show the size of the measuring apertures. The units displayed depends upon the mode setting:

All aperture measurements equate to radius, in dimensions of [pixels]. The mode Radius or Scale (top of window) is specified on the Other Preferences page opened from the button on the Aperture Photometry Toolbar. Values can be set for each of the aperture rings. Actually, these settings control the semimajor axis of each aperture, since Mira considers apertures to be elliptical (Note: for a circular aperture, the semimajor axis equals the radius). From the top control downward, these 3 fields show 1) the inner aperture that measures the object, 2) the inner sky aperture, and 3) the outer sky aperture. Needless to say, these three apertures should have radii in increasing order.

Aperture Ellipticity

The control above the large round dial, changes the Ellipticity of the markers. Ellipticity is defined as E = 1 - b/a, in which b is the semiminor axis and a is the semimajor axis length. All 3 apertures have the same ellipticity, so they also change together. Setting a value of 0 makes the apertures circular.

Angle of Major Axis

The Angle dial orients the direction of the major axis of the marker. This is only useful if the Ellipticity is greater than 0, meaning that the apertures are not circular. The angle is always measured so that it increases from positive x through positive y (see Angle Measurement Definition), except that the astronomical 0 angle is taken along the y axis, which is north (up) if the image is a FITS format image displayed in the normal way.

If you have more than one Aperture Photometry process active, you can see the values in the Tool Window change when you click on the different windows. One useful result of this is that you can easily compare the effect of different aperture parameters by opening the same image(s) for aperture photometry in a number of separate Image Windows. Simply click on each window in turn to connect it with the Aperture Tool and then change the settings. This also shows why each Image Window is connected to a different Photometric Measurements Report window.

Related Topics

Aperture Photometry, Photometric Measurements, Other Preferences, Star Removal Package