Aperture Tool

The Aperture Tool adjusts the local properties of the apertures used in the Aperture Photometry package. The settings are used locally in the image window where aperture photometry is active. That is the window that shows the Aperture Photometry Toolbar from which you opened the Aperture Tool. The properties can be saved as global default using the button at the bottom of the tool.

Open the Aperture Tool by clicking on the Aperture Photometry Toolbar.

If multiple image windows are engaged in photometry, each has its own private Aperture Tool. The tool opens with the current settings for the particular photometry package.

Using the Aperture Tool

This tool changes both the local aperture properties for the current photometry session and the global default properties for future aperture photometry sessions. The aperture Photometry Toolbar opens using the global default properties.

Automatic vs Manual Changes (The Update Checkbox and Button)

The Aperture Tool may be set to automatically update changes to the image as you make them in the tool, or to delay all changes until you manually Update them.

To automatically update changes, check (auto-update) box next to the [Update] button. This updates the image immediately after you change any aperture parameter. It auto-update is enabled (checked) then the[Update] button is disabled. Unchecking this box enables the [Update] button and causes changes to appear in the tool but be withheld from the image until you click[Update] to apply them.

Setting Local and Default Aperture Properties

The [>> Default] button saves the current properties into default values for future Aperture Photometry uses. If you do not click this button, the changes apply to the current session (image window) but not to future sessions.

Aperture Size Mode

The top-most control is the Mode Selection drop box. In this example "Radius" mode is selected. Mira provides two ways to express the size of the measuring apertures: directly in terms of pixel radius or indirectly 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 either in terms of radius or scale. The units displayed depends upon the Mode setting:

Radius: The aperture radii are specified in units of [pixels].
Scale: The aperture radii are expressed as a scale factor which multiplies the FWHM of point sources in the image. The FWHM must have been previously measured for the image and the value stored in the image header as the PSF-FWHM keyword. See FWHM Measurements.

All aperture measurements equate to radius, in units of [pixels]. The mode Radius or Scale (top of window) is specified on the Other Properties 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.