Open the Aperture Tool by clicking
on the Aperture Photometry Toolbar.
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.
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.
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 autoupdate 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.
All aperture measurements are made in terms of radius, measured in units of pixels. 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.
The [>> Default] button saves the current properties as default values for future Aperture Photometry tasks which open using the default properties.
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.
Aperture Photometry, Photometric Measurements, Other Properties