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. Changing any
property causes all apertures already marked in the image to be
updated.
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.
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.
on the
Automatic vs Manual Changes
(The Update Checkbox and Button)
(auto-update) box next to
the 
button on the
