The example below illustrates the basic steps of
doing aperture photometry on point sources. The commands used to
execute the tasks below are located on the Aperture Photometry Toolbar.
The first image below shows 3 objects marked.
Objects 1 and 2 are target objects and object 3 is a standard star
with the label 3*. Objects 1 and 2 were marked using the standard
Marking Mode, enabled by clicking on the
toolbar. Mira tries to get the photometric zero point from a
ZERO-PT keyword in the image header. If that keyword exists, the
values listed at the time you click on the object will be
instrumental magnitudes that include the zero point value.
Otherwise, if there is no ZERO-PT
keyword, raw magnitudes will be listed. They will be very negative,
like -5 or -12. When the zero point is calculated (see below),
these values get updated using the new zero point and they will
change to values more like 17 or 11.
Object 3*, the standard star, was marked on the
image after clicking to enable Standard
Star marking Mode. Auto-centroiding was enabled from thePoints
Preferences page, so the positions were locked onto precise
object positions. When a standard star is marked, Mira requests the
standard magnitude for use in computing the photometric zero point.
The zero point is not calculated until you click the (calculate) button.
On the Other Preferences page of the Aperture
Photometry Preferences dialog, the Verbose Messages option was
checked. As a result, Mira listed the zero point calculation
results in a Text
Editor window like that shown below.
Each new calculation and some other massages add to
this window. You can add your own comments, delete text, etc. to
keep notes of the procedure. This information is also listed for
each image in the table on the Calibration
Preferences page of the Aperture Photometry Preferences
dialog.
After Mira calculates
the photometric zero point for each image, it adds that value to
the header of each image for which it is calculated using the
ZERO-PT keyword. If an image is in FITS format, the zero point can
be saved with the image. If it is saved, then Mira will use it for
that image the next time you do photometry on it. Of course, you
can always add new standard stars to refine the zero point for the
image.
The magnitudes and other measurements are listed in
the Photometry Measurements report. Each Image Window with a
photometry procedure active has its own report window that holds
the measurements for all of its images. You can tell which report
goes with which window by noting the identical Serial Numbers (the
number x in the "[x]" at the beginning of the window
caption).Notice that the apertures in the first figure above
picture are elliptical, not round, and they do not appear to match
the shape of the star images. The apertures were made intentionally
elliptical to illustrate that capability of Mira for the case when
the objects really are not round.
The figure above shows the small Aperture Tool window which
is used to set the aperture sizes, shape, and orientation. This
floating tool window is opened by clicking on the Aperture Photometry Toolbar. [more]In this
example, the Auto check box is
checked. This setting causes a recalculation request to be sent to
the target Image Window each time you release the mouse after
changing a setting. This then causes all measurements to be
recalculated for all images in that window. In addition, all items
in the Photometry Measurements Report are also
updated.
The Photometry Measurements window below
shows 12 of the 22 columns in the measurement table. You can
manipulate the table in various ways as for any Mira Report Window. For
example, you might want to save the results to a text file or copy
it to the clipboard for pasting into Microsoft Excel.
Finally, if a zero point value was calculated
during this photometry session, the zero point for each image was
saved in each of the image headers. However, this is not yet a
permanent change to the images since they may not yet have not been
saved to a file. You can save the images using one of the Save commands
or the related commands for saving an Image Set.
on the
toolbar. Mira tries to get the photometric zero point from a
ZERO-PT keyword in the image header. If that keyword exists, the
values listed at the time you click on the object will be
instrumental magnitudes that include the zero point value.
Otherwise, if there is no ZERO-PT
keyword, raw magnitudes will be listed. They will be very negative,
like -5 or -12. When the zero point is calculated (see below),
these values get updated using the new zero point and they will
change to values more like 17 or 11.
to enable Standard
Star marking Mode. Auto-centroiding was enabled from the
(calculate) button.
After Mira calculates
the photometric zero point for each image, it adds that value to
the header of each image for which it is calculated using the
ZERO-PT keyword. If an image is in FITS format, the zero point can
be saved with the image. If it is saved, then Mira will use it for
that image the next time you do photometry on it. Of course, you
can always add new standard stars to refine the zero point for the
image.
on the 