Aperture Photometry

The Aperture Photometry package is used to make photometric measurements of stars and other small objects using the aperture photometry method. This suite of commands is extremely versatile and can be used for measuring 1 or more objects, with 1 or more standard stars, in 1 or more images. Applications include techniques such as differential photometry, time series photometry, and weighted ensemble photometry. The results can also be used for all-sky photometry, but Mira does not compute or apply the extinction corrections and color transformations.

This command uses the Aperture Photometry Toolbar (see below). This is a standard command toolbar interface to a suite of commands. Aperture photometry results are reported in a standard Mira Report Window.

Getting Started with Aperture Photometry

Aperture Photometry Toolbar

Aperture photometry commands are operated from the toolbar shown below. Some commands are also accessed from the Preferences dialog, which is opened using the button on the toolbar.

The button calculates and/or applies the photometric zero point for all images where objects are marked, then updates the results in the Photometric Measurements table. This calculation uses all specified settings and all standard stars defined in all images. It is important to recalculate everything after making certain types of changes such as changing the weights of standard stars or making adding 1 or more standard stars to an image or you make changes to the GAIN, RDNOISE, or EXPTIME keywords for an image. If in doubt, click .Mira allows some flexibility in which keywords are used for the various photometry parameters. The keywords can be specified using the Photometry Keywords dialog. The airmass is also calculated if the appropriate keywords are found in the image header. The airmass calculation uses the first 6 keywords specified in the Photometry Keywords dialog.

The results of photometric measurements are listed in the Aperture Photometry Report Window. All reported values are defined in Photometric Measurement Definitions and Photometric Error Definitions. You can copy or save the results from this window, plot a light curve, or make a scatter plot of values in two table columns. For example you might plot the Error vs. Magnitude. If doing photometry of an Image Set, then you might plot Magnitude versus Julian Date.


The Image Window below shows an image with 2 marked objects, one a standard star and the other the target object to be measured. Each marker includes 3 apertures. The inner aperture measures the total signal from the object + sky, and 2 outer apertures define an annulus for measuring the local brightness of the sky background. The background is subtracted from the [object + sky] measurement to obtain the net signal attributable to the object. This is converted to a magnitude as described above and is listed in the Photometric Measurements Report window. The Aperture Photometry Toolbar is shown on the left window border.

Additional Features

Automated Processing of Multiple Objects in Multiple Frames

Mira provides a semi-automation capability for measuring objects in multiple frames using the importing facility. In this case, you import the coordinates of the objects from a text file "database" and have Mira process the objects in all images of an image set. Object data may be imported in (column, row) coordinates or, for FITS format images having a WCS (World Coordinate System) calibration, in (RA, Dec) coordinates. See the Import Photometry Catalog command.

Related Topics

Aperture Photometry Preferences, Tutorial: Introduction to Aperture Photometry, Tutorial: Doing Time Series Photometry, Import Photometry Catalog, Magnitude Calculations, Photometric Measurements Report, Aperture Tool, Report Windows, Photometry Keywords, Using Edit Mode in Aperture Photometry, Preparing an AAVSO Report, Plotting a Light Curve, Kwee - van Woerden Solver