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Tutorial: Introduction to Aperture Photometry This tutorial describes the procedure for making a simple photometric measurement using Mira's Aperture Photometry package. This tutorial assumes you understand how to display an image and do elementary operations like moving the image cursor. To get a quick start on displaying images, see Tutorial: Displaying an Image. A separate tutorial describes how to do Time Series Photometry. One application area for time series photometry is finding the time of minimum brightness for variable stars and other objects; for this application, see the Kwee - van Woerden Solver command. Getting StartedTo begin, click File > Open on the menu to open the familiar Open dialog. In the Open dialog, select the image shown below and click [Open].
The image opens as shown below, with the Image Bar at the top of the window. The yellow rectangle shows the part of the image used in this tutorial.
To get more space without showing the
Image Bar at the top of the window, right click on
the image and select Configure > Image
Bar from the menu to close the Image Bar (you can re-open it
the same way). Adjust the position and magnification to show the
field of view below (for help with magnifying, see
Magnifying the Image). On the main
Image Measurements Toolbar, click the
photometry button
Before measuring a star, we want to magnify a portion of the field so that it looks like the view shown below
Now click Preparing the Image for PhotometryIn this tutorial, we are measuring a single image, but Mira can measure any number of images displayed as an image set. Before starting photometry, it is important to know that each image contains information needed for calculating magnitudes and their errors (uncertainties). These quantities, stored in the image header, are the exposure time, electronic gain, and readout noise, and they are stored as the keywordsEXPTIME, GAIN, and RDNOISE. If these keywords are not in each image header, or the values are not correct, then you need to add or edit them. Mira provides a way to do this efficiently, even for many images in an image set; see Fixing Header Problems in Photometric Data. You also can have Mira substitute aliases for the standard keywords; see Photometry Keywords. Making the MeasurementNow move the mouse pointer to near coordinate (160,458) and click the left mouse button. Mira computes a centroid position near this point and draws a marker on the star near where you clicked:
The magnitude is reported with other information in the Photometric Measurements Report Window shown below. This Report Window lists more than 20 columns of data but we see only the first few in this example. To show more, you can adjust the data table using the scrollbar. See Photometric Measurement Definitions to learn more about this these quantities.
Adding a Standard StarIn the Photometric Measurements window shown above,
notice that the magnitude is a large negative number. This results
from the definition of magnitude as -2.5 log (flux) and the fact
that a photometric zero point has not yet been applied (see
Aperture Photometry). We will use a standard star to
calculate the zero point. On the Aperture Photometry Toolbar, click
Into the Standard Data dialog, enter the star's magnitude and a statistical weight. You can leave the weight at the default of 1. If you are doing "ensemble photometry", you may wish to use another weight value based on the magnitude or the magnitude uncertainty of the standard star. For the star you just marked, enter 12.5 for the magnitude and click [OK] to accept the value. The image window updates like this:
Notice that the standard star has an ID number with
an asterisk (*) beside it; this is used in the photometry report to
show that the measurement was made of a standard star. If the
magnitudes in the table are still negative after marking a standard
star, click
To see more data for these measurements, use the horizontal scrollbar. Two photometric error estimates are listed in the report: Error and Error(T). These are the empirical (measured) uncertainty and the theoretical uncertainty for the calculated magnitude. Mira also saves the photometric zero point and its uncertainty to the image header (see ZERO-PT and ZERPTERR keywords in the table of FITS Keywords), but these values are not saved to files unless you save the images using File > Save or an equivalent command. With the zero point saved in the image header, you can do photometry on another occasion and the magnitude measurements will automatically be convert raw to instrumental values with the zero point applied. Measuring More ObjectsNow let us return to measuring magnitudes. Click
Now click
Now that there are 6 measurements in the report table, you may wish to save the se results to a file or export them to other software such as Microsoft Excel where you might do further analysis. This simple procedure is described in Grid Controls. If you save the coordinates to a file, you can use the Import command on the Point Markers page to load the object coordinates at a future date. By now you have doubtless noticed the scrolling Text Editor Window named Photometry Messages. This window lists results of photometry calculations and gives warning messages. The contents of the message window can be edited, printed, copied, or saved to a file.
Changing the AperturesSuppose we did not like the aperture sizes and
shapes that were used for these measurements. These can be changed
using the
Aperture Tool Window. To open the Aperture Tool,
click Going FurtherMira can perform other useful operations that will enhance your scientific capabilities. Here are some examples:
Related TopicsTutorial: Doing Time Series Photometry Fixing Header Problems in Photometric Data
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to open the
on the main
toolbar. This displays the Aperture Photometry Toolbar on
the left border of the window (see picture below).
to enter standard star marking mode,
then click on the standard star. In this case, move the mouse onto
the star near coordinate (57,453) and click to mark the object.
This opens the Standard Data dialog:
to compute and apply the
photometric zero point. The table now has positive magnitudes.
Notice that the standard star you marked has the magnitude 12.5000
that you entered:
on the toolbar to switch to
on the Aperture Photometry Toolbar.
This tool window is a shared resource used by whichever photometry
window is currently active). When you change the aperture
description, Mira automatically re-measures and updates the report
table.