Doing Time Series Photometry
This tutorial describes how to use the Aperture Photometry package to do time series photometry on a set iof images of the minor planet Xanthippe. In this tutorial we use observations of the minor planet Xanthippe to place limits on variations in its brightness over a short time period. In minor planet studies, these variations are used to characterize the rotation or tumbling of the body. The same methods could be used for analysis of variable stars, to search for stellar eclipses caused by extra-solar planets orbiting stars, or for other types of investigation.
We begin by opening an image set containing 9 calibrated images of the minor planet Xanthippe. First, click File > Open on the main menu to open the Open dialog. Now click on the first image of the image set, hold down the [Shift] key, and mark the last image of the image set to highlight all 9 target images as shown below (see Opening Multiple Images).
Make sure the Open as Image Set option is checked at the bottom of the Open dialog, then click [Open] to open the images. They appear in an Image Window like this:
Next, click the on the
main toolbar to open the aperture photometry package and its
toolbar. On the toolbar, click
to open
the Aperture tool
window for setting the aperture parameters. After setting the radii
and other parameters to the desired values, close the Aperture tool
window. Here we used circular apertures with radii of 4, 13, and 23
pixels. These values are listed at the beginning of the results in
the Photometry Messages window (see below).
On the aperture photometry toolbar click to select Target Mode and mark the target object on
the first image. Also, mark a check star to be used to monitor the
stability of the photometry from image to image. Then click
to select Standard Mode and
mark the 3 standards on the first image. Each time a standard is
marked, you will need to enter its magnitude (and possibly other,
optional information) into the Standard Data
dialog as shown below.
In the Standard Data dialog we also added a comment for this object and changed its weight to 0.1 of that used for the other standard stars. The minor planet moves close to this star in the last 4 or 5 images, hence the profiles may be blended.
The Image Window now should show 4 objects as shown below. Object 1 is the minor planet Xanthippe.
Next we want to extend the marked objects from
image 1 to all images in the image set. To do this, click on the toolbar. Next, calculate
everything by clicking
. This
produces a verbose listing of the standard star measurements and
the photometric zero point calculations in a Text Editor window. The
picture below shows the results for the 9-th image.
Next we will plot a light curve from these
observations. On the Aperture Photometry toolbar click to open the Plot Light Curve dialog.
In the Targets field, enter the number 1 so that
object 1 is plotted. Uncheck the Standards and
Check
Stars boxes so that the plot will auto-scale to only
the minor planet we measured. These observations were made over the
course of a night so we could make a plot using "Time (Hours)" or "Time
(Minutes)" on the Horizontal Axis. Instead, we will select
"JD - 2453000" from the drop down
list. The JD option takes its name
from the column title in the Photometry Measurements table. A Julian Date
Offset of 2453000 was entered on the Other
Preferences page before the photometry was done.
Click [Plot] on the Plot Light Curve dialog to produce the following plot containing magnitude error bars:
The error bars here are relatively large because the standard star magnitudes we entered were not nearly as precise as the actual uncertainties of the measurements. Therefore, the uncertainty in the calculated photometric zero point dominates the uncertainty in the measurements of the target object, Xanthippe. You can see this effect by inspecting the standard star data listed in the Photometry Messages text window (see above). For image 9, the zero point value is listed as 21.4819 +/- 0.0049. This value combines the scatter in the standard star magnitudes (see the Residuals column) and also the "random" error for each standard star measurement itself. The random error for each standard star is listed in the Error and Error(T) columns. Notice that the star "Mira-4", which is object 4, has a large random error that is caused by becoming progressively more blended with Xanthippe starting around image 5 of the image set. We set its weight to 0.1, so that object 4 would have only a small effect on the photometric zero points.
Aperture Photometry Preferences, Tutorial: Introduction to Aperture Photometry, Import Photometry Catalog, Photometric Measurements, Aperture Tool, Report Windows, Photometry Keywords, Using Edit Mode in Aperture Photometry, Preparing an AAVSO Report, Plotting a Light Curve