Perform differential photometry on an object moving across a field

Posted by karenacollins on Jan 27, 2015; 11:37am
URL: http://astroimagej.170.s1.nabble.com/Perform-differential-photometry-on-an-object-moving-across-a-field-tp240.html

First become familiar with how to perform standard differential photometry on a times series of images by referring to chapter 10 of the user guide here:

http://www.astro.louisville.edu/software/astroimagej/guide/AstroImageJ_User_Guide.pdf

To calculate apparent magnitude(s) of your target star(s), read this post on how to enter apparent magnitudes for the reference stars:

http://astroimagej.1065399.n5.nabble.com/Enter-apparent-magnitude-of-reference-stars-to-calculate-magnitude-of-target-stars-requires-AIJ-v3-1-td236.html

NOTE: AIJ v3.1.0+ is required for the enhanced single step mode needed to allow changes to apertures from one image to the next and to allow apertures to be repositioned by dragging to a new image location.

To perform either absolute or relative differential photometry on a moving target, it is important to define your moving target with the last aperture rather than the first aperture when setting up Multi-Aperture (MA). This is important because MA initially places all apertures after the first aperture relative to the centroided (if enabled) position of the first aperture, and then optionally centroids the other apertures. So, if the first aperture is moving, the other apertures can get displaced from the desired stationary reference objects because of the movement of the first aperture as it attempts to follow the moving object.

The first aperture that is placed in MA defines a target object by default, and all the following apertures define reference objects by default. However, the sense of target and reference apertures can be reversed by holding the <Shift> key when left-clicking to place an aperture. <Shift>-left-click inside an existing aperture to change it from a target to reference aperture, or vice-versa.

As an example, to place 4 reference object apertures and 1 target aperture for the moving object, perform the following steps:

1) Shift-left-click near the first reference object (enter a magnitude if that option has been selected) -> C1
2) Left-click near the second reference object (" " ")  -> C2
3) Left-click near the third reference object (" " ") -> C3
4) Left-click near the fourth reference object (" " ") -> C4
5) Shift-left-click near the target object -> T5

As long as the moving object in the T5 aperture does not move by more than the aperture radius from one image to the next, and if the centroid feature is enabled for that aperture (i.e. it has a plus sign in the center of the aperture - <Alt>-left-click an object to reverse its centroid setting), aperture T5 should track the moving object through the time series of images. However, if the moving object passes near a brighter object, the target aperture may "stick" to the brighter object and stop tracking the moving object.

NOTE: Disable the option to "Use RA/Dec to locate aperture positions" in the MA setup panel. Since the moving target has a different RA/Dec coordinate from image to image, the target aperture will not follow the moving target if this option is enabled.

If the target aperture will not automatically track the moving object, you will need to use MA single step mode as described in the next section.