The Aperture Photometry Tool is a FITS file interface that concentrates on analyzing the amount of light emitted by stellar objects. It is not a tool to be used to create color images. It can analyze the amount of light being emitted by an object after the light has passed through a color filter.
This is a file of the interface while in use.
Briefly, after opening APT:
Get Image (make sure it is FITS format)
Stretch (you can use a preset contrast or manually adjust using the sliders)
Set your apertures (to capture light from object and background. Use the Radial Profile to fine tune.)
NOW click on an object in the image in the upper right. (You will see it appear in the live window in the lower center.)
Check the coverage of the red centroid over the target object. You may need to adjust the apertures again to get the best fit.
Check the More Settings button. Model B is good.
Recompute Photometry (note any change to the settings or the apertures will cause this button to be red)
Save
List
Listed in the files below are directions specific to using APT for the analysis of an AGN.