How do I force photometry on dimmer of two closely spaced stars

[Phil Evans]

How can I force the photometry aperture to remain on the dimmer of two very closely spaced, but resolved, stars?
As soon as I start the run the aperture jumps to the brighter one.

[karenacollins]

Hi Phil,

To solve the problem, you will need to place an aperture with centroid enabled on the brighter star (i.e. the usual way to place an aperture, it will show a "+" sign in the middle of the aperture indicating centroid is enabled). Then place another aperture on the faint star, but disable centroid first by clicking the centroid icon () above the image display (these apertures are just a circle with no "+" sign in the middle). After clicking the centroid icon, its background should show light gray rather than dark gray indicating it is disabled. You may even need to offset the aperture on the faint star to exclude as much of the bright star as you can to improve the photometry of the faint star (but you still need to include most of the flux of the faint star in the aperture).

If you need to place additional centroided apertures, you should click the centroid icon again to enable the centroid feature before placing the additional apertures.

Even though centroid is disabled for one or more apertures, as long as one or more other apertures has centroid enabled, the non-centroided apertures will follow the average movement of the centroided apertures from one image to the next. So, even if your tracking/guiding is not perfect, the non-centroided apertures will follow the image shift detected by the other centroided apertures.

Karen

[asteroidguy]

Dear Karen,

I have exactly this scenario, but for a faint satellite near a bright planet. I let the planet aperture follow the centroid, and position the satellite aperture relative to the planet aperture (centroiding off). However, I would need to define two different aperture sizes. Is there a way (through scripting, aperture settings files, etc.) to accomplish that somehow?

[karenacollins]

Unfortunately, AIJ does not currently support different aperture sizes for each aperture. The only way to potentially handle such at situation currently is to offset the non-centroided aperture relative to the faint object (if there is not another object in the way). Would something like the following work for your situation? It's not ideal since it captures more sky background noise than necessary, but seems to work okay for me most of the time.

I hope to someday have time to implement per-aperture aperture sizes, but the option should only be used for very specific cases since theoretically all point source PSFs (especially if near each other) are the same size in an image, no matter the relative brightnesses of the stars. However, sky background swamps for outer edges of faint star PSFs, so do I realize there are situations were this feature would be useful.

Karen