How to size the aperture (Generic)

(This content is pretty generic in that it can be used with any of the tutorials for aperture photometry software)

Aperture Photometry: Setting the aperture size
In order to compute magnitudes for stars from images, the software needs to know which pixels of the image should be considered to belong to a certain star. This is not trivial, because to get better photometry results, you often want to spread the light of a star (especially for bright stars) across many pixels by slightly defocussing, so the star in the image often will be a disc of several pixels across. Just how many pixels is something you can tell the software.

Even thoses pixels will not contain the light of only this one star, but also some "background" light coming from far away stars from the same direction or from the glow of the athmosphere in the night sky.
To compensate for this, you will also show your software a patch of the sky near the star that represents a starless sky background. The software can then compute the general background brighness in that part of the sky and subtract that from the brightness of the star pixels, so that only the real starlight is measured in the end (to a good approximation).

The easierst way to define the "star-pixels" and the "background-pixels" in one go is via an "aperture" consisting of three concentric circles:

...screenshot....

The inner circle defines the area where the star has to fit in. The pixels inside this area must contain all (or at least almost all) light from the star. It should contain a bit of extra sky, tho, but never a second bright star.

The outer ring between the middle and the outer circle defines the area that is considered to be "sky background". As a rule of thumb, the radius of the outer ring should be about the same as the radius of the inner circle, but on the other hand it should not be so big that stars near any of the stars that you measure will be included in this ring. 

The area between the outer ring and the inner circle is just there to separate the two areas, pixels in this area are ignored. You will set one aperture to fit all stars (variable and comparison stars) that you will measure in an image.

This picture shows an example where the inner circle is set too small, as bright pixels of the star image lie on the outside  of the inner area.

...example....

This picure shows an example where the outer ring is too big: It's so huge that a bright star falls within it.

..example....

Now this looks  right: 

..example....

As long as you are working with the same camera, lens, and focus setting, you need to make this decision only once and use the setting all over again for your measurements. When doing the measuremts, you use the aperture like a reticule to "capture" the light of the star you want to measure.

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