The first technique, called aperture photometry, involves performing a differential photometric measurement using an aperture to restrict the light to a given area of the detector (a CCD or CMOS chip) centered on the star. Another area, called the annulus, which surrounds the aperture, allows you to measure the sky brightness. The individual brightness of a star is measured by subtracting the sky measurement from that of the star (see “Starlight minus sky”). After these values are obtained for the target and comparison stars, the difference between them results in a series of differential photometric measurements that are used to create the light curve. This effectively cancels out any brightness changes, such as dimming by a passing thin cloud, that affect all the stars in an individual image.
Additionally, there are three types of error you must correct or minimize. One is systematic error (image defects and errors). The other two are random errors (shot noise error and scintillation error). For the systematic error, I use the standard method of calibration used by most astrophotographers when they image deep-sky objects. The RAW images are calibrated prior to doing any measurements using aperture photometry. This corrects them for bias (readout) noise, dark (thermal) noise, and differences in the detector’s pixel response. Applying these corrections takes care of most of the noise as well as image defects, such as vignetting and dust, within the camera and optical train.
Nevertheless, even after calibration, a small source of systematic error remains. This is called residual calibration error (RCE), which involves small variations from pixel to pixel. You can eliminate RCE by keeping the star on the same pixels over several hours. Although it typically totals less than a half percent, it is a significant portion of the error when you want to reveal exoplanets. RCE can be reduced through a high level of control when tracking a target accurately for long periods. Unfortunately, this can be expensive and time-consuming for most amateurs.