In practice, the aperture correction also depends on defocus. The interplay between aperture correction and defocus may be complex, since the optimal focus changes with field position. In general, we recommend that a minimum aperture radius of 2 pixels be used whenever possible, in order to reduce the impact of variations of the aperture correction with focus and field position. If the field is too crowded and a smaller aperture is needed, we recommend that users verify the validity of the corrections on a few well-exposed stars. The following section includes a discussion of aperture corrections as a function of OTA focus.
Open symbols indicate spurious data.