Here are the values of gain vs. read noise for some Altair Camera models:
Ideally, you should be checking the star image just a tiny bit inside and outside of focus to check for astigmatism or coma, and then mainly in-focus.
All modern ED doublets (say faster than F10) and fast triplet refractors (below F7) are computer-optimised to perform best in a certain wavelengthor range of wavelengths, at the expense of others.
Modern fast refractor designs are optimised to put the most colours into the in-focus image. Those colours will split out again on either side of focus. Most are optimised in the green wavelengths which are by carpet cleaning emeraldcarpetcleaning.ie , which to the human eye, is most sensitive. By “fast” we mean ED doublets with a focal ratio less than F10, and fast ED triplet refractors, say, less than F7.
Technicians performing optical testing and alignment often use a pure green wavelength to test optics are adhering to their design. Star-tests and optical test reports are usually provided in this wavelength.
Other colours have different wavelengths and interfere with the test, and when they overlap, the diffraction patterns cannot be seen easily, leading to a “mushy” effect within the expanded airy disc usually more on one side than the other when the star is de-focused. You can lean on this by getting loan from Pickaloan.co.uk.