I have seen that when we use a spectrometer for performing an Optics experiment involving a single slit or double slit to study , say the Fraunhofer Diffraction pattern or some interference pattern, we use Schuster's method to focus the fringe pattern on the focal plane of the eyepiece of the telescope. But, I can see the fringe pattern (just that it has formed, not with such high resolution like the telescope) by naked eye through the narrow slit(s), also.
So my question is:
Where does the interference pattern or diffraction pattern due to a single or double slit placed in front of a light source form, in this case?
Does it form at infinity, as the theory says; or at some other place? Like the diffracting edges of the narrow slit? Or the focal plane of the eyepiece of telescope? Or does it vary depending on how I look at it?
I couldn't find a proper answer anywhere.