If one stacked many Barlow lenses/used several focal extenders, what would the resulting image look like?
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$\begingroup$ Well it depends on the telescope aperture, but likely the image would be very blurred. $\endgroup$– amateurAstroOct 16, 2022 at 17:34
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There is two effects which will happen:
if you magnify beyond the useful magnification you are essentially magnifying the aperture's diffraction pattern (thus the airy disk for circular apertures without obstructions). Thus you make the light sources (or rather everything) larger, but you don't add resolution - which will result in a blurry impression. That's similar to zooming-in to a pixelized image: you don't get more resolution than the pixels, irrespective of how large you make or display those. See also e.g. here for some numbers based on the f-number (ratio of aperture to focal length).
every surface in the optical path (a lens has two, a mirror one) adds optical errors due to manufacturing limits in smoothness and curvature. Additionally you add some degree of scattering in impurities within the lenses glass. Both effects reduce the effective contrast and thus the resolution slightly, too.
answered Oct 16, 2022 at 18:09
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1$\begingroup$ Another point to add is that the object gets dimmer as the magnification goes up, as the same light is spread over a larger area. $\endgroup$ Oct 17, 2022 at 13:42