[Wavytone]

Only because taking it to that extreme, you have spread the star over a large area - so its now an extended object, not a point. My initial response was based on stars remaining essentially point sources - as should be the case for most "imagers" (horrible word). If you increase focal length so that the Airy disk is 3 pixels or more this is no longer the case, and they become extended objects.

The photographic limiting magnitude of a local focal length, small aperture refractor IS indeed the same as the limiting magnitude of a wide aperture light bucket having the same focal length. The limitation however is the exposure time to reach it - which may be horrendously impractical for amateurs using low-cost portable gear.

This is the case even for large professional scopes - there are many examples where large scopes are used at high magnification with imaging sessions running for tens or hundreds of hours in order to reach faint objects at very deep magnitudes.

This is exactly how the Hubble was used to take the "deep field" shots, where the exposures ran for 42.7 hours at each wavelength, using high magnification (and high focal ratio). The wide-field camera working at low focal ratio cannot reach those magnitudes.