In sci.astro.amateur, Ioannis wrote:

"Abdul Ahad"
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

I see no reason why it shouldn't be precisely the same magnitude limit that
exists for the larger scopes.

Spectrographs usually have much less throughput than cameras; there are
losses due to the grating (or grism) and re-imaging optics. Furthermore,
the light is spread over many pixels, each of which has read noise and
dark current.

So you do lose substantial SNR with a spectrograph, compared to imaging.

Dave

David Whysong
DWhysong (at) physics (dot) ucsb (dot) edu