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By way of comparison:
Using the SimSpecV4 spreadsheet...
With a 350mm scope in suburbia, a 120min exposure, R=1000.
For a SNR=100 the limiting magnitude would be around +12mag
Go to a dark sky site and up the aperture to 500mm, we'd get to +13mag.
Go for an even larger scope, 1000mm, and the magnitude creeps to +13.7
OK, reduce the resolution to R=500, the same 1000mm scope would get to +14.7
If we reduce the SRN to 50, we could get close to +16mag.
(generally a rule of thumb is the limiting spectrum is about 5 mags higher than the imaging limiting mag...ie to get a spectrum of a 17mag star we'd need a scope (and conditions) to image a 23mag star...)
As you see, nothing is impossible, just very, very challenging!!
You can play with all the variables in the attached SimSpecV4.
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