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Jason,
Practical limits are difficult to quantify but CCD magnitude limits can easily be 5 or more magnitudes fainter than what can be seen visually.
Interestingly the C8 I have has an advertised "limiting stellar magnitude" of 14. In practice a good visual observer could see stars of mag 15 under good conditions with this scope.
As a quick test I tried the C8 under good suburban skies tonight with a cooled monochrome CCD camera. Because I wasn't guiding, I had to limit exposures to 45 seconds, and just took a series of photos and stacked them in IRIS. With 15 exposures, or about 11 minutes total exposure the stellar limiting magnitude was 19.5!
If I were to attempt this more seriously, I would use an autoguider make longer single exposures and with 2-3 hours exposure mag 21 would then be achievable. With an adaptive optics unit, a multi-night exposure, true dark skies you can go deeper still...
Terry
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