How deep with a 5" APO & DSLR?

Comparing my images with The Sky & Megastar, I think I'm seeing stars around
15.5 magnitude in this image... Anyone know offhand how deep a 5" refractor
should be able to go photographically? (I'm using a Nikon D100 DSLR if that
matters.) I'm also seeing other stuff, that isn't in the programs... Is that
noise, or deeper stuff that just isn't there, do you suppose?

Thanks in advance for any info,

-*** Locke
www.dl-digital.com


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