The magnification limit by Dawes' criterion limits the magnification to roughly the objective diameter, or better half of it, in mm, which 300x for your 12" mirror. Your secondary will give a further contrast limitation though, and a diameter of anything above 8" will suffer disproportionally from turbulence. So really from a resolution perspective your scope is really 8" or 200x, with further deteriorating factors (more turbulence in the daytime probably).
The shadow is caused by your daytime pupil being smaller so the light hole caused by the secondary can cover your entire pupil. At night this is effect is also present except the pupil is much larger so it does not get noticed. See Al Nagler's article in S&T,
https://skyandtelescope.org/astronomy-e ... ce-primer/
I just went to Nagler's articles at TeleView and they are very worth while reading:
http://www.televue.com/engine/TV3b_page.asp?id=154
He also has an article that calls the magnification rule that I just used a myth, check it out:
http://www.televue.com/engine/TV3b_page ... vice&id=86
... Henk.
Telescopes: GSO 12" Astrograph, "Comet Hunter" MN152, ES ED127CF, ES ED80, WO Redcat51, Z12, AT6RC, Celestron Skymaster 20x80,
Mounts and tripod: Losmandy G11S with OnStep, AVX, Tiltall,
Cameras: ASI2600MC, ASI2600MM, ASI120 mini, Fuji X-a1, Canon XSi, T6, ELPH 100HS,
DIY: OnStep controller, Pi4b/power rig, Afocal adapter, Foldable Dob base, Az/Alt Dob setting circles,
Accessories: ZWO 36 mm filter wheel, TV Paracorr 2, Baader MPCC Mk III, ES FF, SSAG, QHY OAG-M, EAF electronic focuser, Plossls, Barlows, Telrad, Laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50,
Software: KStars-Ekos, DSS, PHD2, Nebulosity, Photo Gallery, Gimp, CHDK,
Computers:Pi4b, 2x running KStars/Ekos, Toshiba Satellite 17",
Website:Henk's astro images