[von Tom (Tom)]

I remember many years ago at 16 (in 1987) trying to see Sirius in daylight with my trusty 75mm Tasco 3T-RB Newtonian on alt/az tripod.

I figured that if Venus at mag -4 could be seen with the naked eye in daylight, then the daytime sky must take about 10 magnitudes off the apparent brightness of anything else (ie Venus' -4 becomes the equivalent of mag 6 or so for daytime viewing). With that in mind, and because my 3 inch scope had a limiting magnitude of 11.5, then Sirius at -1.46 would become equivalent to mag 9.46 through the scope during the day. Theoretically! Still not sure if that's a valid way of looking at it.

On to the search for Sirius. I thought if I pointed it at a star, the night before, with the same declination and left the telescope outside (covered over) for the length of time that the Right Acension between the two stars was different, I would see Sirius the next morning. It worked a treat and it was an amazing sight to see another sun in the daytime sky.

If the 10 mag difference between night and day is (very!) roughly true, then a 12" scope should be able to see down to 14.8 then Mag 4.8 should be possible. I have seen Io, Europa and Ganymede which were just visible so maybe there's something to it. In an image I could see 20 Psc (at mag 5.5) just barely. Also in my 50mm finderscope Jupiter is just visible at mag -2.2.

Anyway, this probably belongs in the observing forum! : )