Limiting Visual Magnitude and Averted Vision

[chiltonstar]

The classic equation for limiting visual magnitude gives a value for a 127mm scope of mag 13.2 under dark sky conditions; does the equation refer to central field vision, or averted vision, which is I understand can see up to 3 or 4 mags better??

I'm asking because I think I can just about see the central star (Mag 14) in M57, which seems impossible on the face of it...... (may of course be age-related spots before the eyes).

Chris

SW 127mm Mak, 15mm Omni EP.

[Merlin66]

Chris,

The usual figures refer to direct vision. the star being held visible to the eye. Averted vision will give you that little bit more 0.5 mag or so.

[chiltonstar]

Ken

Are you sure the gain is only 0.5 mags? The Wiki article and refs claim up to 3-4 with training/experience.

Chris

[Merlin66]

When I was doing supernova searching with a 12.5" f5 scope, about 20Km from the centre of Melbourne ( 3 million pop.) I could get and hold 15.5mag visual. With adverted vision, checking the Thompson SN charts I could just glimpse low 15 mag stars...I'd say 16.0 was definately my limit. Hence the 0.5 difference. 3-4 mag is a lot of ask!!

Maybe others have managed better? Any takers??