[Ian Robinson]

Originally Posted by glenc

Ian, I was interested in your percentages for reflectivity. Do you have a web site for them?
Magnitude 11.5 with 65mm binoculars suggest that the formula is m = 9.5 + 5log D with D in inches.
That is the magnitude limit for stars of course not a WML for DSO.
My WML is defined as the magnitude where the observer only finds half the DSO available.
Herschel's 18.5" with no secondary mirror was about equal to a modern Newtonian 16.5" assuming 88% reflectivity for Al and 61% for speculum at 550nm.

Here : http://www.iop.org/EJ/volume/0950-7671/24
S Tolansky et al 1947 J. Sci. Instrum. 24 248-249 .

Abstract. The reflectivities of electrodeposited speculum (a copper-tin alloy containing preferably 45% of tin) and of speculum prepared by evaporation in vacuo have been determined over the range 4500-6500 A. The reflectivity of the freshly polished electrodeposited alloy containing 45% of tin, which is the ideal composition from the viewpoint of maximum resistance to tarnishing, varies from 63% at 4500 A. to 75% at 6500 A. After keeping for six months in a damp atmosphere the reflectivity decreases by 10% in the red region and by 2% in the blue region. Evaporated speculum on a glass base has a reflectivity of 68% at 4500 A. increasing to 78% at 6500 A. The higher reflectivity of evaporated speculum as compared with electrodeposited speculum of the same composition is attributed to the greater perfection of the evaporated mirror surface and consequent reduction in the amount of non-specular reflexion.