Sir J. Herschel's definition mirror quality

[Brian W (Brian)]

Hi all, @1850 Sir J. H. wrote that if one could see '6' stars in the Trapezium then one had a quality mirror. Has optical performance greatly improved or is this still a good working definition of a fine mirror?

Brian.

[Robh (Rob)]

For today's larger amateur scopes like a 16 inch, it's too easy. The scopes of today have more accurately defined mirrors and better coatings so the test doesn't really mean much.
An 8 inch scope or bigger should easily pick up the six stars provided collimation is good and skies are reasonably dark. Skyglow and transparency are probably more important factors in discerning the two 11th magnitude stars.
A better test would be to see whether you can split a reasonably close double star in good seeing e.g. the system nu Scorpii, a double-double, with component magnitudes 4.4/5.3 separated by 1.2 arcsecs and 6.6/7.2 separated by 2.3 arcsecs. Can you split either pair. The two doubles themselves are easily separated, 41 arcsecs apart. Of course, the larger the aperture, the easier it is.

Regards, Rob

[Satchmo]

Bare in mind that the Speculums mirrors' reflectance was about 60% and with a speculum secondary brought the total transmission to about 30%. No AR coatings on the eyepieces either. It would have taken a fair bit of aperture to even see the 5th and 6th stars and the slightest mirror aberration they would have vanished.

The vast majority of Herschels scopes were around F13 and only had to be polished spherical although he was known to have made a 24" in later years of F4 focus. Incredible given that the only test available then (before even the foucault test) was to use masks on the mirror and compare the focus point of differnt parts of the mirror on a distant terrestrial scene.

[glenc (Glen)]

Herschels' 18.5" mirror was roughly equivalent to a 16.8" Newtonian in light gathering ability. It had no secondary.
I am assuming 63% reflectivity for speculum and 87% for aluminium.
Here are some numbers on reflectivity. <http://www.iop.org/EJ/abstract/0950-7671/24/9/308>

[pgc hunter]

Originally Posted by Robh

For today's larger amateur scopes like a 16 inch, it's too easy. The scopes of today have more accurately defined mirrors and better coatings so the test doesn't really mean much.
An 8 inch scope or bigger should easily pick up the six stars provided collimation is good and skies are reasonably dark. Skyglow and transparency are probably more important factors in discerning the two 11th magnitude stars.
A better test would be to see whether you can split a reasonably close double star in good seeing e.g. the system nu Scorpii, a double-double, with component magnitudes 4.4/5.3 separated by 1.2 arcsecs and 6.6/7.2 separated by 2.3 arcsecs. Can you split either pair. The two doubles themselves are easily separated, 41 arcsecs apart. Of course, the larger the aperture, the easier it is.

Regards, Rob

Yep the 5th and 6th stars are routine in both my 10 + 12" dobs, and I've seen atleast the 5th component at fairly low magnification in the 120ST.