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[snip]The only question left unanswered in my post is how are the stars and DSOs measured in terms of magnitude with such precision ....to decimal values...do they use some ****meters etc nowadays? How about the pre-electric/electronic times?

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Astronomers use photometric standard stars--stars whose brightnesses are known to about 0.01 magnitude--as references against which other stars are compared to determine their brightnesses. Commonly used reference stars include those known as Landolt standards.

The first catalog of Landolt standards was published in 1983. It included more than 200 stars along the celestial equator--visible from every observatory on the planet--and included stars ranging in brightness from approximately 7.0 to 12.5 magnitude. A second Landolt catalog, published in 1992, includes stars between 11.5 and 16.0 magnitude along the celestial equator. Others have extended the list of standards to include fainter stars at a wider range of declinations.

Standard stars had their brightnesses measured at multiple wavelengths using photomultiplier tubes--old technology that does a much better job of accurately determining the brightness of stars than modern CCD systems.

Here's a link to a page that talks about standard stars and has links to relevant original sources of standards: Photometric Standard Stars

One of the challenges faced when determining the brightness of extended objects is determining where the background sky ends and the object begins. Often, astronomers use a limit of 25.0 magnitude per square arcsecond as the cutoff for galaxies. Since this goes beyond what is typically seen at the eyepiece of an amateur telescope, the size of the galaxy you see is usually smaller than the published size of the galaxy.

One of the best known galaxy catalogs is the RC3 (Third Reference Catalog of Galaxies) by de Vaucouleurs, et al. This two volume catalog (1993, 1995) includes 23,000 galaxies, including most larger than one arcminute in size and brighter than about 15.5 magnitude in blue light. That translates to a magnitude limit of about 14.7 in visible (green) light.

My favorite online resource for galaxy data is linked, here: NASA/IPAC Extragalactic Database

Regards,

Bill in Flagstaff

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Grand Canyon Adventure
Lowering the Threshold

18" Obsession
4.5" Meade 4500
10x50 Swift Audubon

Cosmic Voyage

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Star catalogues from as recently as a decade or two ago now have out of date info as to precision of measured magnitudes. the Tycho Survey has thousands of stars measured to precision.

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