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Maximum Star Counts for various binoculars

Started by Barry Simon, 04/15/2003 06:00AM
Posted 04/15/2003 06:00AM Opening Post
Below is only a small part of the complete article found in the Files Section of the BinocularAstronomy Yahoo Group on potential star counts with binoculars based on field size and magnitude reach. Check out BinocularAstronomy for the full article.

(My apologies if the chart does not format correctly upon posting.)

Star Counts for Binoculars and Rich Field Telescopes

BINO MAG LMT STAR LMT(Total Sky) FIELD SQ.DEG/SKY(Averaged) STARS/FIELD

7X35 10.4 450,000 9.3 deg 68 742

8x42 10.8 600,000 8.2 deg 53 771

10x50 11.2 7 50,000 6.5 deg 33 600
10x50 11.2 750,000 5.0 deg 20 364

12x60 11.6 1,000,000 5.7 deg 25.5 618

16x60 11.6 1,000,000 2.8 deg 6.2 149

15x70 11.9 1,600,000 4.5 deg 15.9 616

16x70 11.9 1,600,000 4.0 deg 12.5 485

20x80 12.2 2,000,000 3.5 deg 9.6 465

20x100 12.7 3,000,000 3.0 deg 7.0 509

25x125 13.2 4,000,000 2.5 deg 4.9 475

30x150 13.6 6,000,000 2.2 deg 3.8 553

(It is interesting to note that star counts stay within a relatively narrow range with binoculars with maximum fields, typically between about 450 and 750 stars per field. As magnitude reach improves, field size decreases keeping the star counts close in spite of increasing aperture. The peak count was seen with a pair of wide angle 8x42 pair of binoculars; so I guess you could say it qualifies as the top "Rich Field" Binocular. Standard angle binoculars really suffer when it comes to star counts because of their dramatically small total field size area)

Some telescopes added to the mix -

SCOPE MAG LMT STARS Poss. FIELD Sq Degrees STAR CT

80mm f/6.25 12.2 2,000,000 4.8 deg 18.1 878
(35mm eyepiece and 14.3x mag)

152mm f/5 13.6 6,000,000 3.1 deg 7.6 1105
(35mm eyepiece and 21.7x mag)
(more than double the star count as compared to equivalent power binoculars)

Barry Simon
Posted 04/15/2003 10:22AM #1
Barry,

Are these counts suposed to represent someone's actual data?
Or are these counts simply fov area / celestial sphere area multiplied times theoretical limit of stars for a given mag.

1st the mag limits. Aren't these the mag limits determined for various aperture sizes? If so, then they are only achieveable at optimum magnification needed to obtain maximum resolution. Maximum resolution usually occurs for any particular objective lens at around e.p. 1mm to 0.75mm, or in other words for a lens of 100mm at a mag of about 100x to 120x, or for an 80mm lens at about 80x to 100x. Magnifications at less than optimum will not achieve these magnitude limits and in fact at typical binocular magnifications will only achieve limits about 1.0 to 1.5 magnitudes less. Right off the bat, doesn't each magnitude double the potential number of stars. Take for instance the mag limit of 13.2 for a 125mm lens. My excellent 5" SCT is just barely capable of seeing the 13.2 mag star at the edge of the Ring nebula under the best of conditions and then only at high magnification. Anybody out there with a 125mm binocular at 25x ever see the 13.2 mag star on the edge of M57?

This table fails to account for the increase in the number of stars seen in a constant aperture while increasing magnification. The difference between 12x and 16x can be anywhere from a 15% to 30% increase while keeping aperture constant. Limiting mag at 16x60 is fainter than limiting mag at 12x60.

Not related to the difference in the counts between the scopes above vs the binocs above as those differences are clearly just related to fov. BUT, two things the assumptions for the scopes are also inccorrect because they are not at optimum magnification and Star counts for binoculars will never equal maximum star counts for telescopes as they can never achieve optimum limits due to the low magnifications in use. This goes right back to the under-utilization of aperture in binoculars. Except for brightness, every other criteria by which we judge performance limits of objective lenses is under-utilized in binoculars due to the limitation of low magnification.

Seems to me a lot of inaccuracies in this data.

edz