Re: [Re: Binoculars & Mag]

Bjoern Gimle (b_gimle@usa.net)
14 Oct 98 09:17:23 MET DST

JRespler@surfnj.net wrote in reply to Vitor Sexto Bernardes:

> That partly depends upon what your naked eye mag limit is. Here, it's about
> 4.2.  8x56 binoculars go to mag 8 1/2 or a little fainter.  7x18 probably
> reach mag 7-8.
> 
> -- 
The other important factors are the magnification, and the ratio
between the binocular aperture (18 mm) and your eye's pupil, usually
7 mm for a young person, but 4-5 for an elderly.

For the 8x56, an old person uses only 8x4 or 8x5 mm = 32-40 mm
instead of 56 mm. In the case of 7x18, all healthy persons would
utilize the full 18 mm.

For 8x56, 4.2 + 5 * log( 8) is 8.7
For 7x18, 4.2 + 5*log(18/5) is 7.0 and 4.2+5*log(18/7) is 6.3

A neglected benefit is that the 'high' magnification of 7x18 increases
contrast by reducing the surface brightness of the sky background. 

Sky&Telescope May-98 had a program to include the effect of
atmospheric temperature, humidity, Sun's and Moon's elevation,
Moon phase and angular distance from Sun and Moon
(though all these values had to be pre-calculated).

See http://www.skypub.com/software/software.shtml#list (VISLIMIT.BAS)
(http://www.skypub.com/software/programs/vislimit.bas)
There was also a similar program for CCD's, CCDLIMI2.BAS.






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