Re: Magnification needed to split tight doubles


canopus56 wrote:
EdZ wrote:
<snip all>

Ed, on a different tack, do you have any sense of what the effect light
pollution has on the resolution of doubles? Many beginners and
intermediates are working doubles from their urban homes instead of
from a dark sky site.

- Canopus56

(P.S. - Maybe we should move this subtopic to a new thread.)

I suspect the answer lies in how far above the LM threshold the double
is.

Light pollution, for example difference between pretty good mag 5.5
skies and so-so/poor mag 4.0 skies, may not totally prevent from
separating a double. Doubles, at least fairly bright doubles, are not
affected nearly as much as other types of objects.

If I were attempting that mag 8.5 double in good mag 5.5 skies with a
70mm or 80mm binocular, the binoc threshold would be about 2.5 mag
deeper than the stars. easy enough to accomplish.

If I were attempting that mag 8.5 double in poor mag 4.0 skies with a
70mm or 80mm binocular, the binoc threshold would be maybe only 1.5-2.0
mag deeper than the stars, now getting more difficult. However If I
were attempting that mag 8.5 double in poor mag 4.0 skies with a 100mm
binocular, the binoc threshold would still be maybe 2.0-2.5 mag deeper
than the stars, still easy enough.

An interesting story, and in keeping with the lowest power to split a
double theme here. I was out once observing Castor (4.2 arcsec) trying
to reach the lowest power and still see a clean split. I was using my
5" Celestron C5. Castor is a very bright double and this affects the
ability to achieve a good low power split. I had reached the lowest I
could go with a 26mm ep for 52x. A cloud bank rolled in and
obliterated the entire sky from view, but I look in my scope and Castor
still was shining thru the clouds. I exchanged the 26mm for a 30mm
Ultima at 45x and I was easily able to see Castor cleanly split. In
this case, with the clouds acting very much like light pollution, it
helped more than hindered on a bright double since it cut down on the
glare of the bright primary.

edz

.

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