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PostPosted: Mon Jul 16, 2012 6:54 pm 
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Here is a Web site containing formulas for calculating your telescope's limiting magnitude. One formula is for "super ideal" conditions; the other for "normal ideal" conditions.

http://users.zoominternet.net/~matto/Ja ... 0Table.htm

Under "normal ideal" conditions, you'll need a telescope with an 8-inch aperture to find Pluto, which is magnitude 14.0 .
Currently (July 16, 2012), Pluto is highest in the sky at 11:30 pm EDT.

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PostPosted: Mon Jul 16, 2012 9:30 pm 
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I wouldn’t have much confidence in the formulae presented by this author. He doesn’t really disclose the assumptions used to generate the two columns, which yield considerably different results. He almost seems to imply that the difference results from the use of millimeters vs. inches for the aperture, which is nonsense. A given aperture, regardless of the units of measurement, will yield the same result for the same conditions (assuming the formula handles the units correctly).

Here’s a handy calculator from Sky & Telescope:

http://www.skyandtelescope.com/scopecalc

It assumes a dark, moonless sky and takes the magnification into consideration (a very important factor!). However, it does not account for other intangibles, such as transparency, how dark is dark, or the skill and acuity of the observer, so you need to consider the output as a guide rather than a absolute value.

In the case of (134340) Pluto, we have another factor to consider in New Jersey -- Pluto doesn’t get very high in the sky. Tonight for instance, Pluto transits at 11:52 pm EDT at an altitude of just under 31 degrees (for a nominal location of 40 deg N, 75 deg W). That results in it appearing dimmer than the standard magnitude 14.0 commonly listed. SkyTools 3 gives it a mean extincted magnitude (as corrected for atmospheric absorption) of 14.9 tonight at transit. That's almost a full magnitude lost.

I saw Pluto back on May 20th at Franklin Parker Preserve (about 6 miles east of Coyle Field) with my 12.5-inch Newtonian using a Nagler zoom eyepiece at 6 mm (265x). At the time (appx. 1 am), Pluto was at 20 degrees altitude with a mean extincted magnitude of 15.3 (significantly dimmer than the standard 14.0!). I could only see it with averted vision, but it was at the exact spot indicated by my SkyTools chart. Interestingly, the S&T calculator shows a limiting magnitude of 15.3 for my scope with that eyepiece. It didn't get much easier to see at 2 am when it reached 26 degrees altitude and an extincted magnitude of 15.0 (perhaps it was glimpsed with direct vision).

So, using an 8-inch scope around 250x would get you to around a 14.4 magnitude limit per the S&T calculator, but that might not be enough to see Pluto from NJ (due to both the low altitude and the lack of truly dark skies here) -- unless you had superb vision.

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