Sierra Remote Observatories is very dark, at 21.78 mag/sq arcsec, Vband. We are at the edge between Bortle 2 and Bortle 3 with the site falling into the Bortle 3 category. It would be difficult to get find a lower Borle Index and still have services such as reliable power, T1 or fiber optic internet and easy accessibility all year.
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mags/sq arcsec VBand
Descriptions are approximate. Your sky may vary.
|< 0.01||22.00 to 21.99||1||Gegenschein visible. Zodiacal light annoyingly bright. Rising Milkyway confuses some into thinking it's dawn. Limiting magnitude 7.6 to 8.0 for people with exceptional vision. Users of large Dobsonian telescopes are very happy.|
|0.01 to 0.11||21.99 to 21.89||2||Faint shadows cast by Milkyway visible on white objects. Clouds are black holes in the sky. No light domes. The Milkyway has faint extensions making it 50 degrees thick. Limiting magnitude 7.1 to 7.5.|
|0.11 to 0.33||21.89 to 21.69||3||Low light domes (10 to 15 degrees) on horizon. M33 easy with averted vision. M15 is naked eye. Milkyway shows bulge into Ophiuchus. Limiting magnitude 6.6 to 7.0.|
|0.33 to 1.0||21.69 to 21.25||4||Zodiacal light seen on best nights. Milkyway shows much dark lane structure with beginnings of faint bulge into Ophiuchus. M33 difficult even when above 50 degrees. Limiting magnitude about 6.2 to 6.5.|
|1.0 to 3.0||21.25 to 20.49||4.5||Some dark lanes in Milkyway but no bulge into Ophiuchus. Washed out Milkyway visible near horizon. Zodiacal light very rare. Light domes up to 45 degrees. Limiting magnitude about 5.9 to 6.2.|
|3.0 to 9.0||20.49 to 19.50||5||Milkyway washed out at zenith and invisible at horizon. Many light domes. Clouds are brighter than sky. M31 easily visible. Limiting magnitude about 5.6 to 5.9.|
|9.0 to 27.0||19.50 to 18.38||6 or 7||
Milkyway at best very faint at zenith. M31 difficult and
indistinct. Sky is gray up to 35 degrees. Limiting magnitude 5.0 to 5.5.
|> 27.0||< 18.38||8 or 9||Entire sky is grayish or brighter. Familiar constellations are missing stars. Fainter constellations are absent. Less than 20 stars visible over 30 degrees elevation in brighter areas. Limiting magnitude from 3 to 4. Most people don't look up. CCD imaging is still possible. But telescopic visual observation is usually limited to the moon, planets, double stars and variable stars.|
Credit: P. Cinzano, F. Falchi (University of Padova), C. D. Elvidge (NOAA National Geophysical Data Center, Boulder). Copyright Royal Astronomical Society. Reproduced from the Monthly Notices of the RAS by permission of Blackwell Science.