How good is the sky at Granite Gap?

Posted by Michael E. Bakich

During my last visit to Rancho Hidalgo in January 2010, Gene Turner asked me to evaluate the sky above Granite Gap, so that was one of my main goals during that trip. After three observing sessions at the Gap, I rated both the seeing and the transparency as superb. Seeing is the term observers use to rate how steady the air is above any location. The world’s finest astroimagers produce high-quality shots because of the great seeing at their sites.


Observers judge the other quantity — transparency — in two ways: by measuring a site’s limiting visual magnitude (the faintest star you can see without optical aid) and by counting deep-sky objects they can see (also without optical aid). The first night at Granite Gap, I estimated the limiting magnitude at 7.4, about as dark as I’ve ever experienced.


I then made a quick tally of the deep-sky objects I could see with just my eyes. A dozen of famed comet-hunter Charles Messier’s open clusters were easy to spot. Among the faintest were magnitude 6.2 M93 in Puppis and magnitude 6.4 M38 in Auriga. The ability to see such faint extended objects speaks volumes about how great the sky over the Gap is.


Then, still without binoculars or a telescope, I held a nebula filter in front of one of my eyes. Through it, I could see the large California Nebula (NGC 1499) in Perseus as well as the Rosette Nebula (NGC 2237-9) in Monoceros. The list goes on.


In summation, I rated Granite Gap as one of the finest observing sites I have ever visited. If you’re considering purchasing a plot there and you want my opinion, I give it my highest recommendation, without reservation. In terms of sky darkness, seeing, and transparency, it is world-class.

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