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AN EASY TO USE MAGNITUDE CALIBRATOR

For years I have used the stars in Ursa Minor as magnitude calibrators to determine the magnitude of the faintest star I could see on a given night. Umi contains stars in convenient (near)one-magnitude steps from 2nd magnitude to our visual limit. At a recent STAR party held at our Collier's Mills observing site the seeing was spectacular (for New Jersey) and I used the stars in this constellation to establish that we had a mag 5 night.

Starting with Polaris, a spectral class F8 star with magnitude of 2.0, climb the handle of the little dipper to delta, a 4.4 mag A0 class, then on to epsilon, a 4.2 mag G5 star, then to zeta, a 4.3 mag A2, the first corner of the cup. Beyond that is beta, the brightest star in the cup at mag 2.1 and class K5. Diagonally across the cup from zeta is a pair, gamma 2 a 3.0 mag class A2 and gamma 1, a 6.1 mag K0 which I have never seen in NJ without optical aid. The final star in our calibration series is nu, a 4.9 mag F0, diagonally across the cup from beta.

As you can see, this easy to find constellation contains identified stars that calibrate the visual range from 2nd magnitude to slightly more than 6th magnitude, and does so in nearly one-magnitude steps. The handle contains three 4th magnitude stars that are a quick fix on the transparency. If you can see all three you may have a good evening. If you can see nu with no trouble you should stay out late. If you can see gamma 1 stay out all night...NJ doesn't get much better than that.

This series of calibration stars can be used to determine your limiting magnitude, estimation of variable star magnitudes, and can be used to estimate the effects of particulate matter in the air. Be careful though, as Umi is in the northern sky roughly only half way to your zenith and if you have smog, light pollution, or heavy mist in the air you may not have the accuracy you might think.

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