With this blog, I'll finish my discussion of the magnitude system, which I began last week. I'm aiming this installment at observers.
When you observe, sky conditions are everything. But even when it's clear, the sky's transparency can vary. Amateur astronomers standardize their observations by estimating the limiting visual magnitude (LM). Not only will this help you determine how good (or bad) the sky is at that particular time, it also will allow you to judge the quality of your recorded observations (if you keep a log) months or years from that session. And making repeated estimates of LM over a long time span will make you a better observer, one more conscious of little details.
Most observers estimate LM by eye, but some use a telescope. Generally, an observer estimates limiting magnitude through a telescope when he or she is observing faint objects at the limit of detection. If you perform a telescopic LM estimate, be sure to note the telescope's aperture and the eyepiece (giving magnification) through which you made the estimate.
Most LM estimates come from the zenith, where sky conditions are usually the best. Some Northern Hemisphere observers like to use the area around Polaris (Alpha [α] Ursae Minoris) to estimate LM. The advantage to using the "North Polar Sequence" is that you'll only need one star chart year-round.
If, however, you're studying a particular object that lies far from either the zenith or pole, you may want to estimate LM near the object. Just be sure to use part of the sky at the same altitude as your target.
Measuring LM at each of your observing sessions helps train your eye to pick up detail and will make you a better observer. It's an example of a little effort yielding big rewards.