10 June 2010

How do you rate darkness?

A poll has been running on the Deep-Sky Planner web page for several weeks that asks the simple question 'What scale do you use to rate sky darkness?'

The results as of now are a bit illuminating (pardon the pun).
  • 20% don't bother with rating darkness at all. Observers really should rate darkness somehow.
  • 12% use the Bortle scale. It is a very coarse scale but it is easy to use.
  • 45% use NELM - the naked eye limiting stellar magnitude at the zenith. This is an easy test to conduct during observing but is subject to one's visual acuity and experience. It is a finer measure than Bortle, but subjective.
  • 20% use magnitude per square arcsecond. This is very likely to be measured by equipment such as a Sky Quality Meter. It is easy to acquire this value during observing but only if you have the equipment. It is also a very objective measurement so it is the most precise measure of darkness, but it does require an investment.
  • 0% just make up a number on their own scale. This is a surprise because observers often say something like 'darkness was 8 out of 10'. Maybe the poll is missing respondents on this question.

If you'd like to participate in this non-scientific poll, visit http://knightware.biz/dsp/ and look in the left column for the Observer's Poll. Please vote only once.