Hi Russ and thanks for your article. I have been using the
Bortle scale for many years as one tool for assessment of the night sky, be it at home in our typical suburban sky or at our dark site house in the western part of the state. The
Bortle has been praised and criticized, but for the most part I have found John’s original criteria reasonably accurate for my situations. I know in reading John’s posts on astronomy sites and in my own communications with him he has staunchly defended his scale as a very accurate tool for real time assessment of one’s sky in any given moment.
He specifically is not a fan at all of some trying to correlate his scale to the colored maps/scales that are sometimes employed,. That is simply because they are static in nature, whereas his scale is a variable one, with real time changes possible over the course of an evening. The maps are of course based on nighttime satellite imagery with an algorithm applied to estimate the spread of sky glow over terrain. While they can be a useful tool for identifying locations that might possibly be darker, they don’t give any assurances. I see them as a broad brush approach to locating sites that still need to be vetted to ascertain their true usefulness. John will also argue against any correlations between his scale and the
SQM methodology. He truly believes his scale stands alone without embellishment.
On a personal note, our dark site varies from as good as a
Bortle 2, with 3 being typical, to increasingly poorer levels as conditions deteriorate. That is the beauty of the scale, it slides with changes in conditions. However, as JT points out, which you echoed, individual optical variation and experience can noticeably impact where one falls on the
Bortle scale. So as with NELM measurements, which I never use personally, the
Bortle scale is not equal for all people all the time.
Mention was made of the
SQM, which I also own, having an
SQM-L. As you rightly pointed out, there can be variation there as well because of the unit’s battery status and its calibration accuracy. I suppose there could be some user error, but I don’t see that as a significant factor as they are quiet easy to use. Again, though there are variables associated with the meter, I still use it to get a sense of the actual sky glow levels at our dark site. I do find it a useful tool, bearing in mind, the potential flaws.
As to your expanded scale, I tend to agree with Andrey that there doesn’t seem to be any significant utility for levels beyond a
Bortle 9. While I understand your thoughts that perhaps for beginners it might be useful for them to gain a perspective, I don’t believe it takes a lot of experience to understand that if you assess your conditions at a
Bortle 9, you are in a seriously compromised area. It is obvious you have given a lot of thought and study to the subject matter to come up with your expanded scale. I personally appreciate and applaud your interest in the subject matter and the obvious desire to take a serious look at the
Bortle scale. However, I am just not sure how much true benefit will come from going beyond
Bortle 9. As I said, if you know you are in
Bortle 9, then you are well aware that you are in deep kimchi and anything worse would just be more depressing.
On a side note, I recall an argument I had with John about observing in general. He stated to someone that if he had to observe from anywhere brighter than a
Bortle 3, he simply would give up the hobby. While I understood his sentiment, I countered that his way of thinking was very defeatist in nature and discouraging to a lot of folks. I told him I simply could not agree with his thinking because even from our
Bortle 5 backyard, I could still observe a vast number of galaxies and other
DSOs with my dobsonian reflectors and would never exhaust the potential targets from that location. Anyway, my stance didn’t persuade him, nor did I expect it to. However, I wanted other readers to not be discouraged because they have to deal with conditions worse than
Bortle 3. So we basically agreed to disagree!
Again, thanks for your hard work and thought on this expansion. I hope some find it useful for their purposes in the field, and just for those folks who may not yet know the full criteria of the original
Bortle scale, it can be found at the below link.
https://www.skyandtelescope.com/astrono ... sky-scale/