No such thing as “Loss of Astro Darkness” in Places of Moderate-High Light Pollution

[Captain Scarlet]

I have a Unihedron SQM-L sky quality meter, and since I got it a couple of years ago I've collected quite a bit of data from two sites I'm regularly at. One is near Baltimore in SW Ireland, which lightpollutionmap.info reckons is 21.85. And the other is in Sunbury-on-Thames West of London in the UK, which the same site says is 19.05. Based on my data, it seems that both are reasonable estimates of what their skies can achieve under the best conditions.

I've collected a lot more data since my post about this a year ago, particularly for periods of twilight, and I've changed my regression model to account for the sun's position below the horizon.

A chart of measured values (and my estimator model residuals) are shown below.

For those that are interested, my estimator-equations for the two locations are:

SQ_sunbury = 18.897 + 0.2992 x sunalt - 0.0609 x sunalt^2 - 0.2496 x moonphase - 0.0186 x moonalt

and

SQ_baltimore = 21.661 + 0.1020 x sunalt - 0.0656 x sunalt^2 - 0.7319 x moonphase - 0.0160 x moonalt

(in each case moonalt and phase are set to zero for moon below 10 degrees below, and sunalt is actually its altitude above -18 degrees elevaltion, and zero if below that elevation)

These expressions, for the data I've collected so far, each give a square-root-of-sum-of-squares value for "actual minus model" of around 0.3 magnitudes.

Cheers, Magnus

 

 

Edited June 25, 2020 by Captain Magenta

[Captain Scarlet]

Some interesting inferences about Astro Darkness:

- As the Sun sets, the "darkness level" is the same at all places regardless of your local level of light pollution, until you reach your local "darkest level". In other words, with the Sun at 6 degrees below, the darkness level is the same in Bortle 6/7 as it is in Bortle 3/4. But as soon as the darkest level is reached, the continued setting of the Sun has no further effect.

- This means than Sunbury here, Bortle 6/7, reaches its darkest level when the Sun is a bit more than 12 degrees below, only just out of Nautical Twilight. In other words, Sunbury-on-Thames never ever gets better than Nautical Twilight, which is reached with the Sun at 12 degress below the horizon.

- This further means that the concept of "losing Astro Darkness" isn't really meaningful in light-polluted places. Astro darkness is never achieved anyway at any time of year.

- It also means that, in Bortle 6/7, there is always some time, even at midsummer, when the sky can be as dark as it can ever get any time of year, which in Sunbury happens at 12 degrees of Sundown, whereas the very highest "low point" of the Sun is 16 degrees at midsummer.

- Another interesting feature is the "dip" in darkness level for the few degrees after "most dark" has been reached, and the "undip" a bit further on: people putting lights on, I guess, and gradually turning them off later on.

Cheers, M

Edited June 26, 2020 by Captain Magenta

[Hallingskies]

Interesting data.  It certainly supports my purely empirical point of view. I was out imaging on June 22nd/23rd and at 1.00 am, the sky looked as dark as it ever does in my moderately light-polluted skies. The only real problem of summer nights for me is the short window of quality imaging time.

[DaveS]

I have a supposedly SQI of 21.66, and the last clear nights when I was out the sun was at about -14 deg and the sky was still noticeably lighter than under full Astro Dark at -18 deg.

[Captain Scarlet]

10 minutes ago, DaveS said:

I have a supposedly SQI of 21.66, and the last clear nights when I was out the sun was at about -14 deg and the sky was still noticeably lighter than under full Astro Dark at -18 deg.

Yes I think that's consistent with this data: it shows that you should expect at least a full magnitude brighter than your full darkness at that sun altitude and with your rather low level of light pollution...

Edited June 27, 2020 by Captain Magenta