The Astronomy Picture of the Day for Apr. 7 showed images to represent Bortle 7 to 2
I'm no expert on the Bortle scale but the image for Bortle 7 looked more like a 9 to me.
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Posted 09 April 2020 - 05:18 AM
Yeah, that's a lousy set of photos, for sure. A properly adjusted camera exposure in Bortle-7 skies should show the Milky Way just fine. And a properly adjusted camera exposure in Bortle-9 skies shows hundreds of stars and a fair number of deep-sky objects -- though not the Milky Way. I can supply plenty of examples if desired.
Moreover, the difference between Bortle 4 and Bortle 5 is too small.
Mind you, if you blindly take all photos at ISO 3200, 30 seconds at f/2 -- just about right for dark skies -- then Bortle 9 might indeed be overexposed as shown. But that's because you're using the camera wrong, and it does not reflect the visual impression at all.
Posted 09 April 2020 - 05:29 AM
I don't think these pictures are meant to represent the visual impression of the different steps on the Bortle scale. It's just a rendition of the effect light pollution has on a camera set up with the same settings on different locations. On my monitor, the picture of Bortle 5 shows a lot more stars than Bortle 2.
Posted 09 April 2020 - 05:54 PM
I don't think its a very good representation at all. As presented one gets the impression a Bortle 2 sky has less stars than my sky at Bortle 5! Also with the Milky Way up at a Bortle 2 or better site the ground should have some lightness, not be black as depicted. I've seen my shadow from the Milky Way on light colored ground.
I've always believed these type of presentations shouldn't show much color, they should be heavily desaturated, so the viewer gets a better representation of the night sky regardless of Bortle scale. Lay people see these pictures and think they will see this colorful night sky when they travel out to a remote location. Whats funny is I read a thread on a non-astronomy (IT) forum few months ago where some were doubting the auroras had much color simply because they had been fooled by these type of nightscapes. Had to chime in with others and say that yes, auroras have remarkable color and pictures rarely do them justice.
However I do like the framing of the pictures with a horizon and rising milky way. So its a interesting composition but not particularly accurate.
Posted 13 April 2020 - 10:13 AM
As presented one gets the impression a Bortle 2 sky has less stars than my sky at Bortle 5! Also with the Milky Way up at a Bortle 2 or better site the ground should have some lightness, not be black as depicted.
This thread got my interest going.
Most of these types images using a single picture don't do a very good representation. For example, in the image below, the center of the Milky Way dominates the Bortle 2, 3, and 4 panels. Additionally, the inclusion of the horizon usually includes skyglow from a distant city which only complicates matters across the panels.
So I decided try to make a little better representation of the effect of sky-glow and the number of stars visible. I used the top of the panel in the above image and added proportionally greater levels of "sky glow" and increased the number of stars visible. This is not meant to be scientifically accurate, but rather perhaps an improved subjective / qualitative display of light pollution effects on the night sky.
Click for larger images.
Edited by BYoesle, 14 April 2020 - 03:41 AM.
Posted 13 April 2020 - 10:22 AM
Posted 14 April 2020 - 03:59 AM
I didn't say the photographer was "stupid" or "incompetent."
While the caption is an accurate description of the process and the image is interesting, and perhaps artistically valid, I agree with others that it does not paint a very accurate portrayal of the Bortle Scale with regard to skyglow intensity or the visible number of stars. Had it not included the Bortle Scale and star magnitude data, it would have been fine. Because it did, and is clearly inaccurate in the image representations shown, there is room for a valid critique of this aspect of the presentation.
If you want to include the Milky Way visibility in such a diagram, it would need to be an identical portion in each pane for a more accurate depiction:
Click for larger.
Edited by BYoesle, 14 April 2020 - 04:01 AM.
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