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Old 20-03-2021, 03:07 PM
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Merlin66 (Ken)
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Light pollution maps

https://www.cleardarksky.com/maps/lp...ution_map.html


Do you agree with the readings given for your area?
(The results for St Leonards, Vic don't seem to match my conditions!!)
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Old 20-03-2021, 03:29 PM
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chrisp9au (Chris)
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Agreed Ken, you and I are classed as Bortle class 4, "Milky Way brilliant overhead", I don't think so!
Eyeing off a property in Glenthompson over near the Grampians, Bortle class 1 skies, 90 minutes from Ballarat, very tempting but the CFO says no!
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Old 20-03-2021, 11:35 PM
Saturnine (Jeff)
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Would have to agree that the scale is misleading, according to the map I'm in a Bortle 4 area as well but even on the darkest most transparent nights I can barely make out the LMC, though the Milky Way in the Eta Carina area is a bit easier.
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Old 21-03-2021, 12:08 AM
JA
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Quote:
Originally Posted by Merlin66 View Post
https://www.cleardarksky.com/maps/lp...ution_map.html


Do you agree with the readings given for your area?
(The results for St Leonards, Vic don't seem to match my conditions!!)
Hi Ken

Your source, cleardarksky reports St Leonards (based on 2016 data -see top of page on your link) as what appears as Bortle 4 based on the blue/violet? colour code. I just had a look on LightPollutionMap https://www.lightpollutionmap.info/#...FFFFFFFFFFFFFF ,a source that I usually use for such info and found sort of similar results (see excerpt image below) for St Leonards. It also shows based on 2015 data this time, St Leonards as a Bortle Class 4 area, but for now take these results with a grain of salt we will correct them somewhat...
Click image for larger version

Name:	Light Pollution Map - St Leonards.jpg
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First an aside.....
I like the Light Pollution Map site in preference as it reports the SQM reading, Brightness and Bortle Number of the exact location IF YOU SELECT the World Atlas 2015 Dataset at the top right hand corner. If you use the dropbox and select any of the VIIRS datasets you will get, to my mind, the less useful/less intuitive Sky radiance reported and a colour code. Of course my preference to use the 2015 World Atlas data means that it is 6 years out of date, but there is a work around to that which I use. I take the VIIRS 2020 dataset sky radiance data & the VIIRS 2015 dataset sky radiance data to help correct the World Atlas 2015 Brightness data to give me effectively World Atlas 2020 Brightness data. I will report the result of this in the following.

Anyway.......
When I started doing that for St Leonards I found something interesting....
Between 2015 and 2020 the sky radiance had gone from 0.84 bananas* to 2.17 bananas* (2.58 times original).... No wonder you feel that the sky is not as dark as reported, it's become significantly brighter!

How much brighter is it NOW is the question? Well aside form measuring it, which is the subject of another one of my discussions, the best we can do is use the available data, that means 2020 data.

OK from LightPollutionMap.....

World Atlas 2015 Brightness data
Brightness=0.286 mcd/m^2 , SQM=21.44 mag./arc sec^2, Bortle Class=4

VIIRS 2015 dataset sky radiance data
Radiance=0.84 x10^-9 W/cm^2*sr, Teal / Darker Green colour code

VIIRS 2020 dataset sky radiance data
Radiance=2.17 x10^-9 W/cm^2*sr, Light Green colour code

Ok so ... if you ask me it's therefore reasonable to take (extrapolate) the
World Atlas 2020 Brightness data to be 2.17/0.84 = 2.58 times the World Atlas 2015 Brightness data so that The World Atlas 2020 Brightness is 0.286 x 2.58 = 0.738 mcd/m^2.

Now using this Sky Brightness to SQM reading converter (http://www.unihedron.com/projects/darksky/magconv.php)on the Unihedron SQM Site that means our brightness of 0.738 mcd/m^2 equates to a reduced SQM reading of 20.41.

Now an SQM reading of 20.41 mag./arc sec^2 equates to a Bortle 5** ish sky (just on the border between 4 and 5, whereas previously in 2015 it was firmly a Bortle 4)

OK That's too much writing, colours, &c. for me, for now

Best
JA

* - Sorry I couldn't resist the bananas, but nonetheless the SkyRadiance units are W/cm^2 * sr

** Bortle /Approx SQM Equivalence Ranges https://en.wikipedia.org/wiki/Bortle_scale

Last edited by JA; 21-03-2021 at 01:34 AM.
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Old 21-03-2021, 11:05 AM
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Sunfish (Ray)
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Interesting topic. I tried JAs link and it puts me on the border of Bortle 4 and 5 which seems about right. I can see most things on a night of good seeing. If I look over toward the Bortle 5 side , over the town, the view is not so good and toward the Bortle 4 side most bright objects look pretty clear.
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Old 21-03-2021, 11:59 AM
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Merlin66 (Ken)
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JA,
I bow to your knowledge
The reason this became an investigation for me was to determine the impact of the sky background noise in a spectral image.
I have a few calculations which are used and have the Sky magnitude in mag/arcsec^2 as a starting point.
When I look at this in detail the definition of "which star" magnitude is assumed? S(10) standard....a red star or a blue star or what????


https://link.springer.com/chapter/10...40-07615-8_450


Basically I need to end up with the sky background in photons/ sec.
Any comment/ ideas???
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Old 21-03-2021, 01:14 PM
JA
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Quote:
Originally Posted by Merlin66 View Post
JA,
I bow to your knowledge
Ha ha , Ken.... Thanks

Quote:
Originally Posted by Merlin66 View Post
The reason this became an investigation for me was to determine the impact of the sky background noise in a spectral image.
I have a few calculations which are used and have the Sky magnitude in mag/arcsec^2 as a starting point.

When I look at this in detail the definition of "which star" magnitude is assumed? S(10) standard....a red star or a blue star or what????


https://link.springer.com/chapter/10...40-07615-8_450


Basically I need to end up with the sky background in photons/ sec.
Any comment/ ideas???
Depends.... Possibly: You mentioned that you do a few calculations involving the Sky Magnitude. If you already have a reliable method that you trust to go from an SQM reading in mag/arcsec^2 to sky background in photons/ sec then I may be able to help somewhat, but it would be good to know whether you get your sky magnitude in mag/arcsec^2 from an SQM Meter, LightPollution Map, other method?

If you don't already get your Sky Magnitude in mag/arcsec^2 from some form of direct measurement of your Sky AND if you have a reliable method to go from the SQM reading in mag/arcsec^2 to sky background flux in photons/ sec, then I believe I could offer some help.

Best
JA
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Old 21-03-2021, 03:37 PM
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Merlin66 (Ken)
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JA,
The sky magnitude is an "estimate"
http://www.nightwise.org/magnitudes.htm
http://memebers.ziggo.nl/jhm.vangast...ictionTool.htm


SB0 = 22 - 5Log(10(1.7-LM/5)-1) as suggested by Jan van Gastel, based on the initial formula by Nils Olof

This doesn't cover the calculations for sky noise.....


We need to use a luminance in electrons(photons)/cm^2/s/Angstrom/arcsec
The Planck Black body and Wein temperature formula seem to be used to determine a spectral flux (photons/cm^2/s/A) based on an (agreed?) stellar magnitude and spectral type.
See also:
https://www.researchgate.net/publica..._the_Night_Sky

Ken

Last edited by Merlin66; 21-03-2021 at 03:57 PM.
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Old 21-03-2021, 03:52 PM
JA
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Hi Ken,
I'm answering from my phone so I'm somewhat limited in response, but I can confirm that I believe I could help to get you more of a direct measurement of your own local sky rather than an estimate based on relying on the limiting magnitude. Then you take it from there and use it as you see fit. If you're interested in that I could detail it later. ... when I get back home to my PC

Best
JA

Last edited by JA; 24-03-2021 at 09:55 PM.
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  #10  
Old 21-03-2021, 04:12 PM
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Merlin66 (Ken)
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Also found this:
The surface brightness of the night sky, denoted as Sa, is given in the commonly used astronomical units of magnitudes per square arcsecond (mag arcsec−2). It is a derivative of the magnitude scale (mag) defining the visual impact of the star's brightness as a point light source. Magnitude scale is a logarithmic, relative and reverse scale, in which a star of magnitude 0 is 100 times brighter than a star of magnitude 5. The mag arcsec−2 scale determines the surface brightness of diffuse astronomical objects, such as nebulae, galaxies, comets, or just a background sky. The derived SI unit of luminance, or perceived brightness, is the candela per square metre (cd m−2), which is a measure of light emitted per unit area. It is possible to approximately convert mag arcsec−2 into cd m−2 using the following formula (Crawford 1997; Ściężor et al. 2010b; Kyba et al. 2011):
[cdm2]=10.810410[0.4(magarcsec2)].





(1)
Because of the very low surface brightness of the night sky, the commonly used unit is millicandela per square metre (mcd m−2). In order to maintain compliance with other publications that deal with this problem, as well as with readings from SQMs, I mainly use the scale (mag arcsec−2), also giving the appropriate values in the scale (mcd m−2).
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Old 21-03-2021, 04:15 PM
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Merlin66 (Ken)
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Came from:
https://academic.oup.com/mnras/artic.../1/303/1109630
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Old 21-03-2021, 04:19 PM
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Merlin66 (Ken)
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JA,
Obtaining a better "estimate" may help...but I'm trying to understand how to apply the result to assessing the noise (across the visible spectrum) from the night sky.
I cannot find a definitive reference for the formula currently used. It would be nice to be able to confirm it is correct.
http://astrosurf.com/buil/us/spectro8/spaude02_us.htm
See the N(sky) entries.......
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