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General Astronomy >> Light Pollution

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REC
Post Laureate
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Reged: 10/20/10

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SQ Reading
      #5851572 - 05/10/13 08:51 AM

Ok, last night I had my first chance to try out my new App, Dark Sky Meter on my Iphone. The sky was pretty decent for my suburban backyard that is. I could see three of the stars in the little dipper, Mizar and the last star in Corona Borialis which is close to a Mag. 5 I think.

So one reading toward the east where there is LP measured at about 22. Then I pointed it straight up and got a reading of 21.5. Description said moderate light domes, lit clouds and some Milk way overhead.

So, any comments as how to read this for a newbie?

Thanks,

Bob


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csa/montana
Den Mama
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Reged: 05/14/05

Loc: montana
Re: SQ Reading new [Re: REC]
      #5851717 - 05/10/13 10:31 AM

Quote:

So one reading toward the east where there is LP measured at about 22.




22 is about the best SQM reading. Getting that reading where there's LP is not an accurate reading; unless the Iphone appt has a different scale for SQM.

I'm under nice dark skies, & my SQM readings average 21.5+


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Tony Flanders
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Re: SQ Reading new [Re: REC]
      #5851746 - 05/10/13 10:42 AM

Quote:

So one reading toward the east where there is LP measured at about 22. Then I pointed it straight up and got a reading of 21.5.




That doesn't sound right! Overhead is invariably darker than down low, especially in the direction of the major light dome. And no normal suburban sky comes anywhere close to as dark as 21.5. That would be a good reading in the green zone.

One common problem with the Unihedron SQM is trees blocking part of the sky. They will typical make it read much darker than it really is.

Obviously, if you point the thing dead level, at least half of the sky is blocked by the ground, again leading to a false reading.

Not having used this App, I can't comment on how it works.


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GlennLeDrew
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Re: SQ Reading new [Re: Tony Flanders]
      #5852551 - 05/10/13 07:02 PM

Does this app use the camera's full field of view? If so, you must not include the ground, and so must aim at some upward angle.

A suburban reading of 21.5 is rather too good (dark.) I'd expect something more like 20.5 overhead.


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jchaller
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Reged: 05/29/08

Loc: Tenino, WA.
Re: SQ Reading new [Re: GlennLeDrew]
      #5852721 - 05/10/13 08:54 PM

Doesn't seem accurate. I can see all the stars of the Little Dipper and the best reading I've had (last night) is 20.95.
That is with SQM model-no trees near by, so I think it's pretty accurate. I just purchased the app and will compare vs. my SQM tonight.


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csa/montana
Den Mama
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Reged: 05/14/05

Loc: montana
Re: SQ Reading new [Re: jchaller]
      #5853018 - 05/10/13 11:27 PM

Quote:

I just purchased the app and will compare vs. my SQM tonight.






Jim, that will be an interesting comparison, looking forward to it!


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Tonk
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Re: SQ Reading new [Re: csa/montana]
      #5853293 - 05/11/13 05:59 AM

Quote:

unless the Iphone appt has a different scale for SQM.




The scale should be "bightness magnitude per square arc second". A BMPSAS value is idealised as dividing the sky into squares of 1 arc second on each side and place 1 star of the required ideal brightness into each and every square, then the sky has that overall brightness.


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REC
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Reged: 10/20/10

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Re: SQ Reading new [Re: Tonk]
      #5853525 - 05/11/13 10:26 AM

Well, thanks for the inputs. So the higher the number the darker the sky should be then? It probably does use the camera angle, so I'll have to consider that the next time I try it. I'll just point it straight up and take a reading. I believe on a very transparent evening I can see about Mag.5 is about it.

Bob

Bob


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GlennLeDrew
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Re: SQ Reading new [Re: REC]
      #5854127 - 05/11/13 04:59 PM

Bob,
Indeed, just as for astronomical objects, the higher the number the fainter (dimmer/darker.)

To convert SQM readings from arcseconds to arcminutes squared, add 8.89.

To convert from arcseconds to degrees squared, add 17.78.

(Arcsec to arcmin = 8.89, and arcmin to deg = 8.89, hence the 17.78 when going from arcsec to deg.)

The darkest possible sky on Earth is 22 MPSAS, or 13.11 MPSAM, or 4.22 MPSD.

This darkest sky in total light approaches -8 magnitude, or about 25 times the brightness of Venus.

The foregoing does not include the resolved stars. All stars to 6.5 magnitude and which are above the horizon together shine at about -4.4 magnitude, which is the brightness if Venus. We see that the glow from even the darkest of skies greatly outshines the stars. This is why even at pristine sites you can most easily see your hand silhouetted against the sky, and indeed can walk about without a flashlight.

Outside of the darkest 22 MPSAS sky, most rural locales have skies of about 21.2-21.5.

A suburban site might be in the region of 20-20.8

'Inner suburbia' might be 19.5-20.

Urban skies are brighter than 19 or so.

A full moon night sky is about 18.

The eye's color detection threshold is 18.5-19, depending on color purity.

The brightest nebulae (core of M42, as well as numerous planetaries) get as bright as 14.

Neptune: 9.4. Uranus: 8.2. Saturn: 6.8. Jupiter: 5.2: Mars: 3.9-4.7. Mercury: 2.1-4.6. Venus: 1.2: Moon: 3.4-7.1. Total lunar eclipse: 12.5-17.


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Tony Flanders
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Re: SQ Reading new [Re: GlennLeDrew]
      #5854847 - 05/12/13 04:43 AM

Quote:

A suburban site might be in the region of 20-20.8

'Inner suburbia' might be 19.5-20.

Urban skies are brighter than 19 or so.




This goes to show that Ottawa is a pretty small city!

In Boston, my astronomy club, which I consider to be on the outer edge of the suburbs, only gets as dark as 20.0 late on a pretty good night in the summer. (Summer is darker than winter because of the leaves.)

More typical outer suburbs of Boston range 19.0-19.5, inner suburbs around 18.5, and the urban area around 17.0 (downtown) to 18.0 -- all of this on a good night.

You might be interested in my article Rate Your Skyglow. Also relevant are Ground Truth for the Light Pollution Atlas and Has Your Observing Site Gotten Brighter.


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Tonk
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Re: SQ Reading new [Re: Tony Flanders]
      #5854967 - 05/12/13 08:17 AM

The centre of Leeds were I live has skies measuring ~14 on my SQM!!. I've only seen Jupiter and Arcturus from the Leeds Milleniun Square. That patch of sky from my house 7 miles away is a roughly circular bright white patch on the clouds - it can bee seen easily from 40 miles way. My back garden is 19.60 at the zenith (but has a strong gradient) while my "dark" sky site 37 miles to the north of Leeds is still 20.80 - occasionally reaching down to 20.95 when the air is dry. This is the darkest skies I can visit within 1 hours drive as further north the cities of Middlesborough, Sunderland and Newcastle start to dominate

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Tony Flanders
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Re: SQ Reading new [Re: Tonk]
      #5855043 - 05/12/13 09:00 AM

Quote:

The centre of Leeds were I live has skies measuring ~14 on my SQM!!.




Are you sure, sure, sure you weren't picking up streetlights in the field of view? That renders the reading meaningless.

My photos indicate that midtown Manhattan rates around 16.5, which makes it hard for me to believe that Leeds is really 10 times brighter.


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REC
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Re: SQ Reading new [Re: Tony Flanders]
      #5855078 - 05/12/13 09:27 AM

Thanks for the article Tony!

Bob


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GlennLeDrew
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Re: SQ Reading new [Re: REC]
      #5855553 - 05/12/13 02:00 PM

The greater Ottawa population approaches one million, although the central conurbation is not that large. I was trying to be a bit more representative of more typical cities, leaving the mega-metropolii, which we all know to be rather hopeless, out of the picture.

I guess I should consider myself fortunate. In spite of residing in (but near the edge) of a red zone, from my (horribly lit) balcony on the best nights I can just glimpse the North America nebula in unfiltered 10X50 binos when located in my darkest part of the sky.


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REC
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Reged: 10/20/10

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Re: SQ Reading new [Re: GlennLeDrew]
      #5857165 - 05/13/13 10:33 AM

Ok, so I gave it another try last night and was getting a reading of 19.5 most times and was shooting at the zenith. When pointed lower to the light dome it got lower, so I guess it was working ok?

For some reason the sky seemed brighter last night and not as dark as I thought it should be? Maybe some clouds in the sky was reflecting more light down to my area....Tonight it will be very clear and don't think we will have any clouds overhead. I will take another reading.

Bob


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jchaller
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Reged: 05/29/08

Loc: Tenino, WA.
Re: SQ Reading new [Re: REC]
      #5857620 - 05/13/13 02:10 PM

That sounds about right for red/orange. I used to live in a red/orange zone and got readings on my SQM at between 19.0 and 19.6.

Towards the bottom of the page on the following link, there is estimated correlation between SQM readings and light pollution zones.

http://www.skyandtelescope.com/howto/visualobserving/35614744.html


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Tonk
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Re: SQ Reading new [Re: jchaller]
      #5866882 - 05/17/13 11:05 AM

Quote:

Are you sure, sure, sure you weren't picking up streetlights in the field of view? That renders the reading meaningless.




I disagree that this is "meaningless". Its a fair measure of the naked eye view of the sky allowing for the situation at that location. So I'm measuring as close as possible the naked eye view using the assumption that the field of view of my eyes and the SQM are very roughly similar. The measuring point was in the centre of the square furthest from lights and buildings - there is no other suitable measuring location in the city centre. Yes looking directly upwards puts brightlights into the periphery of my vision (the lights are on 100 foot poles - you simply can't get away from them - so its not meaningless) and hence I can see only Jupiter (barely!). The SQM value under these conditions is agreeing with that - bugger all visible


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GlennLeDrew
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Re: SQ Reading new [Re: Tonk]
      #5867510 - 05/17/13 04:05 PM

A measure of sky darkness *must not* include discrete lights or obstructions, only sky. Otherwise the result is indeed invalid.

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