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Moonshane

Tell us your sky quality

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On 02/01/2019 at 15:34, clarkpm4242 said:

Says 21.6 for us up in Swaledale.  Recent meter reading managed 21.4 in the early hours of a really clear night.

I am very fortunate! Paul

Have recently done some readings for the Yorkshire Dales National Park. 

Separate nights have had 21.45 and 21.5 (averaged) in our garden.

?

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50 minutes ago, bobmoss said:

Would love the get hold of a proper SQM to see how accurate the website is. It is pretty dark here though, must get around to trying to properly estimating how dark it actually is.

I see Canon 600D listed in your kit in signature, so I'm guessing you take odd picture now and then with that camera and the scope?

If you are, then there is simple way to measure SQM your self (approximately). Take an image of sky near zenith (where you want your SQM value measured) and another one of bright reference star. Use green channel from your image and measure star intensity - photometric measurement - you can use AstroImageJ for this. Then take empty patch of sky and measure average value there (in first image). Make sure you subtracted dark frame from your image (you can do flat calibration as well but you don't need to do it for this rough measurement).

Divide average background value with pixel scale squared (so if you image at 1.5"/pixel  - divide with 1.5*1.5 = 2.25). Divide total star brightness with background value and calculate magnitude difference based on this number

-2.5*Log(star_intensity / background_intensity) = magnitude difference between star and sky brightness. Subtract this value from star magnitude.

This should give you SQM reading.

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I'll know more accurately on Thursday if clear. I get a SQML from my birthday 

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You can actually check other people's readings on that website as well - just turn SQM overlay to get recorded measurements and other info.

One can add their own measurements as well.

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You do simply need a SQM - L. I have just checked the actual coordinates on the light pollution map based on a dark sky session last Saturday: 55' 13' SQM 21.73. The reality is that for this session based upon a frequency of readings, periodically taken throughout, the average reading and repeated log concluded decisively at 20.96, the top reading on this particular night was 21.1. This is based on the circumstance for that given night, which was very cold with snow and ice, its probable that a combination of ice crystals, some moisture resulted in good but not excellent transparency. It is not the darkest area I could get to, but this particular location is convenient, being less than an hours drive from home, visited frequently over a number of years when occasionally gaining readings of 21.3.  

A SQM-L will record readings at a 40 degree point of sky you observe in. Logging the variations becomes quite addictive, it is not a chore more so a pleasurable task that is complementary to the outcome of your deep sky observing session. The online map resource as said, is just about OK as a rough guide indicator, but it is on a specific night that counts, we should each of us have a Unihedron SQM-L in our tool box.

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1 hour ago, vlaiv said:

I see Canon 600D listed in your kit in signature, so I'm guessing you take odd picture now and then with that camera and the scope?

If you are, then there is simple way to measure SQM your self (approximately). Take an image of sky near zenith (where you want your SQM value measured) and another one of bright reference star. Use green channel from your image and measure star intensity - photometric measurement - you can use AstroImageJ for this. Then take empty patch of sky and measure average value there (in first image). Make sure you subtracted dark frame from your image (you can do flat calibration as well but you don't need to do it for this rough measurement).

Divide average background value with pixel scale squared (so if you image at 1.5"/pixel  - divide with 1.5*1.5 = 2.25). Divide total star brightness with background value and calculate magnitude difference based on this number

-2.5*Log(star_intensity / background_intensity) = magnitude difference between star and sky brightness. Subtract this value from star magnitude.

This should give you SQM reading.

Excellent, thanks for that. Will give it a go next time I get a clear night.

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1 hour ago, vlaiv said:

You can actually check other people's readings on that website as well - just turn SQM overlay to get recorded measurements and other info.

One can add their own measurements as well.

Just checked, no other people's readings near me unfortunately. Only 2 on the whole of the Skye!

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36 minutes ago, scarp15 said:

You do simply need a SQM - L. I have just checked the actual coordinates on the light pollution map based on a dark sky session last Saturday: 55' 13' SQM 21.73. The reality is that for this session based upon a frequency of readings, periodically taken throughout, the average reading and repeated log concluded decisively at 20.96, the top reading on this particular night was 21.1. This is based on the circumstance for that given night, which was very cold with snow and ice, its probable that a combination of ice crystals, some moisture resulted in good but not excellent transparency. It is not the darkest area I could get to, but this particular location is convenient, being less than an hours drive from home, visited frequently over a number of years when occasionally gaining readings of 21.3.  

A SQM-L will record readings at a 40 degree point of sky you observe in. Logging the variations becomes quite addictive, it is not a chore more so a pleasurable task that is complementary to the outcome of your deep sky observing session. The online map resource as said, is just about OK as a rough guide indicator, but it is on a specific night that counts, we should each of us have a Unihedron SQM-L in our tool box.

Interesting, thanks. I guess you take a reading and then decide if its worth getting a scope out?

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Rain and more rain and grey skies--no its not England, when it is clear about 8 or 9 months there is still fog to contain and lots of humidity from the Pacific, but there are somme very good evenings.
For now we "enjoy" the rain and it will make the garden happy this spring which is around the corner.

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I queried three independant sources for the relevant figures, with varying results:

- Pants

- Forget about it

- Don't bother

I make the best of it, though. :happy11:

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9 hours ago, bobmoss said:

Interesting, thanks. I guess you take a reading and then decide if its worth getting a scope out?

On Skye?

I would suggest getting a scope out (a) it's actually going to get dark (b) it isn't cloudy and (c) the sheep are able to remain upright.

?

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6 minutes ago, Stub Mandrel said:

On Skye?

I would suggest getting a scope out (a) it's actually going to get dark (b) it isn't cloudy and (c) the sheep are able to remain upright.

?

So true! We got one out of three last night - it got dark...

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Skye is class 1 Bortle.

Edited by Saganite

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10 hours ago, bobmoss said:

Interesting, thanks. I guess you take a reading and then decide if its worth getting a scope out?

On my recent session, attention was centred on a variety of subjects that included planetary and emission nebulae. I had also intended to pursue a few dark nebulae, which became a challenge. Periodically taking sky brightness measurements was informative firstly clarifying periods where transparency was more favourable and to confirm that for particular subjects such as dark nebulae, would not be the most applicable conditions. Readings also vary between different locations that I use and it is useful to log and be aware of. My location on Saturday is a very good place accounting for convenient distance from home, an 360 vista and peace of mind. There is sky glow in the S/E which on nights such as my recent encounter can become more magnified. Another location I head to can gain 21.4 and potentially more and anywhere is determined by weather such as snow or wind. 

So its not at all about whether its worth getting the scope out, you can of course make that judgement easily enough. But it will inform you how much fluctuation there is in transparency, whether or if certain targets are a struggle and it will highlight any distant light pollution if this is magnified on a night of high humidity. Most importantly it will convey a true and accurate reading and not an overly generalised and overstated one such as the online map implies. Feeding the information on here such as in reports can be valid and enough collated data and information on localised sky brightness might be relevant in the challenge to protect what there is of dark sky quality. Which is not just for our own indulgence and wellbeing but for the protection of wildlife habits.  

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4 hours ago, Saganite said:

Skye is class 1 Bortle.

Skye has its own challenges. Right at the start of getting into this hobby I tried to get some photos with the limited long exposure on my bridge camera. Parked up somewhere on the coat road I didn't do very well, and when I started hearing large animals breathing loudly and moving around on the rocks nearby (I guess they were seals) it did get more than a bit spooky...

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17 hours ago, Moonshane said:

I'll know more accurately on Thursday if clear. I get a SQML from my birthday 

Great stuff.  I got one for Christmas, superb present, been having great fun with it :thumbright:

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There is no single reading that defines your sky darkness for all time. I take SQM readings from my site occasionally and can say that it varies a lot. Mostly I get values in the mid-21's 21.4, 21.5, 21.6

However the value changes with the seasons, with the sunspot cycle with the time of night and with many other factors. Also SQM can be very high when the place is completely clouded over! ? While SQM reading also give some idea of transparency - with little in the way of particles / water vapour to reflect back light from sources of pollution, it tells us little about steadiness, which seems to me to be a more valuable metric.
The best ever was at the beginning of January this year when it was -8°. Here's the photo

SQM-21_82.thumb.jpg.584fec409cb97ed42af8a6024cbe0c90.jpg

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10 hours ago, Saganite said:

Skye is class 1 Bortle.

Some parts are. Unfortunately I live near-ish to the main town of Portree so get a little bit of light pollution from there but it only knocks it down to a 2.

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5 hours ago, Stub Mandrel said:

Skye has its own challenges. Right at the start of getting into this hobby I tried to get some photos with the limited long exposure on my bridge camera. Parked up somewhere on the coat road I didn't do very well, and when I started hearing large animals breathing loudly and moving around on the rocks nearby (I guess they were seals) it did get more than a bit spooky...

I've only been here a year and the darkness still gets me. We live in a small township with not a lot of houses so I can literally walk out of our house at night and not be able to see the car parked 20 ft away! One your eyes get light adapted after a minute you can see but its scary at first. Its also sooooop quiet.....

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5 hours ago, Skipper Billy said:

I am just across the way from Skye !

 

dark.jpg

Hello! I'd give you a wave but there are some big mountain's between us!

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4 hours ago, pete_l said:

There is no single reading that defines your sky darkness for all time. I take SQM readings from my site occasionally and can say that it varies a lot. Mostly I get values in the mid-21's 21.4, 21.5, 21.6

However the value changes with the seasons, with the sunspot cycle with the time of night and with many other factors. Also SQM can be very high when the place is completely clouded over! ? While SQM reading also give some idea of transparency - with little in the way of particles / water vapour to reflect back light from sources of pollution, it tells us little about steadiness, which seems to me to be a more valuable metric.
The best ever was at the beginning of January this year when it was -8°. Here's the photo

SQM-21_82.thumb.jpg.584fec409cb97ed42af8a6024cbe0c90.jpg

Interesting, thanks!

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7 minutes ago, bobmoss said:

Some parts are. Unfortunately I live near-ish to the main town of Portree so get a little bit of light pollution from there but it only knocks it down to a 2.

 

The skies above my home are class 4 Bortle, as is Kelling Heath I believe, and I am happy with that, so class 1 to 2 is observing heaven. It has become the second home to our Dob Mob, so enjoy those skies Bob !!

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45 minutes ago, bobmoss said:

Unfortunately I live near-ish to the main town of Portree

Well that's a phrase you don't hear very often...

Edited by Stub Mandrel
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