SQM readings

[bomberbaz]

Ok so last night I went to a place which wasn't the best for viewing. The south was wiped out by skyglow. Overall SQM was 20.90. Lovely skies at zenith, pants lower down especially south.

Now a little further on from last night (10 minutes drive) I have found a spot that is 5 miles deeper, offset by a few miles to the side of the LP and has a SQM of 21.10.

I intend recce-ing this place. Smack in the middle of know where, no known localised LP and well sheltered but with great aspects. I know the place (roughly speaking) and think it would be a far better option.

Anyhow, SQM is something I could do to get my head around more. Seems it starts high up the scale anyway (18+ in central manchester) and goes up in the smallest increments the darker it gets.

Can anybody though explain in simple terms the SQM readings and how they work please?

Steve

[Waddensky]

The SQM measures sky brightness in magnitudes per square arcsecond, it's the same unit used to describe the surface brightness of deep-sky objects. The larger the number, the darker the sky. The darkest natural skies are around SQM 22.00, by using a telescope you can get even darker backgrounds due to the darkening you get at higher magnifications.

The difference between the sky brightness and object brightness determines the 'contrast' of the object and the background sky. The larger this contrast, the easier to spot the object. Since the brightness of deep-sky objects does not change, making the sky background brightness darker (by moving to a darker site) will result a larger contrast with the deep-sky object, and therefore this object will be easier to see (and more objects are within reach).

The Wikipedia page on the Bortle scale relates the different sky brightnesses to Bortle. As you can see, the sky brightness of an inner city sky is smaller than 18.00. Daylight sky brightness is around SQM 4.00.

[bomberbaz]

Ok so that's reasonably straight forward then. I have read the bortle scale chart before, so not completely niave.

However, does the sqm reading take account of skyglow from nearby towns and can this adversly affect the reading.

That is to say in my own example, you have a lot of skyglow lp from the south but next to nothing from the north. This lowers the reading to the detriment of the northern views which are actually far superiour to your southern options. Hope thaat makes sense.

[Waddensky]

Measurements are usually done in zenith, right above your head. Results depend on the type of SQM device you have. I own an SQM-L and they have quite a narrow field of view: they measure the brightness of a narrow piece of sky where you point the device to. I usually take a few measurements around the zenith to avoid the light of the Milky Way or other bright objects to influence the results.

The 'classic' SQM device has a larger field of view and will be affected by skyglow of nearby towns.

[Captain Scarlet]

The unit of measurement is magnitudes per square arc-second. In other words, a reading of 20 will be equivalent to the brightness of  a single mag-20 star occupying every square arc-second of sky...

[pete_l]

Quote

does the sqm reading take account of skyglow from nearby towns

No. It only measures the darkness of what you point it at. Although the "width" of what it sees is quite broad.

It is an interesting question about skyglow. It is caused by light that is shone upwards getting reflected off particles of dust or moisture in the air. The more cr*d you have in your air - the more pollution, for example - the more light will be reflected back down. So greater transparency will also darken the sky background.
It will be interesting to see if amateurs have noticed better sky transparency and darkness during this time when there is acknowledged to be mush less air pollution. Especially if compared to this time last year.

[vlaiv]

SQM is not very precise - it has measurement cone FWHM of about 45 degrees. SQM-L is a bit more precise - it has about 20 degrees of FWHM for measurement (more narrow part of the sky is measured).

Both can give somewhat inaccurate readings because they average out reading over said part of the sky. This is why it is advised to aim at zenith - the least changes of brightness can be expected there.

To put it into perspective - difference between mag20.9 and mag21.1 is 0.2 mags. You can use standard magnitude formula to see the ratio of intensities between the two: mag = -2.5 * log(I1/I2) or

I1/I2 = 10^ ( mag * (-2.5) ) = 10^ -0.25 = ~0.56

Intensity at darker site is about half that on brighter one. Mind you - we don't perceive it as half as bright - our vision is closer to magnitude scale - it is same difference as between mag6 and mag6.1 star - very small in terms of perceived brightness.

Probably best thing to have is something like this:

It does however require specialized lens / camera and computer to generate.

From here you can see how SQM reading changes depending on direction and altitude of measurement.

In my view - it's worth having such chart for observatory / regular observing site

[Captain Scarlet]

43 minutes ago, vlaiv said:

It does however require specialized lens / camera and computer to generate. ...

It seems that image was produced using a Canon 6D, i.e. a full-frame DSLR (I have one). Software is just software, so I could get it somewhere? And I'd need a fisheye or perhaps stitch together a few frames from a sufficiently wide-angle lens (I have a Samyang 14mm/2.8)?

So all I, for one, need is the software ... ?

[vlaiv]

25 minutes ago, Captain Magenta said:

It seems that image was produced using a Canon 6D, i.e. a full-frame DSLR (I have one). Software is just software, so I could get it somewhere? And I'd need a fisheye or perhaps stitch together a few frames from a sufficiently wide-angle lens (I have a Samyang 14mm/2.8)?

So all I, for one, need is the software ... ?

I've found these images online - specifically on LightPollutionMap.info website - you select different SQM/SQM-L/SQC readings to be shown on the map.

Most of them are by Andrej Mohar.

I searched for software that could convert all sky images to SQM readings, but I have found none.

Anyone having suitable planetary camera and short focal length lens should be able to do such a map - provided software is available for that. In principle, software is just a bit more complicated than measuring sky background on regular images. This is because for the most part - regular images taken with telescope are more or less uniform in geometric distortion (large zoom - means low distortion).

Here we have "fisheye" type lens that has very high distortion. Pixel in center does not cover same surface area as pixel at the edge of image and this needs to be taken into account when creating map because we need amount of light per surface area.

Other than that - filtering bright stars - which can be accomplished by doing multiple exposures over the course of half an hour for example and then doing sigma clip stacking - rejecting any brighter pixels as having bright star in them. Not sure how one would subtract Milky way from the image though.

But you are right - if you can do all sky image with your gear like this one:

You can produce report like this one:

If you visit lightpollutionmap.info - and download these measurements, you'll find even more images that are interesting, for example this one:

Which happens to be cylindrical projection of brightness together with markings of large cities/towns and their distance - very cool.

I've just seen that there is SQC 1.9.3 marking on these images - could be software name and version number .... of to search the net to see if I can find it.

[bomberbaz]

Interesting answers and explains a lot too. I know of a place which on the face of it should be pretty rubbish but actually gives better results than it should. Mainly due to the fact most of the bad LP is northern, the southern aspect is clear. 

I mean really it should be obvious and I did expect last nights viewing to be affected by the LP, but not as badly as it turned out. 

It was gone midnight and above me the skies were super dark, inky black and I was struggling to spot some known stars as there were so many others, not bad considering there is no true astronomical twilight for my latitude at the moment.

However if I looked south it turned to dark blue, lightening to what almost felt like civil twilight by the horizon. Hence my nights viewing was cut short as half my list was in the south section. 

I am guessing an SQM-L pointed towards the southern skyglow would have given a pretty low reading and to be fair, it is that which I find the most important given that is where most of the DSO's are best viewed.

[ScouseSpaceCadet]

 

On 21/05/2020 at 11:01, bomberbaz said:

Ok so last night I went to a place which wasn't the best for viewing. The south was wiped out by skyglow. Overall SQM was 20.90. Lovely skies at zenith, pants lower down especially south.

Now a little further on from last night (10 minutes drive) I have found a spot that is 5 miles deeper, offset by a few miles to the side of the LP and has a SQM of 21.10.

I intend recce-ing this place. Smack in the middle of know where, no known localised LP and well sheltered but with great aspects. I know the place (roughly speaking) and think it would be a far better option.

Anyhow, SQM is something I could do to get my head around more. Seems it starts high up the scale anyway (18+ in central manchester) and goes up in the smallest increments the darker it gets.

Can anybody though explain in simple terms the SQM readings and how they work please?

Steve

bomberbaz, assuming you drove north from Burnley to your secret spot, if you keep driving another 30 minutes, you will hit possibly the darkest spot in Lancashire, around Slaidburn in the Forest of Bowland AONB. SQM around 21.25 & no distracting LP intrusion from nearby cities.

I'd kill for 21.10 with some horizon sky glow ten minutes drive away! 😉

 

Edited May 25, 2020 by ScouseSpaceCadet

[BGazing]

On 21/05/2020 at 17:15, bomberbaz said:

Interesting answers and explains a lot too. I know of a place which on the face of it should be pretty rubbish but actually gives better results than it should. Mainly due to the fact most of the bad LP is northern, the southern aspect is clear. 

I mean really it should be obvious and I did expect last nights viewing to be affected by the LP, but not as badly as it turned out. 

It was gone midnight and above me the skies were super dark, inky black and I was struggling to spot some known stars as there were so many others, not bad considering there is no true astronomical twilight for my latitude at the moment.

However if I looked south it turned to dark blue, lightening to what almost felt like civil twilight by the horizon. Hence my nights viewing was cut short as half my list was in the south section. 

I am guessing an SQM-L pointed towards the southern skyglow would have given a pretty low reading and to be fair, it is that which I find the most important given that is where most of the DSO's are best viewed.

I have SQM-L and two of my friend have them, too. Most of the readings on lightpollution map in Serbia are done by us and the readings side-by-side are pretty accurate.

Over the years some things of note:

- When cooling it gives a slightly higher reading. So keep it ambient.

- Do avg of 3 readings at zenith, just in case.

- You can point it sideways to have a better idea of the quality further from zenith.

- Milky Way significantly affects it, easily 0.3 at dark place.

- SQM reading varies by season and atmosphere conditions. But it never lies. The BEST Swan I have ever seen was on a brutally cold and damp night in June on our dark site when fog was crawling in the gullies and we shivered, in C8 Swan literally burned our retinas. I kid you not. It was 21.77, the highest reading we recorded.

- SQM scale is logarithmic. Meaning that the difference between 21.0 and 21.1 is not nearly as big and pronounced as between, say, 21.5 and 21.6. Once you are over 21, every 0.10 will be a significant improvement and deserves a bit more hassle (i.e. driving).

- SQM cannot read transparency. There will be nights when the readings will be high but the views not as good as they were on nights of similar or even lower readings. 

We had a 21.65 night last Friday. It was magic.

Edited May 25, 2020 by BGazing

[bomberbaz]

2 hours ago, ScouseSpaceCadet said:

I'd kill for 21.10 with some horizon sky glow ten minutes drive away! 😉

 

Actually nearer a half hour, the ten minutes is on top of the initial 20. Still good though.

46 minutes ago, BGazing said:

We had a 21.65 night last Friday. It was magic.

Now your just bragging 😉

I have observed under skies of 21.85, I got lost because there were that many stars.

Thanks for the responses guys

[ScouseSpaceCadet]

1 minute ago, bomberbaz said:

Actually nearer a half hour, the ten minutes is on top of the initial 20. Still good though.

Now your just bragging 😉

I have observed under skies of 21.85, I got lost because there were that many stars.

Thanks for the responses guys

Yea half an hour drive for a few hours observing in a dark spot is doable... 30 minutes for me headed north would at best be Bortle 5 and sqm around 19.5, the other directions, the Mersey, the Irish Sea or even worse, Manchester! 😆

[BGazing]

16 minutes ago, bomberbaz said:

Actually nearer a half hour, the ten minutes is on top of the initial 20. Still good though.

Now your just bragging 😉

I have observed under skies of 21.85, I got lost because there were that many stars.

Thanks for the responses guys

'Fcourse I'm bragging LOL

Would you believe that I got lost, too. I was not sure which quadrangle is actually Hercules.

21.10 is good for 30 minutes drive, actually very good, wish I had it for short hops. This site is 2 hrs drive for us, luckily there is a house we are renting on the spot. Pefect place, if a bit wet at times. 21.2 site of another astro group (another direction) is another 2 hrs drive, it is just less brutal in winter, sort of reserve position.

[bomberbaz]

6 minutes ago, BGazing said:

21.10 is good for 30 minutes drive, actually very good, wish I had it for short hops. 

Not checked it out properly yet, but in brief it is twice as deep into the country as my wash out place the other night and also moves the LP source a further 6 km towards the west. I am hopeful that this will make it as you said a decent place for a quick jaunt.

I have access to skies at 21.7, these are just over an hour away but I am not going there at the moment until the return of proper astronomical dark. 

[scarp15]

2 hours ago, BGazing said:

- SQM scale is logarithmic. Meaning that the difference between 21.0 and 21.1 is not nearly as big and pronounced as between, say, 21.5 and 21.6. Once you are over 21, every 0.10 will be a significant improvement and deserves a bit more hassle (i.e. driving).

 

Yes I agree with that and also as you mentioned to take a succession of three readings and base on the average. Not having a SQM-L device at dark sky locations, to determine an accurate sky brightness assessment, is a disadvantage in my opinion.

[PeterW]

Upwards of 21 is good, beyond 21.5 the numbers don’t change much (Comparing to the borle descriptions is better). Transparency is key, being at a mag21.5 site and seeing the stars just vanish as cloud Rolls is is depressing... same SQM reading. I have an issue at home with street/building lights that are outside the SQM field of view, scattering off the SQM window and Giving me a lower value, now I have a stray light shield to help.

 

peter