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Old 04-01-2019, 03:21 AM
glend (Glen)
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Share your SQM?

I was just out closing up my observatory (3am here), and as the Seeing was particularly good for mid-summer, so I took a SQM reading: 20.85 . This is not far off my best ever recorded reading at my place on Lake Macquarie (20.92) which was from a few years ago. Increased development in the area has affected sky quality, but it is pleasing that on a good night it's still pretty good.
Have you noticed and recorded changes in sky quality at your place? What is acceptable to you?
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Old 04-01-2019, 08:43 AM
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multiweb (Marc)
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I can only dream of an SQM reading. My sky is rated in term of color glow. It varies between orange sodium to the south and blue neon to the west. I can make out about 3 stars in crux visually.
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Old 04-01-2019, 11:58 PM
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mental4astro (Alexander)
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There is one thing I don't get about SQM readings - do they take into account the quality of the transparency of the sky too, or just "how dark" it is?
Because an SQM reading can mean bugger all if transparency is stuffed.

I've been to one dark site that is really dark, but transparency is terrible most of the time. Another site I use is much less "dark", but transparency is significantly better than the much "darker" other location, so I can actually see more and fainter stars and objects. I routinely see the Horse of Sagittarius at the brighter site, and next to never at the other darker site, as an example. So will an SQM give me the quality of transparency too?

Sorry if my question is a bit of a digression from Glen's original intentions, but I as the topic has come up now, I felt it necessary to ask here.

Alex.
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Old 05-01-2019, 01:54 AM
AnakChan (Sean)
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Maybe this will help answer the transparency question :-

http://unihedron.com/projects/darksky/faqsqm.php

"The SQM's readings are assuming 'best transparency'.

You can get an updated definition of the transparency in your area from:

Attila Danko's Clear Sky Clock for North America.
Andrew Cool's SkippySky Astro-Weather Forecast for Europe, Australia, and North America.

Also, frequently local weather stations can provide "visibility" and "relative humidity" numbers which could potentially be used as surrogates for actual transparency measurements (which aren't possible with a handheld meter)."

Anyhow my "best" SQM was with some of the Takahashi Star Party meet in Yass back in 2016. I didn't measure it but someone did and quoted 21.67.

But around Japan, in a gondola carpark in Tateshina, I had 21.30, whist my usual astro spot in Chiba is around 20.97 at best. Whereas central Tokyo is a joke 15.xx or 16.xx.

In my family home in suburban Perth, I had 18.95. Can't remember what I had for Canning Dam, W.A. though.
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Old 05-01-2019, 08:03 AM
glend (Glen)
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Quote:
Originally Posted by mental4astro View Post
There is one thing I don't get about SQM readings - do they take into account the quality of the transparency of the sky too, or just "how dark" it is?
Because an SQM reading can mean bugger all if transparency is stuffed.

I've been to one dark site that is really dark, but transparency is terrible most of the time. Another site I use is much less "dark", but transparency is significantly better than the much "darker" other location, so I can actually see more and fainter stars and objects. I routinely see the Horse of Sagittarius at the brighter site, and next to never at the other darker site, as an example. So will an SQM give me the quality of transparency too?

Sorry if my question is a bit of a digression from Glen's original intentions, but I as the topic has come up now, I felt it necessary to ask here.

Alex.
The SQM is just a tool that takes the human subjective assessment out of equation. Take fifty people and your likely to get 50 different Bortle ratings from them, regardless of experience. People have different eyes, some good , many bad. As a tool the SQM can be a standard that does not require a human to make an assessment.
Sure transparency is a factor, and the SQM can be somewhat duped by poor transparency, which is why readings made over several night's is a better indicator than one offs. In the case of my location I have years worth of readings to consult. At least the SQM, no matter where it is used around the world, makes exactly the same reading and it is not looking through cataracts, dirty glasses, or the bottom of a Port bottle.

Last edited by glend; 05-01-2019 at 11:21 AM.
  #6  
Old 05-01-2019, 10:08 AM
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OneCosmos (Chris)
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SQM readings

I have returned from a family holiday to Ballandean in the Granite Rock area of SE QLD and I took not only my SDM 16” but also my sky quality meter.

I tested the sky at different times on successive nights (4 nights) and, at midnight got readings of between 21.67 and 22.34! That came as a surprise as I didn’t even know it could read 22 anything. It was the darkest sky I have ever seen. Does anyone know if there is a max reading possible? Would the middle of the Simpson desert read 23 something?

I smiled when taking a reading about an hour after the sun set because it was 18.4 which is about what I get at best in my garden in Brisbane!

Duckadang and the QLD Astrofest reads typically 21.7 or so.

Regarding transparency I think if there was say mist the ambient star light would be diffuse causing a worse reading so to that extent the meter will act as an indicator. The SQM is most useful when used in your usual observing site multiple times across the year because better readings on some nights may well indicate less smearing of ambient light.
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Old 05-01-2019, 03:05 PM
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Hi Alex & All,

Quote:
Originally Posted by mental4astro View Post
There is one thing I don't get about SQM readings - do they take into account the quality of the transparency of the sky too, or just "how dark" it is?
Because an SQM reading can mean bugger all if transparency is stuffed.
There are several components that make up an excellent sky and they include minimum of artificial light pollution, minimum of natural light pollution (Zodiacal light, gegenschein, sky-glow etc) altitude and lastly solid particles in the atmosphere that scatter light (dust, soot, water-droplets volcanic aerosols etc). To rank them in order of importance, (1) Artificial light pollution (2) Solid particles, (3) Natural light pollution and (4) Altitude. (Though 3 & 4 are probably a close-run thing).

The Earth's atmosphere (assuming a complete absence of solid particles) is uniform over the surface of the globe in composition and therefore equally transparent. The only variable here is thickness -- increasing altitude means you have to look through less of it. It is largely transparent except over long distances.

The amount of solid particles is going to vary a lot from location to location and indeed from day to day, month to month and year to year depending upon the weather, climate and how many "Plinian"-type volcanic eruptions have occurred in recent times. These types of eruptions inject large quantities of very fine silicate dust into the stratosphere. At the present time, the stratosphere is pretty clean and the last truly big event was Mt Pinatubo in the mid-90s. At least living in the bush means you only have to live with "natural" solids.

The Unihedron SQM & SQM-L aren't the absolute end of the story, but I believe are a very strong and importantly objective indicator of the quality of a site. They measure the brightness of the background sky in magnitudes per arc-second Sq. They can't take into account the amount of star-light blocked-out by solids in the atmosphere (upper and lower) but they do, for all practical purposes take into account all the other factors that will make the sky brighter or from scattering incoming light.

One Cosmos, you mentioned readings in excess of 22.0. Normally, a reading much over 22.0 shouldn't occur. How long have you had the unit? I returned mine recently (after 11 years) for re-calibration and replacement of the I.R filter because I was sometimes getting readings in the 22.1 - 22.25 range. I compared it with a new unit that was giving a different reading. Over time this filter can become frosted due to interaction with water (ie dew, fog etc etc). It was retested after replacement of the filter and is now accurate again. In winter on moonless nights around midnight returns reading in the 21.6 to 21.8 range (usually depending on the amount of humidity) and in summer 21.8 to 22.05. The cost of the service was just the cost of return postage to Unihedron in Canada.

I live in a site that is very, very low in artificial light pollution, 70km south of Orange, 30km north of Cowra and 260km from Sydney (as the photon flies) at 321m elevation. To find a "perfect" site (leaving aside elevation) I'd think you'd need to be at least 500km from any major city, about 150km from any major rural centres (think Orange, Wagga, Tamworth, Dubbo etc etc) and 70km from any good-sized towns with populations over 5000.

Plenty of places to choose from that meets these criteria, but actually living there on a day-to-day basis is a chore as you have to travel for at least an hour for "anything" -- there are little to no facilities. A compromise must be made on that front.

Best,

L.
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Old 05-01-2019, 03:11 PM
glend (Glen)
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I might add to the excellent post below, apparently the SQM can give strange readings if the battery is not providing sufficent power.

This archived thread suggests readings in excess of 22 are due to a bad battery.

http://www.iceinspace.com.au/forum/a...p/t-71823.html
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Old 06-01-2019, 09:04 AM
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Thanks everyone for the replies.

I'm still struggling to see how comparing SQM readings is valid though. A sky quality meter can only gauge "how dark". It cannot tell you anything about those other factors that go into making one site better than another, even when they share the same SQM value. It can't, even by your own thinking Les. Instead, it can only give a false sense of security if one hasn't undertaken an appropriate and thorough site selection process. Worse is if your site selection process solely relies upon SQM readings. It means your site selection process is no better than just finding an open grassy field. By just comparing numbers is meaningless if the other factors that go into accessing an astro site are not also taken into consideration. An SQM reading may be objective, but it is mute as it assumes EVERY site is equal in sky transparency and every location has the exact same geographic and microclimate conditions with only artificial light as the one point in common. As one factor of many in investigating a potential astro site, yeah ok. As a tool by which to compare sites, no.
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Old 06-01-2019, 12:16 PM
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Hi Alex,

Quote:
Originally Posted by mental4astro View Post
Thanks everyone for the replies.

I'm still struggling to see how comparing SQM readings is valid though. A sky quality meter can only gauge "how dark". It cannot tell you anything about those other factors that go into making one site better than another, even when they share the same SQM value. It can't, even by your own thinking Les. Instead, it can only give a false sense of security if one hasn't undertaken an appropriate and thorough site selection process. ...
With respect, I disagree. The SQM and SQML-L reading will be affected one way or another by three of the four criteria I listed -- Artificial LP, Natural LP and to some extent, altitude.

Sure it won't be able to read how much air you have to look through nor how "dirty" it is, but in my experience with using it, you won't get a good reading at a crap site, nor will you get a crap reading at a genuinely good site.

I do agree that it probably should not be used as the sole criterion in assessing a site and thee are other factors that play in, but it is at least objective (provided the readings are faithfully recorded) whereas any other method is going to be mostly subjective and subject to the frailties of memory.

On a side note: It is one of the things that disappoints me about our particular discipline within the wider astronomical community is that it relies very frequently on the honesty and objectivity of the observer. Sure, you can on many occasions have another confirm what you see, or claim to see in the eyepiece, but many times, if not most that isn't available and you have to try very hard to be objective (ie don't inadvertently mislead yourself) (averted imagination) and not to tell out-and-out "porkies". I've read (to be honest it is mostly "read' people infrequently make outlandish claims "in-person" -- face to face) hundreds of patently silly claims about what can and cannot be seen/done in this, that and the other aperture. When I was a younger man I used to often argue the toss on these ones but as the years have rolled by I'm more inclined to sprout a "knowing grin" and pass by claims of that type.

... and this is the point of the above diatribe: More often than not on the point of whether something can or cannot be seen/done, it falls to claims about how good the observing site is or the conditions on the night were. But these factors, again, are easy to either "pump-up" or simply lie about. ie I could see magnitude 7.4 stars at zenith, the Milky Way cast shadows, the Gegenschein could be seen at only 20 degrees altitude etc etc. The SQM if properly and faithfully used introduces a significant degree of objectively to assessing the conditions. As I said, not the be-all and end-all and does (again) require the claimant to be truthful about the reading. But it doesn't lie about what it "sees".

Please anyone reading do not see what I have written above in a general sense, as an accusation against you or a person you know of gilding the lily or lying about what you do or don't observe. It is a general observation about the nature of our observing discipline and the reporting of what is or isn't seen.

Best,

L.
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Old 06-01-2019, 01:05 PM
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"Averted Imagination" - I love it!


Last edited by glend; 06-01-2019 at 01:17 PM.
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Old 06-01-2019, 02:24 PM
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Atmos (Colin)
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Quote:
Originally Posted by glend View Post
"Averted Imagination" - I love it!

I rely in averted imagination when going galaxy spotting with my 4" in Melbourne and some nights I have a very vivid imagination

9ish months ago when we were at Allans place in Coolah I experienced very dark skies. After a few nights there I spent a night at Heathcote on the way back to Melbourne and the skies looked like dirty dirty mud!

I've never done any readings in Heathcote but others over the years have managed 21.2-21.5 as an average so that last little bit towards 22 must make quite a big difference.
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Old 06-01-2019, 03:52 PM
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Light pollution maps

With regards to a meter reading greater than 21.99 this site is interesting as it gives the readings for each site and, for example the Simpson desert is listed as 22 site!

https://www.lightpollutionmap.info
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Old 06-01-2019, 08:24 PM
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We are presently staying at Kingscote on Kangaroo island , South Australia. I don't know what the reading is here, but it's very dark. I will look it up later on the light pollution website.

I just checked.... Around kingscote 21.99 and a few km away at the North Cape 22.00. Needless to say the naked eye view is wonderful.
Best
JA

Last edited by JA; 06-01-2019 at 10:24 PM.
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Old 06-01-2019, 08:35 PM
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Quote:
Originally Posted by OneCosmos View Post
With regards to a meter reading greater than 21.99 this site is interesting as it gives the readings for each site and, for example the Simpson desert is listed as 22 site!

https://www.lightpollutionmap.info
Using the link on the attached map Iím pulling 21.89 sqm...........and Iím 60min from Melbourne. Pretty happy with that
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Old 06-01-2019, 09:07 PM
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Well, according that map:


Observatory Hill (Sydney) 18.16 and Bortle 8-9;

At my place it’s 20.37 and Bortle 5;

15 minutes away the NSAS site at Terry Hills is 20.56, Bortle 4;

Two hours drive to Shipley Plateau - 21.78, and Bortle 3 which is understandable with the sky glow from Sydney in the east.

Eight hours drive to Mt Kaputar - 22, and Bortle 1.

IMHO the Bortle scale is more meaningful as across these locations the SQM value varies by only 3.84 (and to be quoted 4 significant figures is frankly absurd when measuring sky brightness) whereas the Bortle values range from 1 to 9.

Last edited by Wavytone; 06-01-2019 at 10:21 PM.
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Old 06-01-2019, 11:33 PM
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Hi Wavytone & All,

Quote:
Originally Posted by Wavytone View Post
IMHO the Bortle scale is more meaningful as across these locations the SQM value varies by only 3.84 (and to be quoted 4 significant figures is frankly absurd when measuring sky brightness) whereas the Bortle values range from 1 to 9.
But, the magnitude scale is a logarithmic one -- with each whole step being effectively 2.52 times fainter than the preceding whole step. Whereas, the Bortle scale is linear, so the reality is ... (I'll let you find a pencil & an envelope).

Best,

L.
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Old 06-01-2019, 11:41 PM
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I am ready to have this thread locked. It has been hijacked in my opinion.
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