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SQM measurements.

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#1 Piggled

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Posted 24 September 2019 - 05:58 PM

So I just got my meter a few weeks ago. I live in the center of the city of San Francisco and last night, no moon, it was 18.5. Kind of depressing. I would like to know what other people have registered at their sites. It would be nice to know that there is some kind of hope out there........

PS I did search for this but couldn’t find anything in the first couple pages......

#2 Redbetter

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Posted 24 September 2019 - 06:19 PM

That is darker than I would have expected for the city center.


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#3 scadvice

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Posted 24 September 2019 - 06:31 PM

Three miles west of Lodi, Calif. the meter has read 20.20 to 20.34. Do a sampling of your area as it will change depending on where you point the meter.

 

You most likely should state which meter. Mine is a Unihedron Sky Quality Meter with Lens - Narrow Field of View SQM-L


Edited by scadvice, 24 September 2019 - 06:40 PM.

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#4 Darren Bly

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Posted 24 September 2019 - 06:48 PM

I get about 18.5 in the middle of Bakersfield, CA. My normal observing site is 48 miles from my house and is about 21.2. The best I've seen was once at Panament Springs Resort l got 21.85. BTW: These readings were overhead with the original SQM.

Edited by Darren Bly, 24 September 2019 - 06:52 PM.

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#5 krisSeattle

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Posted 24 September 2019 - 06:56 PM

I live 5 mile NE of Seattle and my measurement using regular Unihedron are usually in 17.5-17.9 range. The darkest I got was 18.1.

I used to complain a bit till a friend moved 7 houses down the street and mentioned that he can see many more stars compared to his previous location that was 1 mile north of downtown!

 

At granpa's place - East Port Angeles, WA SQM was about 19.1 last time I measured.

Ka'anapali, HI was 18.9 few weeks ago



#6 TOMDEY

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Posted 24 September 2019 - 07:01 PM

I'm on the outskirts of Springwater... "Upstate" New York. I measure between 21.2 and 21.5 on clear, moonless observing nights... and feel relatively blessed to have that right in my backyard! I moved here a few decades ago to excape the suburbs and cities.    Tom


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#7 Tony Flanders

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Posted 24 September 2019 - 08:00 PM

Yes, San Francisco is a pretty small city, surrounded by water on three sides and a substantial mountain range to the south, making it abnormally dark for a major U.S. city. 18.5 is quite good near a city center. A few years ago, I recorded 17.6 from the Cambridge Common and 17.2 on the Boston Common, using an SQM-L. I usually get around 18.5 in Arlington, 7 miles from the center of Boston.

 

My country home typically runs in the low 21s -- 21.5 on a really good night, maybe as bad as 20.8 on a bad one. Still very far from really dark, but a lot better than 18.5.


Edited by Tony Flanders, 24 September 2019 - 08:01 PM.


#8 havasman

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Posted 24 September 2019 - 10:56 PM

17.8 is the highest SQM-L reading I've had from the driveway. I bought the house long before I started observing.

 

The dark site is 106 miles away and averages @ 21.35 and is often quite transparent.



#9 scadvice

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Posted 24 September 2019 - 11:23 PM

For those who are not familar with the SQM and Bortal scale. Here is a link.

 

https://en.wikipedia...ki/Bortle_scale



#10 ausastronomer

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Posted 25 September 2019 - 05:19 AM

We are very fortunate in Australia with our prevailing Sky conditions.   You only have to fly across Australia once from Sydney to Perth and then fly across the USA from Chicago to LA or from say Orlando ----> Denver ----> San Francisco to appreciate the difference.

 

We basically have Bortle 1 Skies (SQM > 21.6) within 3 hours drive of every major city in Australia.  The big difference is that Australia has a large land area, but all the major cities are right near the coast and 95% of the Australian Population live within 50km of the coast.  There isn't much away from the Coast. In the USA there are large cities all over the entire Country.

 

I live right on the ocean about 2.5 hours South of Sydney.  I live in a small village with a population of about 1,000 people with no commerce. I have decent skies at home and usually get 20.4 to 20.8 on my SQM after 10pm at night, when the neighbouring domestic lighting gets switched off.  It's certainly dark enough in my back yard to make all of the brighter DSO's pop and give aesthetically nice views. If I want to observe dimmer targets, I have Bortle 2 Skies (SQM 21.3 to 21.5) within a 40 minute drive from home and Bortle 1 skies (21.6+) within 2 hours drive from home.

 

Cheers

John B


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#11 Piggled

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Posted 25 September 2019 - 02:51 PM

I am surprised that people think that’s high for the city! I guess once adjusted you can actually see quite a few stars, but it is always such a shock when you get somewhere with truly dark skies. Gonna be picking up a Ioptron CEM40 today for traveling. The LX850 has proven to be quite the PITA to move around. I think a lot of the problem for my observing is that I live in a neighborhood called Cole Valley and just like it suggests we are in a little valley with lots of houses up on the hills around us so the light comes down at us even though my scope is on the roof.

And I have just the Unihedron SQM standard version.

#12 RLawson

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Posted 26 September 2019 - 09:10 PM

I live about 10 miles south of Knoxville, Tennessee and get an average of 19.2

I take 4 readings and average them.



#13 Redbetter

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Posted 27 September 2019 - 08:28 AM

I take 4 readings and average them.

You would get a more accurate result by throwing out the first reading and averaging the other three.  The initial reading is biased to the high side by the way the circuit stabilizes on start up (per Unihedron's site.) 

 

I don't even bother with averaging anymore.  I just throw out the first reading then take two or three more.  Typically the 2nd and 3rd read identically or within 0.01 MPSAS or so.  If the numbers show more spread/and a further decline then I will take another reading to see if stabilizes.


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#14 TOMDEY

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Posted 27 September 2019 - 09:35 PM

You would get a more accurate result by throwing out the first reading and averaging the other three.  The initial reading is biased to the high side by the way the circuit stabilizes on start up (per Unihedron's site.) 

 

I don't even bother with averaging anymore.  I just throw out the first reading then take two or three more.  Typically the 2nd and 3rd read identically or within 0.01 MPSAS or so.  If the numbers show more spread/and a further decline then I will take another reading to see if stabilizes.

Yeah, me too, on that. The 1st reading is always sort of a warm-up exercise... and then they stabilize nicely.    Tom



#15 Allan Wade

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Posted 13 October 2019 - 05:19 AM

My suburban home averages 19.8 to 20.2 on the SQM. That’s in the middle of 300,000 people.

 

My astro property that’s three hours away ranges from 21.6 to 22.0, depending where the Milky Way is at. When the winter Milky Way is directly overhead in the southern hemisphere it has a big influence on how bright the environment looks. My experience is that observing sites in the northern hemisphere don’t vary in brightness so much through the seasons like they do in the southern hemisphere.

 

This time of year when the Milky Way lays flat on the horizon later in the night is when I get the 22.0 readings. There is a really big difference in how dark the sky and observing environment looks going from 21.6 to 22.0.


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#16 BPO

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Posted 13 October 2019 - 05:09 PM

The SQM-LEs at my observatory in New Zealand record around 22 on moonless nights, while here in Japan I get about -30... (I kid, it's not that terrible where I live, and some places are surprisingly dark, considering. Los Angeles is far worse.)




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