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Sky Quality Meter


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There's a USB version? Hmmm... I've got one of the original versions which is quite useful for comparative sky brightness. It can be affected though by the position of the Milky Way - when Cygnus is overhead then the sky background appears brighter...

I've had some fun doing some comparative readings - Kelling is about the same darkness as my main observing site south of Taunton, and Exmoor is substantially darker than Kelling... and for the record Kielder is indeed darker than Kelling :( (At least according to my SQM).

I'll bring mine along to SGL5 and we can see how it compares to SteveL's.


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I think the best I have ever seen from my back garden imaging location was 20.25, which using the maths to convert from SQM to NELM, makes that about Mag 5.6.... which I do seriously doubt, but hey, who am I to argue with maths :(

The USB version appears as a COM port (via prolific drivers) and is easy to knock up an application to read the details from it. The maths is a bit more complex but certainly doable (I wrote mine in Perl). Awaiting bonus to see if I have anything left to buy the USB version after the essentials have been paid for...

They do a ethernet network version too for those with permenant setups and some network knowledge.

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Seem to agree to around 0.1 between units. The Isle of Wight Starparty this year was regularly giving us 21.3-21.4 from 2 units. The newer lens version has a narrower field of view, so allows you to compare the sky in different directions. They should be required equipment at starparties... see how the sites change with time. I got mine as I live under 18.5

(mag/arcsec^2) skies and wanted something to be able to tell me how good a dark sky was when I was lucky enough to get to one, rather than going "wow I see a lot of stars"! I am also a physicist and like numbers!

Maybe we need to collate together the SQM readings from all the different starparties that have popped up... "substantially darker"==???!!!



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...and Exmoor is substantially darker than Kelling..

"substantially darker"==???!!!

In this case substantially darker is derived from the following averaged readings :( :

Exmoor (eastern end of, not the darkest area): Best reading: 21.71, Average 21.62

Kelling Heath: Best reading 21.58 Average: 21.45

Kielder: 21.72 (based on several readings over one night)

My back garden: 19.80 (on a good night)

My back garden 2 years ago on a good night: 20.05

Don't forget the scale isn't linear - i.e. the difference between 21.45 and 21.62 is greater than the pure numbers suggest.


Edited by James
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they are a great little device.

have owned one for about 2 years

the best reading i have seen from my garden is 19.60

an average moonless night with good transparency is around the mid to high 18's

and from the dengie peninsula supposedly the darkest site in essex

the best reading i have seen is 20.65

mine usually lives in my glove box , so i am prone to suddenly leaping out of the car

to wield it , much to someones annoyance, she is usually sitting there with her head in here hands!!!


Edited by essex sky
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hi pete

although i am a dyed in the wool visual observer it is very helpfull

as over time you get to know by what readings you get.

what the faintest object you are likely to see is

i know for a fact that at this time of year if i want to see m97

through my c8 i need a reading of at least 18.90

so the same princible can be applied to imaging.


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I definitely have to get one! Think I'll go for the lensed version.

Manufacturer's FAQ says: "A reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80." Forestry Commission Scotland says of Galloway Forest: "The Forest Park’s scores range from 21 to 23.6" Hmm...

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Hi All,

I have done some work with Sky Quality Meters so I can contribute some thoughts.

First, I would definitely recommend a model with lens. That restricts the area of sky measured to a circle of about 20 degrees. This helps keep trees, over-hanging structures and bright celestial objects from affecting the measurement. The meter is sensitive enough to detect a bright planet or milky way in the field of view.

Second, the newer models have computer communication capability but no button for manual readings. The ethernet model can be used over a network (wi-fi or wired), or directly connected to a computer with an ethernet cross-over cable. The USB model requires some free third party drivers. I wrote an article about how to setup the drivers.

I really like the USB meter for field use because it does not require an extra power source - it gets its power over the USB cable from my laptop computer. I mount it on a photo tripod and run it while I observe.

Also, the formula that converts the SQM reading to a NELM value is theoretical. On the other hand, NELM itself is a very subjective value. This is the beauty of the SQM - it takes personal factors out of evaluating sky darkness.

Finally, people are having success correlating SQM readings to a minimum surface brightness for visual and imaging use. I'm currently experimenting with that.

- Phyllis

Edited by knightware2
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  • 2 weeks later...

OK, I got myself one, the SQM-L, from Teleskop Express in Germany. Cost 142 euros (£128) including postage, and arrived in two days.

Teleskop-Express: Astro-Shop + Fotografie + Naturbeobachtung

Only been able to play with it in the back garden so far - registering 18.6, which comes out as NELM 4.5. That's what I estimate the limit to be on a good night but it was hazy when I tried it and I couldn't even see the third star of Ursa Minor (mag 3). It'll be interesting to see how dark-site readings compare with my NELM estimates: I anticipate getting consistently higher readings from the machine than from my middle-aged eyes. The unit is a very handy size and I'll be using it on every observing session. Will be really useful.

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I have a sky quality meter bought from Starizona, (tucson, AZ)

and I use it to measure the output of bright lights in my area.

I use a small refractor and focus on a yardlight in my area and then

take a reading with the meter. I also use a laser rangefinder to get the distance to the light and compare it with a table I made for legal light output at different distances, say 500 ft. it the meter reads higher than my table, the light gets turned into the county if the

light owner refuses to split the cost of a shield. What a way to make

new friends...:D

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Interesting - do you mean you aim the SQM through the refractor? Is it an unlensed one? I've been wondering if mine could have an application in monitoring streetlight output.

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  • 10 months later...

Re accurate and genuine SQM readings, the highest I've ever recorded from my mountaintop observatory in the South Island of New Zealand at almost 2,000m is 22.02.

On clear, moonless evenings/nights/mornings, Venus and Jupiter are dazzling and cast obvious shadows, and the MW is bright enough to cast faint shadows.

The SQM's manufacturer, Unihedron, suggest that a reading of 22 or above is unlikely. A reading of 23 should probably be considered highly suspect.

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Re accurate and genuine SQM readings, the highest I've ever recorded from my mountaintop observatory in the South Island of New Zealand at almost 2,000m is 22.02.

Now that's just bragging!

My aim for the month is to get out in the Peak District on a decent night with one of our SQM-L's and see if I can uncover some proper dark skies. I'm told they're out there...

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