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SQM Limitations

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#1 Tony Flanders

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Posted 02 May 2019 - 06:24 AM

The Sky Quality Meter (SQM) and its close cousin the lensed Sky Quality Meter (SQM-L) have revolutionized the study of artificial sky glow. For the first time in history, amateurs can purchase a relatively inexpensive device that can measure sky brightness well down below the level produced by natural sources such as airglow, starlight, and the zodiacal light.

 

On a purely individual basis, this makes it possible to calibrate one's observations against an objective measure of light pollution. Like many others, I take SQM and/or SQM-L readings for every serious observing session. These help me confirm my own gut feeling about which nights are better than others at my three or four customary observing sites, about how those sites compare with each other, and about how they stack up against places that I only visit once or a few times during the course of my travels.

 

At the social level, the SQM (and SQM-L) make it possible to calibrate other people's observations against one's own experience. If I hear somebody say "I can see NGC 3628 from my suburban backyard," I have very little idea how bright or dark that backyard really is. A suburb of a small city in the western U.S., surrounded on three sides by desert, is a very different thing from a suburb of New York City. And even within a single metropolitan area, suburbs have a huge range of brightness. But if someone tells me "I can see NGC 3628 from my backyard, SQM reading 19.0," then I have a very good idea how bright that backyard really is.

 

However, there is a tendency to overinterpret SQM readings, because SQMs have numerous problems that severely limit their accuracy.

 

First of all, SQMs are based on a sensor whose sensitivity varies depending on the temperature. The SQM then adjusts those raw sensor readings according to its own internal thermometer, so that in theory it always gives the same result regardless of the temperature. Lab tests indicate that it does this pretty well. But the real world isn't a lab. It is a known fact that SQMs give wildly variable readings until they equilibrate thermally, just as telescopes give poor images until they equilibrate. For this reason, I and many other take a series of readings, and only start recording the results once the readings have stabilized, which in my experience usually happens within a few minutes.

 

Once the SQM has equilibrated, I then take five consecutive readings, and record my results with a standard deviation, e.g. 21.12+-.03. My deviations are usually down to +-.02 or +-.03, but occasionally as big as +-.06 or worse. That's a pretty big range!

 

Second, SQMs have a truly gigantic field of view. They are most sensitive to the region within 45 degrees of the optical axis, but they do get some light from as far as 75 degrees from the optical axis. That means that any directly visible light more than 15 degrees above the horizon can render the results all but meaningless. It is, for instance, almost impossible to get meaningful SQM readings when the Moon is up. In those cases, of course, the SQM reads too bright. More subtly, visual obstructions such as trees -- including trees behind one's back that one's not even aware of -- can make the SQM read too dark. In my part of the world (the eastern U.S.), it's a pretty rare observing site that doesn't have some obstruction high enough to affect SQM readings.

 

That is why the SQM-L was invented. It still has a vast field of view by telescopic or binocular standards, but it does have near-zero sensitivity to light 45 degrees off axis, making it usable (with care!) in typical suburban settings, or when the Moon is up but not too high.

 

A less obvious fact is that the optical axis of both SQM and SQM-L units is not necessarily well aligned with the physical axis. When you think they're pointed to the zenith, they may in fact be reading 10 or 15 degrees from the zenith.

 

Finally, although any individual SQM or SQM-L gives pretty stable readings over moderate time periods, there is quite a bit of variation among different units. I am now proud owner of two SQMs that read roughly 0.15 magnitude different from each other, and two SQM-Ls that read roughly 0.19 magnitude different from each other. In a recent thread (sorry I can't find it!) someone else reported inter-unit variations as high as 0.4 magnitude.

 

To make matters even worse, individual units can drift over time. I began to suspect this of the first SQM that I ever purchased, because its difference from my SQM-L had clearly changed over a matter of years. I shipped the SQM back to Unihedron, who confirmed that it was no longer calibrated correctly, and supplied a new unit for free.

 

Now for some purposes a variation of +-.2 around the norm is not a big deal. A typical darkish urban site or very bright suburban site reads around 17.8 to 18.2; there's not a huge difference in the appearance of deep-sky objects within that range. Both are quite different from skies in the 18.8 to 19.2 range, which is characteristic of medium-dark suburbs.

 

But at the dark end of the rage, where most of skyglow is natural and artificial light pollution is a trace contaminant, a difference of +-.2 is very significant indeed. I can vouch that there is a vast difference in the appearance of the Milky Way -- or of galaxies through a telescope -- between skies that read 21.3 on my original SQM and skies that read 21.7 on the same unit.

 

I have to conclude that attempting to draw conclusions about two different dark sites based on readings of two different SQMs are futile. If one site reads 21.8 and the other 22.0, that's as likely due to variations between the two SQM units as it is due to actual differences between the sites.

 

Likewise, drawing conclusions based on comparison between an SQM unit and an SQM-L unit at the same site are meaningless. Some time ago, Don Pensack noted that his SQM-L always read darker than his SQM -- which one would expect based on theoretical considerations. But my own SQM-L usually read brighter than my SQM, which baffled me considerably. Now I know that was due to the fact that my SQM had drifted dark. But Don's observations on this subject are equally suspect; there's no way to determine how much of the difference between his SQM and SQM-L is due to physical reality and how much is due to inaccuracies of the units themselves.

 

And if (like me) you want to measure how skyglow is changing at any given site over a matter of years or decades, you are well advised to own at least two different SQM units, and preferably more, to compensate for any possible instrumental drift.


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#2 Jon Isaacs

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Posted 02 May 2019 - 07:16 AM

But at the dark end of the rage, where most of skyglow is natural and artificial light pollution is a trace contaminant, a difference of +-.2 is very significant indeed. I can vouch that there is a vast difference in the appearance of the Milky Way -- or of galaxies through a telescope -- between skies that read 21.3 on my original SQM and skies that read 21.7 on the same unit.

 

:goodjob:

An excellent post, very insightful.

I just wanted to add to your point above. Not only are the dark end differences probably more important important but they're also more difficult.  Typically an instrument is rated with an +/- error but it's an absolute error related to the noise floor.  When one is using magnitudes then it's a relative measurement.  A +/- 0.2 magnitude difference at magnitude 21.5 is a much smaller amount of light than it is at magnitude 18.5, about 1/16th.  That means the S/N ratio is much lower and one would expect more relative variation.

My only experience comparing SQM's was one night comparing my SQM-L with a SQM at the Grand Canyon Caverns. Both measured the sky at about 21.8 mpsas but the sky in the direction where the horizon was visible measured about 21.8 near the horizon on my SQM-L.

Jon


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#3 ks__observer

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Posted 02 May 2019 - 07:49 AM

I picked up an SQM-L about a year ago.

It was one of the best astronomy purchases I made.

I visit various sites, as well as stay local, and I always check the SQM regularly.

I usually do 2 to 4 measurements back-to-back when taking reading -- there is always a little variation.

I was able to get some confirmation as to my relative accuracy: One night at a dark site with a friend doing astrophotography, around midnight he noticed a marked drop in his background average signal; we checked my SQM and it had gone from 21.3 earlier to 21.6.

As you note, the SQM is good but has limits -- I wonder how much more it would cost to manufacturer an instrument to the next level of accuracy and uniformity.



#4 knightware

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Posted 02 May 2019 - 08:38 AM

I have also done side-by-side experiments with different models of SQM. Given uniform aiming, etc I've had differences of a few hundredths which I believe is reasonable. If readings between units under these conditions are different by several tenths, I would send my meter back to Unihedron to be re-calibrated. Just but be sure to eliminate the possibility of operator error in taking readings wink.gif

 

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#5 calypsob

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Posted 02 May 2019 - 09:01 AM

There is a company called Astromechanics that now produces an SQM monitor which reads a 10 degree fov. https://astromechanics.org/lpm.html  You can use it like a unihedron SQM or SQM-L, although, imo these non usb Unihedron devices are only good for rough evaluations. They help me determine if there are thin transparency issues I cannot detect with the naked eye. I revisit the same location throughout the year and know what to expect coming in, and I have learned that if readings are below what I expect, I probably have a transparency issue.

 

To truly create an accurate sky reading you need to consider a few variables. The most important is the time of night, Midinight-1am is usually going to be the darkest hour here in Virginia, I can watch the SQM slowly creep from mag 21 at 9pm to 21.6 by midnight.  A friend recently visited a darksite in Arizona which was 21.8 at 9pm, I guess that is a testament to how dark those skies are! We usually do 5 readings every 10 minutes.

The other variables include sensor temperature as you mentioned earlier and transparency issues which can be from fog, clouds, dust, smoke, etc.

 

The new product by Astromechanics interests me alot because it can do a time series over the course of the night, this means that although temps could effect the image, you are getting multiple readings and you would have a very goo visual refrence of transparency and sky darkness. Its other strengths are that it operates like a normal sqm-l with a push button reading and it has a 10 degree fov, as an astrophotographer this will help me get a much better zenith reading, which is where I try and shoot anyway to avoid gradients in my images.

 

I agree with your feedback regarding the unihedron product, it is an amateur instrument, but indeed a useful tool if your require its capabilities. It is not extraordinarily precise but gets close enough to get the job done if you understand its limitations and quirks.


Edited by calypsob, 02 May 2019 - 09:04 AM.

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#6 Ron359

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Posted 02 May 2019 - 11:38 AM

According to the Unihedron website the SQM are calibrated to a NIST standard.  But if you dig a bit further you'll find the "NIST standard" is basically a commercially made photographic light meter, designed to measure in tens to hundreds and thousands of lumens.  That is -several orders of magnitude brighter than anything the SQM is intended for.  I don't see how a sensor calibrated to those levels so far off can be called 'calibrated' for very tiny levels of light. The sensor and electronics would have to be extremely linear in the response over many magnitudes of brightness. Also, the sensor is not sensitive to blue wavelengths, which is where the vast majority of lumen output of LED streetlights falls, along with increased scattering of bluer light, the SQM will 'not see' much of contribution as more and more LEDs contribute to light pollution and so underestimate the amount of light pollution.  It may even lead to the wrong conclusion that light pollution is decreasing over time.  An example of bad data is worse than no data.  

 

 I see many discussions on CNs of observers 'splitting hairs' in arguments about dark skies and tenths of readings completely unaware of the difference between precision and accuracy v. full scale calibration range of instrument readings.  A SQM precisely fits the old engineering & science adage of "measure it with a micrometer, mark it with a chalk, cut it with an axe."  


Edited by Ron359, 02 May 2019 - 11:41 AM.

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#7 Procyon

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Posted 02 May 2019 - 01:27 PM

Tony, or anyone, quick question. The maps show I live in a borderline white/red zone. But my backyard is a forest, 2 side neighbors blocked by fence or tall trees. Does that mean I still live in the dead zone? I think I should get one and measure.



#8 havasman

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Posted 02 May 2019 - 04:45 PM

Good points Tony.

 

All gauges are less than absolutely reliable and yet our civilization can be said to be reliant on their data. Water and electrical power, for instance, would not arrive at our outlets w/o them. Back in my manufacturing days we learned to doubt and trust our gauges and operated on the principal that if your gauge said something was perfect then it was time to get a better gauge. At the same time, when the gauge was adequate to the measurement it was the right gauge.

 

So, though there's no real reason to know it is a better device, I am ordering one of those Astromechanics devices to join my SQM-L. It is not encouraging to find "Each LPM PRO is factory-calibrated. The absolute precision of each device is believed to be ±0.10 mag/arcsec2" in the spec table in the user manual as belief is essentially incompatible with measurement resolution. But @ $129 it's worth a try.

 

In all cases and despite the limitations of these consumer gauges, I very much prefer relying them to some completely uncalibrated individual's objective reading of sky brightness.


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#9 Arcticpaddler

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Posted 03 May 2019 - 07:03 PM

The original post, and my long experience with the shortfalls of electronic sensors, only reinforces why I have not purchased an SQM.  The sky from my backyard here in NE Minnesota is solidly Bortle 3, and I determine the transparency from a few objects.  For example, if M92 (in summer) or M33 (in fall) are naked eye objects, I will definitely take the scope out.  I've seen Leo I and the Horsehead with my 8-inch SCT on excellent nights.  That said, the Bortle 1 and 2 skies an hour to the north and in parts of the West actually make my skies seem bright when I come back home.

 

If I was convinced that a SQM would not drift over time and was extremely similar to other units out there (so I could compare apples to apples) I would likely buy one.  But like all scientific instruments, calibration and repeatability is critical.


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#10 havasman

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Posted 08 May 2019 - 10:40 PM

I am ordering one of those Astromechanics devices to join my SQM-L. 

Ordered May 3. Shipped today, May 8. Good communication all the way from them. Original shipping promise was 10 - 15 days, under promise and over deliver. waytogo.gif



#11 Redbetter

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Posted 08 May 2019 - 11:43 PM

I just haven't seen the sort of wild variations that some have.  My SQM-L seems to track pretty well with what I would expect compared to other's readings, etc.  I have had it for nearly 3 years.  A 0.2 MPSAS departure would be quite noticeable, but I wouldn't be surprised if mine reads up to 0.1 MPSAS low on average, because I do not get 22.0 MPSAS values with it, high 21.8's, at Bortle 1 to Bortle 2 sites at times, but nothing above 21.9 that I can recall. 

 

I haven't seen much if any temperature sensitivity other than the initial reading, and even that is typically only 0.05 to 0.07 MPSAS high.  The circuit always seems to stabilize after the first reading and it doesn't seem to change noticeably as the temp of the unit tumbles by 40 F as it heads toward night time ambient.  It has gotten to the point that I only take a 3rd reading about half the time (e.g. early in the session, if there was some sort of transient light effect during the reading, something seems out of whack on a reading, or if conditions have changed and I want to re-establish a baseline.)  Perhaps half of the time I just take the 2nd of a pair of readings every few hours.  I do keep my SQM-L covered by a blanket on my observing table except when I take a reading, this is to prevent any subcooling that might result in dew on the lens. 

 

I am planning to buy a standard SQM for comparison, although I don't know how useful it will be at the treed sites I frequent.  And I am really mostly interested in the sky darkness of a cone overhead stretching down to 45 degrees.


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#12 Tony Flanders

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Posted 09 May 2019 - 05:08 AM

I just haven't seen the sort of wild variations that some have.  My SQM-L seems to track pretty well with what I would expect compared to other's readings, etc.


I can vouch from personal experience that it's essentially impossible to assess an individual unit in isolation. I only became aware that my original SQM had drifted by carefully comparing its measurements against my SQM-L over a course of many years. And even then the signal wasn't easy to tease out of the noise.

On its own, any given unit gives highly repeatable measurements. It's just that a different unit may give readings that are (in my case) consistently different by well over 0.1 magnitude. Anybody who doubts that is welcome to borrow my units!
 

I wouldn't be surprised if mine reads up to 0.1 MPSAS low on average, because I do not get 22.0 MPSAS values with it, high 21.8's, at Bortle 1 to Bortle 2 sites at times, but nothing above 21.9 that I can recall.


I don't think you can deduce anything at all from that fact. There's nothing magical about that nominal 22.00 MPSAS value; it's just some dude's wild idea of a dark sky, conveniently rounded to a number that has two zeros after the decimal point. In fact most people never get 22.0 readings from SQM or SQM-L units, including some whose skies have essential zero artificial light pollution.

Remember that there's a very big variation in natural skyglow due to latitude, solar activity, position of the zodiacal band, Milky Way, etc.
 

I am planning to buy a standard SQM for comparison, although I don't know how useful it will be at the treed sites I frequent.  And I am really mostly interested in the sky darkness of a cone overhead stretching down to 45 degrees.


I do not recommend using a lenseless SQM at any site where the treetops reach more than 45 degrees above the horizon in any direction.


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#13 chris3c273chris

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Posted 09 May 2019 - 07:32 AM

There is a smartphone app, dark sky meter, that uses the phones camera to measure sky brightness. Based on the comments in this thread I wonder how accurate the measurements are. For example, I don’t think it compensates for temperature.

Has anyone compared dark sky meter values with SQM/SQM-L readings?

#14 Redbetter

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Posted 09 May 2019 - 03:56 PM

I don't think you can deduce anything at all from that fact. There's nothing magical about that nominal 22.00 MPSAS value; it's just some dude's wild idea of a dark sky, conveniently rounded to a number that has two zeros after the decimal point. In fact most people never get 22.0 readings from SQM or SQM-L units, including some whose skies have essential zero artificial light pollution.

On the contrary, after something like 150 nights in a mix of Bortle 1/2/3 conditions, combined with information from other's meters, various dark sky maps, etc. I can deduce quite a bit from it.  The most obvious is that it is not an outlier.  The second is that it has not drifted to any noticeable extent.  The third is that it doesn't read appreciably high, which is the most frequent error noted.  The potential remaining error bracket is rather narrow and all on one side,  it isn't +/- 0.1 if it is off, instead it could only be reading spot on or a little low.

 

When one has a sample set of thousands of readings over a hundred or more nights in dark to very dark skies, and none are reaching 22.0, it yields an effective upper limit for the meter/sky in real world conditions.  This is after all a logarithmic scale, so 0.1 mag is about 10% difference in brightness.  Thin/scattered loud has almost always brightened the sky per the meter, presumably because of illumination from man made sources unless the cloud was low (relative to the elevation) and blanketing in a very obvious way.

 

It doesn't seem likely that 22.0 is that far off as a baseline for pristine dark sky either, despite being a conveniently round figure.  Most things will effectively brighten the result, so it should be rare.  Reliable values at or above 22.0 for an SQM or SQM-L seem to be rather rare--recent TSP readings being an example.    


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#15 Arcticpaddler

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Posted 09 May 2019 - 05:51 PM

I wonder how well the predicted sky quality at the zenith on the excellent, Dark Site Finder site compares with reality.  From what I can tell from by own observations in the Rockies and Upper Midwest, the categorial ratings (Bortle Scale) are quite good, but I have no idea if the predicted SQM readings are accurate.  For my back yard, it rates the sky as Bortle 3 (it is), but the predicted SQM reading of 21.88 seems too dark.  It would be fun and useful to see more actual SQM data from a geographically diverse range of sites compared to the predicted values.


Edited by Arcticpaddler, 09 May 2019 - 05:52 PM.

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#16 chris3c273chris

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Posted 09 May 2019 - 09:01 PM

It would be fun and useful to see more actual SQM data from a geographically diverse range of sites compared to the predicted values.


Have you looked at the globe at night website? They collect a lot of sky quality data. There is a section specifically for SQM-LE data.

https://www.globeatnight.org/maps.php
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#17 Tony Flanders

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Posted 10 May 2019 - 05:59 AM

On the contrary, after something like 150 nights in a mix of Bortle 1/2/3 conditions, combined with information from other's meters, various dark sky maps, etc. I can deduce quite a bit from it.  The most obvious is that it is not an outlier.  The second is that it has not drifted to any noticeable extent.  The third is that it doesn't read appreciably high, which is the most frequent error noted.  The potential remaining error bracket is rather narrow and all on one side ...

These are all excellent points.

I do suspect based on my own experience that the skew in SQM and SQM-L units is lopsided toward the dark -- numerically high -- side. Certainly, when my SQM drifted, it did so by starting to read too dark. Which is what one would expect if the sensor tilted away from the optical axis.

A good way to investigate this would be to launch a coordinated campaign in an astronomy club or at a major star party. Get together everyone with an SQM or SQM-L in one place, record all their readings, and there would be enough data to do some serious statistical analysis.

 

It would also make it possible for the participants to know how to adjust their readings to compensate for deviations in individual units. It's fine if my SQM reads 0.15 high, as long as it does so consistently, and I know that it does so.


Edited by Tony Flanders, 10 May 2019 - 06:00 AM.


#18 Jon Isaacs

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Posted 10 May 2019 - 09:26 AM

A few thoughts:

 

- Multiple readings: I take several readings, I am not comfortable with just two consistent readings. I find very often the first two consistent readings are not supported by the third. 

 

- A dirty lens will cause the sky to measure darker.

 

- The calibration scheme interests me.  Having worked in research and devised calibration scheme's of my own, I would use some sort of light with a calibrated light source and carefully characterized/calibrated filters.

 

Apparently the SQM devices are calibrated against a NIST certified device but it suspect that certification does not reach 22.0 mpsas.  A standard solar filter is 100,000:1, that's 12.5 magnitudes. With a calibrated filter and a light box with a evenly illuminated surface, again characterized and calibrated, the NIST meter could be reading much brighter surface that was appropriate for it's range while the SQM could be reading a much darker surface.

 

- As I mentioned previously, the error would normally consist of and overall accuracy plus a error due to noise.  The fact that this is a logarithmic measurement means the accuracy cam be expressed in magnitudes but the noise or least significant bit cannot be.  

 

At the lowest light levels, the darkest skies, the error as measured in magnitudes will be much greater than under bright skies.  3 magnitudes = 16 x so I would expect the noise error to be 16 times greater in absolute terms because noise is an absolute number..

 

- These are black boxes to most of us. Knowing how an instrument works is an important part of making accurate measurements.  About all I have been able to deduce is that this device is temperature compensated and that it integrates the signal until a given level is reached and then seems to time as the measured value.

 

Thermal calibration at low light levels would seem to be able important since one way or the other, the signal is the smallest so dark current and noise in the sensor are the greatest.

 

Building a calibration box, even if it were only relative and not absolute, could be interesting. Drift and thermal compensation could be investigated.

 

Jon


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#19 Ron359

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Posted 10 May 2019 - 04:22 PM

A few thoughts:

 

 

Apparently the SQM devices are calibrated against a NIST certified device but it suspect that certification does not reach 22.0 mpsas.  A standard solar filter is 100,000:1, that's 12.5 magnitudes. With a calibrated filter and a light box with a evenly illuminated surface, again characterized and calibrated, the NIST meter could be reading much brighter surface that was appropriate for it's range while the SQM could be reading a much darker surface.

 

- As I mentioned previously, the error would normally consist of and overall accuracy plus a error due to noise.  The fact that this is a logarithmic measurement means the accuracy cam be expressed in magnitudes but the noise or least significant bit cannot be.  

Building a calibration box, even if it were only relative and not absolute, could be interesting. Drift and thermal compensation could be investigated.

 

Jon

There isn't much point in going to all that trouble if there is no way to adjust the meter's calibration.  Which it doesn't seem to have.   And yeah, the 'NIST standard' is just a 'light meter' measuring light in tens to hundreds of lumens.  Easily found for sale if you look for it.  



#20 Kendahl

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Posted 10 May 2019 - 04:33 PM

Last spring, I bought a lensed Sky Quality Meter. Readings at my favorite dark sky site typically range from 21.15 to 21.31. Last time out, I got 21.48. These variations don't bother me since the meter's accuracy is rated at ±0.1 and it's obvious to the naked eye that sky conditions vary significantly even over a few days. According to lightpollutionmap.info, the site is supposed to be 21.66 which it has never achieved. Overall, it's solidly Bortle 4.

 

What does disturb me are the readings I got last year at the Nebraska Star Party which everyone agrees is held under Bortle 1 skies. My readings ranged from 21.48 to 21.59. That's within the meter's rated accuracy but is Bortle 4, not Bortle 1.



#21 Jon Isaacs

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Posted 10 May 2019 - 05:11 PM

There isn't much point in going to all that trouble if there is no way to adjust the meter's calibration.  Which it doesn't seem to have.   And yeah, the 'NIST standard' is just a 'light meter' measuring light in tens to hundreds of lumens.  Easily found for sale if you look for it.  

 

A look up table is all that is necessary, a calibration table, a measured value in one column, the calibrated "actual" value in the next column. One could do a curve fit and use 2 variables, that's what the SQM is doing.

 

Jon



#22 Jon Isaacs

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Posted 10 May 2019 - 05:31 PM

Last spring, I bought a lensed Sky Quality Meter. Readings at my favorite dark sky site typically range from 21.15 to 21.31. Last time out, I got 21.48. These variations don't bother me since the meter's accuracy is rated at ±0.1 and it's obvious to the naked eye that sky conditions vary significantly even over a few days. According to lightpollutionmap.info, the site is supposed to be 21.66 which it has never achieved. Overall, it's solidly Bortle 4.

 

What does disturb me are the readings I got last year at the Nebraska Star Party which everyone agrees is held under Bortle 1 skies. My readings ranged from 21.48 to 21.59. That's within the meter's rated accuracy but is Bortle 4, not Bortle 1.

 

I think the light pollution maps are optomistic.  And the variation is quite normal. In November, for whatever reason, my sky is abnormally bright, barely 21.0 at the Zenith. Don Pensack, 50 miles to the north, was measuring something very similar. I sometimes see 215, more typically 21.3.

 

These are sites rated at 21.65 -21.85 by the maps.  I have never seen my SQM-L read what the map says. I believe the map is an estimation based on satellite images and the increased sky glow due to the reflectivity of the atmosphere is difficult to predict.

 

Jon


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#23 Ron359

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Posted 10 May 2019 - 06:38 PM

Last spring, I bought a lensed Sky Quality Meter. Readings at my favorite dark sky site typically range from 21.15 to 21.31. Last time out, I got 21.48. These variations don't bother me since the meter's accuracy is rated at ±0.1 and it's obvious to the naked eye that sky conditions vary significantly even over a few days. According to lightpollutionmap.info, the site is supposed to be 21.66 which it has never achieved. Overall, it's solidly Bortle 4.

 

What does disturb me are the readings I got last year at the Nebraska Star Party which everyone agrees is held under Bortle 1 skies. My readings ranged from 21.48 to 21.59. That's within the meter's rated accuracy but is Bortle 4, not Bortle 1.

A prime example of the difference between precision and accuracy.   



#24 Redbetter

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Posted 10 May 2019 - 07:12 PM

Last spring, I bought a lensed Sky Quality Meter. Readings at my favorite dark sky site typically range from 21.15 to 21.31. Last time out, I got 21.48. These variations don't bother me since the meter's accuracy is rated at ±0.1 and it's obvious to the naked eye that sky conditions vary significantly even over a few days. According to lightpollutionmap.info, the site is supposed to be 21.66 which it has never achieved. Overall, it's solidly Bortle 4.

 

What does disturb me are the readings I got last year at the Nebraska Star Party which everyone agrees is held under Bortle 1 skies. My readings ranged from 21.48 to 21.59. That's within the meter's rated accuracy but is Bortle 4, not Bortle 1.

The variations in the first are likely from actual conditions (real) vs. a meter difference.  My lower altitude Bortle 3 site runs anywhere from 21.0 on a night with very poor transparency and the summer Milky Way overhead to 21.5+ MPSAS in good conditions without the Milky Way.  The light pollution maps give only a relative sense, rather than absolute.  They do decently in brighter conditions but are likely to over estimate MPSAS in darker skies.  (The light pollution map says 21.76 MPSAS for the same site I mentioned above.)

 

As for the Nebraska Star Party, it would be possible to get readings in the range you note, depending in actual conditions.  The real question is what were the actual conditions and what were others seeing?  Were they getting hammered by smoke from fires in the west?  If the conditions at the time were considered very good and other meters were reading in the 21.8 range or so, then your meter is likely off.  There is some substantial seasonal variation, particularly if pointing at the summer Milky Way or when the Milky Way if high in the sky, since this also impacts other parts of the sky.  At a Bortle 1 site my meter will read 21.8+ in average transparency then drop several tenths as the summer Milky Way begins to dominate.  At Bortle 1/2 transition I see similar results, sometimes starting a little below 21.4 and rising all the way to 21.7+ as the Milky Way sets.  On more favorable nights it quickly peaked in the high 21.8 range in excellent transparency before falling as conditions changed.


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#25 Ron359

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Posted 10 May 2019 - 10:40 PM

A look up table is all that is necessary, a calibration table, a measured value in one column, the calibrated "actual" value in the next column. One could do a curve fit and use 2 variables, that's what the SQM is doing.

 

Jon

If every user had to go through your calibration routine and create an individual table every year or few months, the instrument is useless for general practical use.   Users only need is to understand its not a precision laboratory instrument and to take the readings with a big grain or several big grains, of salt as described in my earlier post.  




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