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Arduino Sky Quality Meter - working!


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My understanding is that commercial ones use auto-ranging(?) 
and sample upto 90 seconds? Personally I ended up adding two
slide switches on the front panel! One just switches between 10
and 60 seconds. The other is for single-shot "manual" (push the
button!) readings or "auto" continuous output. The coding can
then branch depending on what you want to do. That may still
be "subject to modification" (in my case) as they say... :p

Hmmm... I forget how to propagate(?) "errors of measurement"
now! As I say, the count rate is 20-60Hz with my Mag 5-6 skies.
Isn't the error "Root N"??? But I leave you do the math(s)! lol :D

But frankly "about a minute" seems MORE than adequate for
the "best" skies? I use a mere 10s for quick readings. At some
stage I might check for consistency of the results. I sense the
biggest error is likely to be "systematic" and depends on the
value of the "constant" used (To be define by experiment...) ;)

Edited by Macavity
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So, finally my DIY arduino SQM is finalized, it is not calibrated yet, but hang in there, i will do so in the near future... But first i want to point out that i didn't want any scientific grade SQM b

I recently had a chance to use the Unihedron SQM and SQM-L meters and decided I wanted to build my own. A search led me to this thread, thanks for all this great info! In researching this project I di

Here’s a few preliminary sketches of my DIY SQM that I’m building. Just need my 3D Printer to show up now. I may have to revise the ‘turret’ area where the lens is to make it a bit more printer f

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  • 1 year later...

If you go to the sourceforge repo and dig inside the Documentation : https://sourceforge.net/projects/arduinomysqmskyqualitymeter/files/mySQMPRO/Documentation/

The "mySQMPRO.pdf" has all the build instruction (multiple schematics, ...).

You can also find the Schematics in https://sourceforge.net/projects/arduinomysqmskyqualitymeter/files/mySQMPRO/Schematic and Layout/

 Rodolphe

 

Edited by rpineau
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  • 2 weeks later...

Looking to build one of these myself, but finding a UK supplier with stock is becoming tricky.

Has anyone got hold of any recently? Mouser have the TLS238 (SMT version), but little else. Shipping is £12 too!

If anyone has any 'spares' I'd gladly buy them off them!

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I started some time ago building something similar, although for now I put it on hold (way too many projects!)

I'm gonna use a TSL2591 instead https://cdn-shop.adafruit.com/datasheets/TSL25911_Datasheet_EN_v1.pdf, which should have the two following very desirable characteristics:

 - very high dynamic range

 - separate L and IR sensor, which should be able to give visible light only readings without installing filters.

It's also much easier to find on the market than the TSL237, and it offer a very easy to use i2c interface.

I also plan to add to the project a temperature/humidity/pressure sensor (BMP280), which beside giving useful information, can also be used to compensate the TSL readings for temperature variation, and a GPS sensor.

Furthermore, I'd like to add an SD Card module, to save all the readings to a CSV file, and possibly a wireless/bluetooth module.

Now, the project itself is not too complicated, as all the involved modules use I2C or SPI, and therefore they're pretty easy to wire to the Arduino (although I'll be using a Blue Pill instead, in my case).

Last time I was planning this I was a bit stuck for the calibration part, as I'd need a commercial SQM sensor for that.

 

I'll keep posting updates here as I go further with the project.

Edited by GuLinux
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On paper the TSL2591 is whole lot less sensitive in low light than the light-to-frequency sensors. The reason being that it output a number of counts proportional to light level, and at low light levels the difference between 1 or 2 counts is getting pretty big. Some rough calculations give a count of 1 equal to mag 22.5 and count of 2 equal to 21.8, so it would be impossible to measure reliably at low light levels. The maximum dark signal is also a maximum of 25 counts which is in the region of mag 19 (in reality it will be lower). It may work well enough in towns and cities, but below mag 19/20 it will struggle to give meaningful results.

The TSL235 is a better alternative to the TSL237. The responsivity is lower so the output frequency will be ~4x lower. The TSL37 would have an output period of around 10s at mag 22, and the TSL235 around 40s. However the dark current of the TSL237 is about 15-20x greater, which will have a big impact at low light levels unless you can compensate for temperature. My experience has shown the dark current is often less than specified in these sensors though.

Counting periods of seconds with an arduino gives excellent resolution, and actually increases in magnitude terms the fainter you get, whilst your resolution will decrease dramatically with the i2c sensors under darker skies.

I probably have a bunch of TSL237s somewhere I can put up for individual sale if there's interest.

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Another important thing to note is if these are in permanent installations (i.e. exposed to the sun all day) make sure you have a good UV (and IR) filter in place. If exposed to UV over long periods the transparent packaging of all of these sensors will go yellow. This will reduce sensitivity to blue light and ruin any long term light pollution measurements.

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@12dstring interesting.

I was planning to use high gain, long exposure, and an average of multiple exposures to compensate for measurement errors, in order to get a decent accuracy.

I'm not sure if that's enough or not, and I'm not very skilled at reading components datasheets, so I can only trust someone more capable than me, or at least give it a try, and see how it works.

 

I think replacing the TSL2591 with a frequency component will be relatively easy anyway, so it's worth setting up a base project, and swapping components if needed.

Worth actually trying a comparison, I guess.

I saw the TSL235 is relatively easy to find on ebay, while TSL237 can be found in some uk online stores (low price, but relatively high shipping cost).

 

I'm not sure how I'll go forward, but since I already have the TSL2591, I'll probably try that first, then run a comparison with either the TSL235 or the TSL237 (I was already planning to compensate for temperature anyway, since my project already includes a temperature sensor).

 

The blocker for me right now is calibration, though, it's quite difficult to do it without a calibrated reference SQM reader. This is why I temporarily stopped my project (that, and the fact that I also have tons of other open projects, and little time to finish any of them).

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I didn't mean to discourage trying the TSL2591. As I haven't tested it yet I can't comment on how it works in the real world, only on paper. I didn't want people to think that the i2c sensor is automatically an 'upgrade' as it's easier to make readings with, as there is a big sensitivity difference.

Taking multiple readings to average will be the best method. Just like imaging, more light gives better signal-to-noise. The frequency output sensors naturally increase effective exposure time for lower light, 10s or more under the darkest sky.

Calibration is an issue, and it's not necessarily solved by having a commercial SQM to compare with. Unless you have the same filter, lens and spacing, you won't be able to calibrate them properly if you want to make accurate comparisons. DIY SQMs are best used for relative measurements, i.e. is tonight darker or lighter than average for my site, is my site getting brighter..etc

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On 23/02/2018 at 13:57, 12dstring said:

I probably have a bunch of TSL237s somewhere I can put up for individual sale if there's interest.

I’m definitely interested in a couple of these so please pm me. 

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  • 2 weeks later...

The filter used in the Unihedron SQM is a Hoya CM-500, however I don't think it's a particularly good filter for the job. It claims to match the human eye response but passes light below 400nm which the human eye can't see, and will also cause the sensor plastic to yellow if exposed to the sun for long periods.

You can pick up cheap IR/UV filters from China/Hong Kong. These are the ones I use and plan to measure the transmission curve properly at some point, but the specs say Tavg > 95% @ 440-620nm which will allow LED and Sodium light to pass fully whilst blocking invisible wavelengths.

There's lot of LED lenses around, e.g. this one. The one in the SQM doesn't have a part number so not sure exactly which one it is. Note that the field of view given for an LED lens won't be the same as when using a light sensor, as they will be difference sizes (in the same way different sized cameras with the same lens will give different field of views). The main thing is to reduce the influence of light from nearer the horizon, and a lens concentrates the light from the zenith more than a simple blocking aperture would.

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Here’s a few preliminary sketches of my DIY SQM that I’m building. Just need my 3D Printer to show up now.

I may have to revise the ‘turret’ area where the lens is to make it a bit more printer friendly. Might split that part into two assemblies and glue them together afterwards.

I have the Arduino code finished now and outputting data to APT via usb (using a TSL237 as per the original - seems to work well). The window area is for a small OLED screen that displays SQM data as well as temperature, humidity and dew point.

 

F4D0ABF0-9E0D-4D6F-98BB-B4E75B66493C.jpeg

C9567333-8946-41F9-A86A-DC18541F62B3.jpeg

02A934D7-DDAB-411C-9297-7F0CD585931C.jpeg

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