Author Topic: Dark Sky Meter Live map now online  (Read 610 times)

Greg Fleming

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Re: Dark Sky Meter Live map now online
« Reply #15 on: January 23, 2018, 02:48:07 PM »
Mauro: is the close agreement with the SQM after you "calibrated" the app against the SQM? Is the SQM the narrower FOV version, or the original?Also, very good information, thank you; and I look forward to your further tests.

Chuck Johnston

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Re: Dark Sky Meter Live map now online
« Reply #16 on: January 30, 2018, 02:14:51 AM »
No, I did no particular calibration. My SQM is on average 0.1 mg optimistic and is the standard version. However the sky at my home is pretty uniformly polluted so the difference in the FOV is in influential.However, despite the agreement at ~SQM 19, there is no longer agreement above SQM 21, and, what is worse, the DSM gives non repeatable readings.

Mark Patterson

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Re: Dark Sky Meter Live map now online
« Reply #17 on: January 30, 2018, 11:29:22 PM »
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However, despite the agreement at ~SQM 19, there is no longer agreement above SQM 21, and, what is worse, the DSM gives non repeatable readings.

That's exactly what I would expect. The DSM has to rely on the built-in camera, which is designed for compactness and high pixel count -- precisely the qualities that make a camera work poorly at low light levels. On the whole, it's amazing that it works as well as it does.

Eric Mannasseh

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Re: Dark Sky Meter Live map now online
« Reply #18 on: January 31, 2018, 03:34:54 AM »
This is quite obvious. There is no detail about how the sensor data are processed, but I believe that by lumping all the pixel together one may reduce noise and increase signal. After all the CCD of the iPhone is similar in size to the sensor element of the SQM. So it looks to me that a clever processing might obtain similar results.

Use in Dark sites is important. Moreover another useful feature might be that of collecting readings at intervals the whole night and producing a chart (similar to the SQM-LE).

Dan Square

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Re: Dark Sky Meter Live map now online
« Reply #19 on: January 31, 2018, 06:34:54 AM »
Mauro, the phone gets *hot* inside, the SQM does not. The heat you know makes a lot of noise in the image. Kill all the unneeded apps and services, see if there is a way to turn off the mobile radio hardware (big problem), turn off the display hardware (big problem), etc. Wrap a cold pack around it and let it chill. It may help in both variance and going deeper.

togoseera

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Re: Dark Sky Meter Live map now online
« Reply #20 on: January 31, 2018, 12:25:19 PM »
That is a good advice! I will shut down everything not needed (I think the the "airplane use" switch shuts down every radio link) and test it for repeatability  the next time I go to a dark site.

nijambaci

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Re: Dark Sky Meter Live map now online
« Reply #21 on: January 31, 2018, 12:46:29 PM »
Quote
That is a good advice! I will shut down everything not needed (I think the the "airplane use" switch shuts down every radio link) and test it for repeatability  the next time I go to a dark site.
Since you will likely be at the site for a while, you might also at one point try turning the phone off for 30 min to let it cool to ambient, then turn on and immediately try it.

uncritoges

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Re: Dark Sky Meter Live map now online
« Reply #22 on: February 02, 2018, 06:14:23 PM »
Yesterday I went to this site in the middle of the Alps. http://goo.gl/maps/eqOFW

The sky was clear. A traditional (non-L) read abut 21.64 in the first part of the night aiming the Cygnus (at zenith overhead). The SQM was mine, which is probably 0.1 mags optimistic (so the correct readings might have been ~ 21.55).

Turning the iPhone off did not work much probably because as soon as the iphone is turned on and used to gather data, it undergoes a temperature transient. However the ambient temperature was ~6 °C and i worked in a way to keep the iPhone activity constant and at minimum.

I got the following three series:

a) between 00.55 and 00.58: 21.77, 22.03, 21.93, 21.80, 22.07, 21.92, 21.96, 21.77, 21.72, 21.71, 22.00. i.e. 21.88 +/- 0.39 (mean 21.88 standard deviation 0.13).

b) between 02.10 and 02.15: 21.99, 21.50, 21.82, 21.54, 22.26, 21.62, 21.85, 22.06.
i.e. 21.83 +/- 0.80 (mean 21.83 standard deviation 0.27)

c) between 03.24 and 03.26: 21.84, 21.82, 21.54, 21.48, 22.07, 22.07.
ie: 21.80+/- 0.76 (mean 21.80 standard deviation 0.25)

The mean value is consistent with a systematic error of say (+0.25 mags) which may be corrected in the calibration settings. Fluctuations are large (typically +/- 0.8 mags at 3 sigma) but can be averaged.

It might be useful having a systems that takes a series of reading (necessarily with only one dark) and averages.

nepaletha

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Re: Dark Sky Meter Live map now online
« Reply #23 on: February 03, 2018, 02:36:16 AM »
Tonight I made 6 measurements in my backyard.

Between hours 22.15 and 22.19: 18.95, 18.83, 18.85, 18.94, 18.91, 18.91.
ie 18.9 +/- 0.15 (Mean 18.9, standard deviation 0.05

My SQM reads: 19.08, 19.11, 19.13, 19.11, 19.12.
ie 19.1 +/- 0.05 (Mean 19.11, standard deviation 0.019

Matt Gibson

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Re: Dark Sky Meter Live map now online
« Reply #24 on: February 03, 2018, 03:58:46 AM »
If I understand your resultsapp = 21.84, SQM = 21.64 (app +0.2)app = 18.9, SQM = 19.1 (app -0.2)So the app calibration needs to have at least two offset points, since it seems to be at best a linear slope difference v. the SQM, not a constant offset, and 0.4 mag over a range of 2.6 mag is, to me at least, significant. If this is repeatable, of course.

David Felkel

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Re: Dark Sky Meter Live map now online
« Reply #25 on: February 04, 2018, 10:35:03 AM »
Yes, but there are two calibration settings for above and below 21. However it is not clear how they works (maybe there are two different algorithms?).

John Jankowski

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Re: Dark Sky Meter Live map now online
« Reply #26 on: February 04, 2018, 11:55:33 AM »
@Mauro,Your measurements and comparisons are great. I'm wondering what kind of device are you using?Please note that a unihedron SQM is made for dark sky measurements, with dedicated hardware. The DSM software is made to correct a lot of camera-specific items like gamma correction, dark noise etc. Because an iPhone programmer doesn't have direct access to hardware and cannot stat things like temperature the reading will always be a little fuzzy compared to a dedicated hardware SQM meter.

erexgila

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Re: Dark Sky Meter Live map now online
« Reply #27 on: February 08, 2018, 05:55:57 PM »
And a little update about the map. The map visible in the app is live. The  map on the website ( www.darkskymeter.com/map ) is updated weekly. This is because i am validating measurements. The map now contains 1500+ measurements. The amount of measurements is growing rapidly because most of the users are in the Northern Hemisphere and darkness is coming back

Todd Vann

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Re: Dark Sky Meter Live map now online
« Reply #28 on: February 09, 2018, 12:02:05 AM »
Hi No, I think the DSM has an advantage compared to the SQM, which is immediate transparent data logging (try to input measurements by hand to the Unihedron Database!).One feature that would be very useful wold be the possibility to query all reading in a location (all within a give radius) and plot them against time.I am using a standard SQM. Mine seems to be a bit optimistic. Compared to others it reads on average 0.05-0.1 mags darker.As for the SQM if it were possible to close the camera diaphragm and shot (ie to take a dark automatically) then it would also be possible to take 5 darks and 5 readings in a row, and by averaging to get a much better estimate.The spectral response of the SQM is likely to be different from that of the camera, and thus the two devices will respond differently to polluted skie (mostly polluted in red) and pristine skies. So far it seems that if I take 5 readings and average them, and correct for the bias i can get a good estimate of the sky.

Ronald Bryan

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Re: Dark Sky Meter Live map now online
« Reply #29 on: February 09, 2018, 12:30:22 AM »
Quote
One feature that would be very useful wold be the possibility to query all reading in a location (all within a give radius) and plot them against time.
[...]
As for the SQM if it were possible to close the camera diaphragm and shot (ie to take a dark automatically) then it would also be possible to take 5 darks and 5 readings in a row, and by averaging to get a much better estimate.
[...]
So far it seems that if I take 5 readings and average them, and correct for the bias i can get a good estimate of the sky.
I too hope this feature comes, and also building a map as the number of readings greatly increases.

I think Mauro meant DSM app above where he wrote SQM. I too hope you can add a function where you automate taking 5 consecutive readings and averaging them together. Then, if calibrated, you seem to get a quite consistent result.

It would be nice to hear about what the calibration actually does to readings of various levels.