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Has anyone tried to cheap device to detect LP?

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19 replies to this topic

#1 Ballyhoo

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Posted 05 August 2021 - 09:34 PM

https://www.amazon.c...551169658&psc=1



#2 scadvice

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Posted 05 August 2021 - 10:34 PM

links not working Ballyhoo....hummmm? May be on my end...another link I tried didn't ether...


Edited by scadvice, 05 August 2021 - 10:41 PM.


#3 treadmarks

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Posted 05 August 2021 - 11:17 PM

The link works for me. It's a light meter that measures in lux. I doubt it would be made to measure the extremely low lux levels of the night sky. Product page states the range as 0.1 lux to 20,000 lux. A clear moonless sky is 0.002 lux so that's out of its range as I would expect.


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#4 Jim Waters

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Posted 05 August 2021 - 11:52 PM

Sky Quality Meter

http://www.unihedron...ojects/darksky/

 

$120 - $287


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#5 Kfrank

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Posted 06 August 2021 - 02:04 PM

I don’t need anything more than my eyes to detect light pollution. Just look around!

Of course, quantifying the degree of pollution is another matter but I’m not sure what the value in that would be unless you were doing a study or researching LP.

#6 Jon Isaacs

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Posted 06 August 2021 - 03:32 PM

I don’t need anything more than my eyes to detect light pollution. Just look around!

Of course, quantifying the degree of pollution is another matter but I’m not sure what the value in that would be unless you were doing a study or researching LP.

 

Knowing the brightness of the sky as measured with an SQM can be useful for a number of reasons.  I think we're all doing our informal research studying light pollution.

 

Understanding the night to night and longer term variations is useful as well as the site to site differences.. You drive 300 more and discover the skies are brighter than at home.. probably not making that trip again.

 

Jon


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#7 LDW47

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Posted 06 August 2021 - 06:03 PM

Knowing the brightness of the sky as measured with an SQM can be useful for a number of reasons.  I think we're all doing our informal research studying light pollution.

 

Understanding the night to night and longer term variations is useful as well as the site to site differences.. You drive 300 more and discover the skies are brighter than at home.. probably not making that trip again.

 

Jon

Studying / understanding the nite to nite variations as well as ............. might be useful but from my experience you will never know, you can't be sure until you get out there and start your viewing  A ten mile, a 50 mile, a three hundred mile jaunt is still, can be a shot in the dark (pun intended), lol  If you can't count on the nite by nite Dark Sky Charts to be wholely accurate you sure can't count on .........  But its fun trying for some



#8 bokemon

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Posted 14 August 2021 - 09:56 PM

There should be an easy way to figure this out.

Use a camera and lens of a certain F number and focal length, e.g. your 105mm F2.8 macro lens.

Oh, well maybe not with the aperture fully open because that will cause vignetting.

Given a certain exposure time and ISO, this should tell you photons per second per area on the sensor.

You don't even have to focus correctly or hold the camera still.

Take a picture and look at the histogram.



#9 Tony Flanders

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Posted 15 August 2021 - 06:21 AM

There should be an easy way to figure this out.

Use a camera and lens of a certain F number and focal length, e.g. your 105mm F2.8 macro lens.

Oh, well maybe not with the aperture fully open because that will cause vignetting.

Given a certain exposure time and ISO, this should tell you photons per second per area on the sensor.

You don't even have to focus correctly or hold the camera still.

Take a picture and look at the histogram.

Better yet, query the photo in Photoshop or a different tool to find the "level" of the pixel that you care about. That yields more precise results because different parts of the sky have different brightness levels.

 

However, there are numerous pitfalls and problems with this method, which I explored in considerable detail before the SQM was invented -- and also after. The single biggest problem is calibration. Contrary to the quoted post, no device gives readouts in photons per area per time -- nor would it be useful if it did, because how would you correlate that with more commonly used units such as magnitude per square arcsecond? More realistically, how do you correlate a pixel level of 100 out of 255 to mpss?

 

In a long-ago article in Sky & Telescope, I explained one way to do that using reference stars of known brightness. With considerable effort I got results accurate to about +-25%, which isn't terrible but isn't great either. In practice I find it both easier and more accurate to correlate pixel levels to SQM-L readings.

 

The ideal would be to use a 180-degree fisheye lens, which would give brightness levels across the entire sky. Fisheyes also have the huge benefit of having less light falloff toward the edges, which is a major issue with conventional wide-angle lenses.

 

Obviously, all of this requires a camera with full manual mode.


Edited by Tony Flanders, 15 August 2021 - 06:22 AM.


#10 dx_ron

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Posted 16 August 2021 - 03:47 PM

Ballyhoo - have you ever tried to calculate it from images? I keep meaning to, but it never seems to be the most important use of my time (the real reason is probably more that I would get depressed).

 

Method --> https://www.cloudynights.com/topic/684954-calculating-sky-glowfogwhatever-from-fits/?p=9792713



#11 bokemon

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Posted 17 August 2021 - 06:23 AM

 

However, there are numerous pitfalls and problems with this method, which I explored in considerable detail before the SQM was invented -- and also after. The single biggest problem is calibration. Contrary to the quoted post, no device gives readouts in photons per area per time -- nor would it be useful if it did, because how would you correlate that with more commonly used units such as magnitude per square arcsecond? More realistically, how do you correlate a pixel level of 100 out of 255 to mpss?

 

In a long-ago article in Sky & Telescope, I explained one way to do that using reference stars of known brightness. With considerable effort I got results accurate to about +-25%, which isn't terrible but isn't great either. In practice I find it both easier and more accurate to correlate pixel levels to SQM-L readings.

 

The ideal would be to use a 180-degree fisheye lens, which would give brightness levels across the entire sky. Fisheyes also have the huge benefit of having less light falloff toward the edges, which is a major issue with conventional wide-angle lenses.

 

For calibration, you can look up the quantum efficiency of the sensor.  But in a way, it doesn't matter that much because because you'd be interested in the relative readings between two locations, or two different directions of the sky.



#12 Jon Isaacs

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Posted 19 August 2021 - 09:16 AM

Studying / understanding the nite to nite variations as well as ............. might be useful but from my experience you will never know, you can't be sure until you get out there and start your viewing  A ten mile, a 50 mile, a three hundred mile jaunt is still, can be a shot in the dark (pun intended), lol  If you can't count on the nite by nite Dark Sky Charts to be wholely accurate you sure can't count on .........  But its fun trying for some

 

I drive 300 miles to the Kelso Dunes. I stay there a week. I measure the sky brightness several times each night. I find it's very similar to my high desert site. I'll be looking for something darker with 300 miles..

 

Jon



#13 LDW47

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Posted 19 August 2021 - 09:44 AM

I drive 300 miles to the Kelso Dunes. I stay there a week. I measure the sky brightness several times each night. I find it's very similar to my high desert site. I'll be looking for something darker with 300 miles..

 

Jon

And the best guesstimate is the Dark Sky Charts knowing their less than full accuracy  As I said one can take all the readings they want but the + / - factor is always there, there is no question sometimes you can hit it lucky but many times .......... luck



#14 Jon Isaacs

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Posted 19 August 2021 - 12:18 PM

And the best guesstimate is the Dark Sky Charts knowing their less than full accuracy  As I said one can take all the readings they want but the + / - factor is always there, there is no question sometimes you can hit it lucky but many times .......... luck

 

Do you have actual experience making measurements? How many SQM readings do you have for your observing sites?  

 

A measurement is not luck, it's a data point. It's a measurement of the conditions at a given time.  Sky brightness changes night to night, hour to hour. Charts won't tell you that, measurements will.

 

Jon


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#15 LDW47

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Posted 19 August 2021 - 12:30 PM

Do you have actual experience making measurements? How many SQM readings do you have for your observing sites?  

 

A measurement is not luck, it's a data point. It's a measurement of the conditions at a given time.  Sky brightness changes night to night, hour to hour. Charts won't tell you that, measurements will.

 

Jon

I use my SQM-L all the time but I don't record the readings for posterity, thats not the intended use of my meter  I know my favored sites well enough that my memory is sufficent  But those charts will give you a good reference start, there are so many great places locally ie Bortle 1-2 locations within 10-20 mi., wilderness settings, that I don't have to travel long distances to improve my viewing quality and record the .......  I'm just plain lucky   PS  But I sure love my SQM-L and what its for



#16 LDW47

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Posted 19 August 2021 - 12:35 PM

Do you have actual experience making measurements? How many SQM readings do you have for your observing sites?  

 

A measurement is not luck, it's a data point. It's a measurement of the conditions at a given time.  Sky brightness changes night to night, hour to hour. Charts won't tell you that, measurements will.

 

Jon

But if I had to travel longer distances to get better skies, better views I still wouldn't record my SQM-L findings, they would never get used again and I wouldn't miss them  You have to be certain types to continually record everything in .............. because it is .........  PS  I don't want / need charts that tell me what is unimportant the next time around, unless maybe it was a weekly thing


Edited by LDW47, 19 August 2021 - 12:38 PM.


#17 Jon Isaacs

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Posted 19 August 2021 - 05:18 PM

But if I had to travel longer distances to get better skies, better views I still wouldn't record my SQM-L findings, they would never get used again and I wouldn't miss them  You have to be certain types to continually record everything in .............. because it is .........  PS  I don't want / need charts that tell me what is unimportant the next time around, unless maybe it was a weekly thing

 

:lol:

 

I do not continuously record everything. I record enough so I can look back and see what happened. Which I do..

 

If you don't care, I'm fine with that. But I do care and am able to make good use of the information so please just accept that some of us want to be somewhat more scientific in our understanding of our sites.

 

Jon


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#18 LDW47

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Posted 19 August 2021 - 09:23 PM

lol.gif

 

I do not continuously record everything. I record enough so I can look back and see what happened. Which I do..

 

If you don't care, I'm fine with that. But I do care and am able to make good use of the information so please just accept that some of us want to be somewhat more scientific in our understanding of our sites.

 

Jon

Believe me l do respect everyone's way of doing whatever  I don't think I ever intentionally ridiculed anyone for their preferences, I only express my habits which many also disagree with and that's okay  I love scientific this or that, I used to read lots of scientific ......... but I just don't practice it most days



#19 csa/montana

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Posted 19 August 2021 - 10:48 PM

Ok, let's please get back to the topic!  We all are free to use tools or not, when observing.   



#20 barbarosa

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Posted 20 August 2021 - 02:17 AM

Back to the original topic. There is an easy way to measure sky brightness, over a large area of the sky or a smaller selected area. You need the following.

 

SharpCap Pro 

 

One of the  many cameras that work with SharpCap

  1. Altair Astro cameras
  2. AstcamPan cameras
  3. Atik cameras
  4. Basler cameras that use the Basler Pylon drivers, including unique LX mode
  5. Celestron/Imaging Source cameras
  6. iNova cameras
  7. Moravian Instruments CMOS cameras
  8. Player One cameras
  9. Point Grey (now FLIR) cameras
  10. QHY cameras
  11. StarlightXpress Cameras
  12. SVBony Cameras
  13. ZWOptical cameras
  14. Other brands that have an ASCOM driver

 

 

 A ZWO 1/3″ 2.1mm 150 degree lens or a similar lens. (Suggested to get a common cone of measurement)

 

  1. Install lens on camera and point to the zenith
  2. Install SharpCap Pro
  3. Start SC and connect camera
  4. Open Smart Histogram tool.
  5. Click on the Brain. (SharpCap has stored profiles for many sensors. If your sensor is not recognized go to the top menu and select Tools, Senor Analysis and run an analysis before using the Brain.)
  6. Make your choices about channel, ignore edges, unity gain or max dynamic range if you want to see how the Brain handles camera settings or just accept the defaults.
  7. Click on Measure and wait for it to display a value for sky brightness in  electrons per pixel per second



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