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Inexpensive instrument to measure night sky

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#1 Mr. Bill

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Posted 03 August 2005 - 02:24 PM

Just bought this
to get an objective, repeatable reading of the night's sky brightness.

Will be interesting to correlate NELM, the Bortle Scale, and mags/sq arc sec (which is what this meter converts its measurements into.)

#2 lighttrap

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Posted 03 August 2005 - 03:48 PM

Interesting gizmo at a fairly reasonable price. I've been trying to figure out how to convert a Sekonic light meter into something similar, and haven't had much success. Of course, there is the old NELM, which doesn't cost anything but some research, and most of that research has already been done and is posted elsewhere here.

#3 Mr. Bill

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Posted 03 August 2005 - 03:57 PM

Interesting gizmo at a fairly reasonable price. I've been trying to figure out how to convert a Sekonic light meter into something similar, and haven't had much success. Of course, there is the old NELM, which doesn't cost anything but some research, and most of that research has already been done and is posted elsewhere here.


NELM is very subjective....this is a way for everybody to be "on the same page" as all of these are calibrated to the same source that is tracable to NIST (the old Bureau of Standards). We could all then compare apples with apples.

Besides that, it's a new toy to play with that costs much less then a premium ep.

#4 Mark9473

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Posted 03 August 2005 - 04:10 PM

NELM is very subjective....

agreed, but it does capture differences in sensitivity/perception of an observer's vision. It is not uncommon for two observers at the same site to have a NELM differing by up to a magnitude or more. So this device does not really tell you what you will see.

#5 Mr. Bill

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Posted 03 August 2005 - 07:19 PM

NELM is very subjective....

agreed, but it does capture differences in sensitivity/perception of an observer's vision. It is not uncommon for two observers at the same site to have a NELM differing by up to a magnitude or more. So this device does not really tell you what you will see.


You've got it backwards....

it gives you an objective number to match what you see against.

#6 Mark9473

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Posted 04 August 2005 - 12:52 AM

it gives you an objective number to match what you see against.

I don't think I disagree with that, nor have I said anything to the contrary. Then what is it exactly about my statement that you find "backwards"?

#7 Guest_**DONOTDELETE**_*

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Posted 04 August 2005 - 07:22 AM

Mark, maybe you ment to say, "agreed, but it does NOT capture differences in sensitivity/perception of an observer's vision..."??

This looks like a fun toy. I guess I knew such photometric electronics were out there, but I just have never seen one.

#8 Mark9473

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Posted 04 August 2005 - 12:37 PM

Mark, maybe you ment to say, "agreed, but it does NOT capture differences in sensitivity/perception of an observer's vision..."??

no, on the contrary, that was exactly my point. NELM measures what an individual observer actually sees. I follow Bill where he wants to get an objective sky quality measurement, which eliminates the observer from the equation. The limits of this system are reached, when you want to conclude what another observer may or may not be able to see under a given sky even if you know what you would see under that same sky.

#9 Mr. Bill

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Posted 04 August 2005 - 12:56 PM

it gives you an objective number to match what you see against.

I don't think I disagree with that, nor have I said anything to the contrary. Then what is it exactly about my statement that you find "backwards"?


Hi Mark...

Not to imply you are backwards; meant to say FIRST you take the number with the meter as this is a measure of integrated sky magnitude, THEN you estimate the NELM for point source and individual differences.

We both agree that NELM is a highly variable number that is a matter of individual perception. This device allows us all to "be on the same page" when comparing observation notes. It is like using a photometer to measure star magnitudes rather than making visual estimates.One can argue instrument errors, but the "personal equation" is removed from the discussion.

As far as your statement " So this device does not really tell you what you will see ", my contention is that NELM does not really tell you what you will see AS FAR AS DSOs.

OTOH, assuming your NELM method is a reflection of what you can see, if you reliably make the same NELM estimates under the same conditions, the number that the sky meter gives should be the same. Its a matter of repeatability, not the absolute number on the sky meter that allows you to make a prediction on what YOU will be able to see using the sky meter.These are questions that can be answered by empirical means.

One of the problems with using NELM is that it heavily favors point sources rather then extended objects.The Bortle Scale (Feb 2001 S&T) is an attempt to do subjectively what the sky meter does scientifically.

Bortle states,"A modest amount of light pollution degrades objects such as comets, nebulae, and galaxies far more than stars."

#10 Mark9473

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Posted 04 August 2005 - 01:46 PM

We both agree that NELM is a highly variable number that is a matter of individual perception. This device allows us all to "be on the same page" when comparing observation notes.

Bill, this last sub-sentence is where I have trouble following. As I see it, if two observers have this device, one of them may know what he himself would see when he would go to the other's site, however he cannot deduce what the other observer would see at either site.
Can you explain how you see this differently (or not)?

As far as your statement " So this device does not really tell you what you will see ", my contention is that NELM does not really tell you what you will see AS FAR AS DSOs.

that's an interesting statement which I cannot deny... From my 28 years of experience in astronomy, I would say that I have, under a given sky, slightly above-average sensitivity for point sources, but certainly below average for extended objects. Hence my preference to observe open clusters.

if you reliably make the same NELM estimates under the same conditions, the number that the sky meter gives should be the same.

I absolutely agree.

One of the problems with using NELM is that it heavily favors point sources rather then extended objects.The Bortle Scale (Feb 2001 S&T) is an attempt to do subjectively what the sky meter does scientifically.

Bortle states,"A modest amount of light pollution degrades objects such as comets, nebulae, and galaxies far more than stars."

as per the above, I would have to agree. I really should go and get that S&T out of storage. But what I would like to hear from you, is where the sky quality meter fits into this equation according to you. Since we agree that point-source-LM is not the same as DSO-LM, wiil it give you one or the other, or both separately or combined?

#11 Mr. Bill

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Posted 04 August 2005 - 03:41 PM

"We both agree that NELM is a highly variable number that is a matter of individual perception. This device allows us all to "be on the same page" when comparing observation notes."


Q: Bill, this last sub-sentence is where I have trouble following. As I see it, if two observers have this device, one of them may know what he himself would see when he would go to the other's site, however he cannot deduce what the other observer would see at either site.
Can you explain how you see this differently (or not)?


A: I agree it could be used by one individual to predict what he would see at the other's site. As far as the second part of your question, I look at it as being able to adjust the details of the other's observation of the same object you are looking at by comparing dark sky conditions. For example, if he has a lower number (lighter sky) and sees more detail then you, then you have to wonder if it is individual difference or the telescope, or both. This has a certain heuristic value in that it makes me want to get more out of viewing by improving my equipment or observing technique.


"One of the problems with using NELM is that it heavily favors point sources rather then extended objects.The Bortle Scale (Feb 2001 S&T) is an attempt to do subjectively what the sky meter does scientifically."
Bortle states,"A modest amount of light pollution degrades objects such as comets, nebulae, and galaxies far more than stars."


Q: as per the above, I would have to agree. I really should go and get that S&T out of storage. But what I would like to hear from you, is where the sky quality meter fits into this equation according to you. Since we agree that point-source-LM is not the same as DSO-LM, wiil it give you one or the other, or both separately or combined?


A: The sky meter integrates ALL the light sources in a 80 degree cone that is centered on the zenith. It then averages out individual stars (which account for a small portion of the total light received) and heavily weights the background light from manmade sources as well as natural sources such as skyglow. Also, remember these readings are logrithmic; a relatively large change in light results in a small number change.

One interesting experiment is to see if the MW overhead has a significent influence on readings.Initial results (mine) indicates very little. The area of the light cone measured is large compared to the area of the MW at the zenith.

OBTW, another thing to consider is the instrument error, that is the variability between readings taken sequentually. I find good repeatability taking 10 measurements and averaging out. Obviously, if differences in measurements between two units are the less then or same as the standard deviations, the two readings can be considered the same, assuming THEY HAVE BOTH BEEN CALIBRATED TO THE SAME SOURCE and haven't drifted over time. Here is where checking calibration against a standard periodically would be beneficial. These meters do not have that capability, so there is a certain amount of faith required that the meters read the same if we are going to get technical. I guess we would have to send units back to the manufacturer to have them checked against his standard that is traceable to NIST.

Another interesting experiment would be to have two of these meters at the same location taking readings simultainously. That's where I have to talk one of my observing buddies into buying a unit.

#12 Mark9473

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Posted 04 August 2005 - 04:19 PM

thanks Bill for the clarifications.

I went to the manufaturer's site for a quick browse, and noticed the spectral response extends quite a bit into the UV. It would be interesting to see what you get if you additionally mount a UV-block filter, or why not even an LPR.

Looking forward to hear about your results in the coming time.

#13 Mr. Bill

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Posted 05 August 2005 - 01:16 PM

thanks Bill for the clarifications.

I went to the manufaturer's site for a quick browse, and noticed the spectral response extends quite a bit into the UV. It would be interesting to see what you get if you additionally mount a UV-block filter, or why not even an LPR.

Looking forward to hear about your results in the coming time.


Hi Mark....

Excellent idea!! I think I will experiment holding various filters over the sensor chip to allow various frequencies and block others.

I think you need to buy one of these :jump:

#14 Mark9473

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Posted 05 August 2005 - 01:39 PM

I think you need to buy one of these

haha, I was waiting for you to come up with that bright idea!


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