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General Astronomy >> Light Pollution

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ih8usrnames
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Reged: 08/08/11

Loc: Wheaton Illinois
Using photographic meter to measure sky glow?
      #5677374 - 02/13/13 10:30 AM

I have a light meter I use when shooting photos with large format film. I set the ISO of the film, and measure the light, it returns an f-stop/shutter speed for the given light situation.

I wonder if anybody has created a "rule of thumb" for using an ambient light meter to measure sky glow?

Seems like it would be straight forward:
Set it to ISO 100 and measure ambient light.
With F-Ratio set to 1.0 how many seconds does it say for an exposure?

The longer the suggested exposure the lower the light pollution. Just an idea.


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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Using photographic meter to measure sky glow? new [Re: ih8usrnames]
      #5677438 - 02/13/13 10:59 AM

Normal photographic light meters aren't sensitive enough to measure skyglow.

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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Using photographic meter to measure sky glow? new [Re: Tony Flanders]
      #5677542 - 02/13/13 11:53 AM

You use a Unihedron SQM (Sky Quality Meter). This *is* designed to measure sky glow

http://www.unihedron.com/projects/darksky/


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ih8usrnames
super member


Reged: 08/08/11

Loc: Wheaton Illinois
Re: Using photographic meter to measure sky glow? new [Re: Tonk]
      #5679837 - 02/14/13 04:24 PM

I know there are sky meters, I am not about to buy one however.

My idea being;
- ISO ratings are standards (100,200,400,800,1600,3200).
- f/stops are standard ratios.
- time is a standard.

With all these standards it may be possible to figure out a chart for sky glow. Tony already said they are not sensitive enough, I am not debating that. I am only continuing this conversation to fully express my initial idea.

Potentially one could do the following:
Set meter to ISO 3200
Set f/stop to 1
Measure
The meter supplies the number of seconds to create an exposure based on 18% grey.
Create a chart that correlates # seconds with Limiting Magnitude.
30s = lm6
25s = lm5.5
20s = lm5
etc etc.

It would be very cool if it were possible because there are many more photographers in this world with Sekonic LM-358 light meters than there are people with SkyGlow meters.

Apparently it is unrealistic but it was an idea.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Using photographic meter to measure sky glow? new [Re: ih8usrnames]
      #5679935 - 02/14/13 05:36 PM

Have you pointed your meter at the night sky to see if you are getting a definite signal to read? No photographic meter I've used could do this.

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phreon
member


Reged: 09/15/12

Re: Using photographic meter to measure sky glow? new [Re: GlennLeDrew]
      #5680066 - 02/14/13 07:10 PM

I have an ancient Gossen Luna Pro that will indeed twitch a great deal from the sky glow in my area. Most meters will give raw readings in foot candles, LUX, etc. You have to consider how big of a swath your meter is seeing though. How many degrees wide? Averaged, center weight, spot or other?

Doug


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derangedhermit
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Reged: 10/07/09

Loc: USA
Re: Using photographic meter to measure sky glow? new [Re: phreon]
      #5680107 - 02/14/13 07:40 PM

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways. I also doubt the consistency across light meters would allow comparison in a significant way.

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Using photographic meter to measure sky glow? new [Re: derangedhermit]
      #5680411 - 02/14/13 11:24 PM

To be meaningful, sky brightness measurements should resolve to 0.1 magnitude, 0.2 at worst, or 10-20%. For comparison, 1/3 of an f/stop, which some meters only reliably--or are designed to--resolve to, is a 25% difference.

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Using photographic meter to measure sky glow? new [Re: derangedhermit]
      #5680670 - 02/15/13 04:30 AM

Quote:

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways.




Indeed! I have taken photos of the sky using different cameras at exactly the same ISO setting, f/stop, and time, an the brightness varies by a factor of two from the most sensitive to the least sensitive.

The variation in films rated at identical ISO was probably just as great.

There's also the huge question of what wavelength you're measuring. Some cameras/films are more sensitive to red, others to blue.


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