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SQM readings during full moon

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#1 Allan Wade

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Posted 28 June 2018 - 07:49 PM

I tried something interesting with my wide SQM recently. I took readings at my suburban home during full moon and got readings averaging 15.2. Next month at my dark site, did the same, and got readings averaging 15.4. Ordinarily during new moon the SQM reads at best, 20.0 and 22.0 respectively for my two sites, so there is a considerable difference in sky darkness with no moon around. However, the figures indicate that the moon levels the sky playing field greatly.

 

I guess if you take it one step further and record the brightness during the day, the two sites would have identical sky brightness.

 

I thought I would find myself a big bright city centre and take a reading from there during full moon and see what reading I get. It’s an interesting thought that the sky in downtown Sydney/Los Angeles/Tokyo etc might be comparable to a pristine site during full moons.


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#2 GlennLeDrew

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Posted 29 June 2018 - 02:30 AM

With your wide angle SQM, are you definitely not sampling the Moon itself? From a not bright locale, and when the air is relatively 'clean' (not much haze present), I gather a sky brightness during the full Moon is typically/characteristically about 17-ish MPSAS.


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#3 Allan Wade

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Posted 29 June 2018 - 02:34 AM

I was taking readings while the moon was around zenith, so certainly capturing the moon directly in my wide SQM reading. Perhaps an SQM of 17.0 would be likely with a narrow meter that is only sampling the sky without any influence directly of the full moon.



#4 Allan Wade

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Posted 29 June 2018 - 02:37 AM

I will try this again when the full moon is lower in the sky and take some readings with my wide SQM away from its position so as not to sample the moon light directly.



#5 Tony Flanders

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Posted 29 June 2018 - 06:30 AM



I was taking readings while the moon was around zenith, so certainly capturing the moon directly in my wide SQM reading. Perhaps an SQM of 17.0 would be likely with a narrow meter that is only sampling the sky without any influence directly of the full moon.

Yes, using a wide-angle SQM when the Moon is within the field of view is essentially meaningless. What you're measuring isn't skyglow, but rather the response of the SQM to off-center light sources. The skyglow itself is utterly overwhelmed by the light of the Moon.

 

Even when the SQM is pointing 80 degrees away from the Moon, you will still get significant spillover.

 

If you really want to measure skyglow when the Moon is up, you either need the lensed version of the SQM and/or rig up a shield that prevents direct moonlight from falling on the device. See my blog How brightly shines the Moon?


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#6 Kevdog

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Posted 29 June 2018 - 05:38 PM

Using my SQM-L and making sure I was pointed away from the full moon I get (approx)

 

20.00 at home

18.00 at home with full moon

 

21.3 at dark site

18.5 at dark site with full moon.

 

It seems to be about 2 full points lighter regardless of where you are.   So even my dark site with a full moon isn't as good as no moon at home!


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#7 Starman1

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Posted 30 June 2018 - 06:07 PM

With your wide angle SQM, are you definitely not sampling the Moon itself? From a not bright locale, and when the air is relatively 'clean' (not much haze present), I gather a sky brightness during the full Moon is typically/characteristically about 17-ish MPSAS.

Depends where you are.  My typical sky brightness with NO moon in the sky is 16.8-17.8.

In contrast, my dark site usually starts the night around 21.4 and deepens to near 21.6 at 2am, then brightens to 21.35 at the start of morning twilight.

At the dark site, the sky gets noticeably darker when Venus sets.  When there is a Full Moon, no one observes.

It's not even good for Moon viewing.



#8 GlennLeDrew

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Posted 03 July 2018 - 05:34 PM

Don,

When you point out how the sky gets noticeably darker after Venus sets, have you measured the sky brightness to quantify this?

 

Might this include the reduction in zodiacal light? How about those last traces of twilight?

 

I've pondered Venus' potential contribution to sky glow. As we know, it does cast a subtle shadow. Because from that one ~-4.4m point source emits fully as much light as all the stars overhead to about 7m combined.

 

But even a 22 MPSAS dark site has just the sky glow component integrating to about -8m (a 21 MPSAS sky would be about -9m, 20 MPSAS is -10m, etc.)

 

To quantify a source's contribution to sky glow, we have a very handy fiducial which cannot be 'contaminated' by other sources; the Sun! At -26.7m, it induces a sky glow of about 3.5 MPSAS. A simple subtraction here supplies the delta to apply for any other source. And that that is:

 

-26.7 -3.5 = -30.2

 

Let's use the rounded value of 30 magnitudes as our delta to add (to make fainter).

 

And so -4.4m Venus by itself would induce atmospheric scatter of surface brightness -4.4 +30 = 25.6 MPSAS. This is 3.6m, or nearly 30 times fainter than an already pristine sky. Would its subtraction from the sky--and *especially* when already low and hence its sky brightening being less than the ideal calculated here--therefore make a "noticeable" difference to sky brghtness? That's why I surmise other factors.

 

As a check, take the -12.7m full Moon. It's sky glow component will be:

 

-12.7 +30 = 17.3 MPSAS.



#9 GlennLeDrew

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Posted 03 July 2018 - 05:47 PM

Added to the previous...

 

Note that the blue sky resulting from scatter of sunlight will apply for all other sources of even the crudest 'white' spectrum. That includes the Moon, planets and stars.

 

But a dark site's sky color is decidedly a greenish/reddish, resulting from the dominant emitted light of airglow, with zodiacal light coming in at a close second. This demonstrates that the blue component of atmospheric scatter from all those outside sources of stars, planets, milky way and zodiacal light is made minor.

 

It takes a pretty bright source, like the Moon, for the sky color to take on a dominant blue hue.

 

We can take a guess at the required brightness. For a 22 MPSAS sky, let's suppose an atmosphere scatter component of, say, 21 MPSAS will render its blue hue as becoming dominant. The integrated magnitude of the source would then have to be:

 

21 - 30 = -9m.

 

This 30 magnitude delta is a handy number to keep in mind.


Edited by GlennLeDrew, 03 July 2018 - 05:48 PM.


#10 Starman1

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Posted 03 July 2018 - 08:04 PM

Don,

When you point out how the sky gets noticeably darker after Venus sets, have you measured the sky brightness to quantify this?

 

Might this include the reduction in zodiacal light? How about those last traces of twilight?

 

I've pondered Venus' potential contribution to sky glow. As we know, it does cast a subtle shadow. Because from that one ~-4.4m point source emits fully as much light as all the stars overhead to about 7m combined.

 

But even a 22 MPSAS dark site has just the sky glow component integrating to about -8m (a 21 MPSAS sky would be about -9m, 20 MPSAS is -10m, etc.)

 

To quantify a source's contribution to sky glow, we have a very handy fiducial which cannot be 'contaminated' by other sources; the Sun! At -26.7m, it induces a sky glow of about 3.5 MPSAS. A simple subtraction here supplies the delta to apply for any other source. And that that is:

 

-26.7 -3.5 = -30.2

 

Let's use the rounded value of 30 magnitudes as our delta to add (to make fainter).

 

And so -4.4m Venus by itself would induce atmospheric scatter of surface brightness -4.4 +30 = 25.6 MPSAS. This is 3.6m, or nearly 30 times fainter than an already pristine sky. Would its subtraction from the sky--and *especially* when already low and hence its sky brightening being less than the ideal calculated here--therefore make a "noticeable" difference to sky brghtness? That's why I surmise other factors.

 

As a check, take the -12.7m full Moon. It's sky glow component will be:

 

-12.7 +30 = 17.3 MPSAS.

Glenn,

No, I meant what I said.  When Venus sets, it's like a light switch being turned off, and the entire sky, but especially the sky west of the zenith, gets darker and fainter stars are seen.

I've seen this so often, I don't doubt what I'm seeing.  It's almost a physical relief as if someone turned off an annoying light on a pole a mile away.

The Zodiacal light doesn't disappear in seconds, and neither does twilight.

Jupiter is less bright, but I've recently noticed that all the faint stars I normally see in Libra have become very hard to see, if I can at all.

Jupiter's effect is more localized, but still there.

i once called the planets "small balls of light pollution", and, at a dark site, that's what they are.



#11 Tony Flanders

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Posted 04 July 2018 - 05:08 AM

When Venus sets, it's like a light switch being turned off, and the entire sky, but especially the sky west of the zenith, gets darker and fainter stars are seen. I've seen this so often, I don't doubt what I'm seeing.  It's almost a physical relief as if someone turned off an annoying light on a pole a mile away.

I don't doubt the effect. I do doubt that it has anything to do with skyglow. Loss of dark adaptation seems like a much more probable cause. And never underestimate the power of expectations and suggestibility.



#12 Starman1

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Posted 04 July 2018 - 03:44 PM

Tony,

 

It may not have anything to do with sky glow.  It is, however, a real effect.  Note I said it is almost a "physical relief", which implies a physiological cause.

But, whatever the cause, the whole sky gets darker and fainter stars appear, especially in the western sky.

It's not at all dissimilar to the setting of a small crescent moon.  The same thing occurs.

 

At any rate, it is a valid reason for me to not look for the faintest targets when either the Moon or Venus is above the horizon.



#13 Tony Flanders

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Posted 05 July 2018 - 04:33 AM

It may not have anything to do with sky glow.  It is, however, a real effect.  Note I said it is almost a "physical relief", which implies a physiological cause.
But, whatever the cause, the whole sky gets darker and fainter stars appear, especially in the western sky.
It's not at all dissimilar to the setting of a small crescent moon.  The same thing occurs.


An obvious test of whether it's due to skyglow is to try it at a place where the setting Venus is obstructed by a large hill or mountain. If it's due to actually seeing Venus, the effect should take place at the moment Venus disappears behind the hill. If it's due to skyglow, it would happen when Venus disappears below the theoretical horizon. That would be an especially good test if you made a point of not knowing in advance when Venus would sink below the theoretical horizon.



#14 ks__observer

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Posted 07 July 2018 - 07:24 AM

I just got my SQM a few weeks ago.

Have people tested SQM for different parts of sky with the moon?

I presume the above readings were taken pointing toward zenith.

What if you point towards Polaris at a dark site with a moon?


Edited by ks__observer, 07 July 2018 - 07:25 AM.


#15 Starman1

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Posted 07 July 2018 - 10:20 AM

I think Glenn calculated how much the sky brightens with the Moon present.

Obviously, the farther you are from the Moon, the darker the sky will be.

However, the Moon brightens the whole sky significantly, so reserve your galaxy and nebula viewing to when there is no moon present in the sky.

 

If you have an SQM, don't point it below 45° in a flat horizon place because the horizon will start clipping the field.

The SQM-L, however, can be pointed at Polaris and not get any horizon interference.



#16 ks__observer

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Posted 07 July 2018 - 10:23 AM

If you have an SQM, don't point it below 45° in a flat horizon place because the horizon will start clipping the field.

Good to know.

Thanks.



#17 Tony Flanders

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Posted 07 July 2018 - 11:44 AM

If you have an SQM, don't point it below 45° in a flat horizon place because the horizon will start clipping the field.
The SQM-L, however, can be pointed at Polaris and not get any horizon interference.

Quite so. But people who want to map sky brightness in detail should be aware that the optical axis of the SQM-L can be as much as 10 or 15 degrees off from the physical axis. And its field, while much smaller than the SQM's, is still pretty big.



#18 Starman1

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Posted 07 July 2018 - 12:04 PM

pretty big difference:

http://unihedron.com...s/fovcurves.jpg

FWHM from the field readings are:

SQM about 100°

SQM-L about 25-30°

That's field diameter.  Radius is 1/2 that.

The output total is:

SQM about 140°

SQM-L about 50°

I think it's safe to conclude the SQM is close to an "all-sky" reader, while the SQM-L is more of an "area" reader.

I would always aim the SQM at the zenith, but you can do "spot checks" with the SQM-L.



#19 GlennLeDrew

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Posted 07 July 2018 - 12:06 PM

As in the daytime with the Sun, the darkest part of the sky will be *roughly* 45-60 degrees from the Moon, depending on the Moon's altitude above the horizon. Forward scatter causes a brightness enhancement toward the source, and greater optical depth along lower sight lines through the atmosphere causes a brightness increase toward the horizon.


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#20 Redbetter

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Posted 07 July 2018 - 05:54 PM

An obvious test of whether it's due to skyglow is to try it at a place where the setting Venus is obstructed by a large hill or mountain. If it's due to actually seeing Venus, the effect should take place at the moment Venus disappears behind the hill. If it's due to skyglow, it would happen when Venus disappears below the theoretical horizon. That would be an especially good test if you made a point of not knowing in advance when Venus would sink below the theoretical horizon.

In this same vein, there is a fairly strong visual effect from blocking a bright source of glare even though that glare has surprisingly little impact on the sky in the eyepiece or meter--as long as there is no reflection into either.  Venus, and to some extent Jupiter/Mars/Sirius, have this sort of impact to me.  If I can block them by observing in their shadow, then I can see much more.  This can mean cupping my hand around an eyepiece, placing a tree, vehicle, structure, or upheld hand to block the glare (the latter item for naked eye primarily.) 

 

I don't know what the explanation is for this.  Perhaps it is scatter from glancing or off axis angles across the cornea?  Perhaps it is even causing some pupil restriction (natural defense against looking at the Sun) or neurological processing effect that is resulting the eye operating more mesopic/photopic   Whatever it is, it is a rapid response visually.  The same is true in other night time lighting situations with distant but bright point sources such as some light poles about 3/4 mile away at one of my sites.  I found it very noticeable at some Christmas light exhibits where LED light strips were on some of the low fence/handrails.  Simply positioning my forearm to block this allowed me to see into unlit (or at least not directly lit) enclosures that were invisible before.

 

One caution on the below the actual horizon for bright sources, for some minutes they are still illuminating the air above, even while below the theoretical horizon.  The source of ground level glare is removed, and local reflections back into the air as well, but the air above is still being lit by the source at a very shallow angle (but long optical depth that can produce a lot of scatter--like unshielded light fixtures) as if almost nothing had changed, initially.



#21 GlennLeDrew

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Posted 07 July 2018 - 08:25 PM

Our eyes are rather imperfect constructs, and more so as we age. The presence of a bright object in the field of vision, even at quite large off-axis angles, can induce perceptible glare within the eye that could seem to be without--as in sky glow.

 

Indeed, for *at least* us us ol' grey-hairs I feel our eyes are rather worse scatterers than are the optics we use--unless the latter are getting notably filthy. To test this out, in darkness look toward some suitably bright light (like a streetlight partway down the block) that does induce annoying glare. Then interpose a piece of reasonably clean picture frame glass. Then in stages make the glass progressively a bit more smudged with skin oils, to see how bad things must get 'til the dirty glass competes with your own eyes. As a control, so as to separate the two contributions to glare, arrange as well to have your eyes *just* fully shielded from the direct light while the glass is itself fully illuminated.



#22 Tony Flanders

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Posted 08 July 2018 - 02:00 PM

One caution on the below the actual horizon for bright sources, for some minutes they are still illuminating the air above, even while below the theoretical horizon.

More than "some minutes" -- this is, after all, why we experience twilight after sunset and before sunrise. And the various phases of twilight typically last around 90 minutes at middle latitudes.
 
But if you measure the sky brightness on both sides of sunset or moonset, you'll see an extremely sharp kink after the object sinks below altitude zero. For a while, the brightness literally changes by the minute. It's a fair bet with an object as faint as Venus (when compared to the Moon -- not to anything else!), whatever skyglow exists must more or less switch off when Venus reaches altitude zero.



#23 earlyriser

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Posted 11 March 2022 - 08:36 AM

Based on readings I've taken in my light polluted back yard, and some crude math, I've generated the following table which seems to give a reasonably good idea of how much the Moon contributes to SQM readings, at least in my light polluted back yard.  It's based on an assumption that the SQM contribution of the Moon scales linearly with its magnitude and measurements made during the full moon at a site which normally has an SQM of around 18.5.  My guess is that it is less accurate in darker locations. 

 

 

Moon and SQM.png


Edited by earlyriser, 11 March 2022 - 08:41 AM.



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