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SQM Questions

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#1 City Kid

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Posted 16 August 2018 - 09:01 PM

I have an SQM-L and when I take readings at a site with clear horizons I take readings at zenith, north, south, east, west, and average them. For each direction I take three readings. When I take the readings they are regularly all over the place. For example, my first reading might be 21.65 followed by 21.72 and then 21.63. I don't like using three readings that are so far apart so I end up taking readings until I get three that are at least within .03 or .04 of each other. This can be very time consuming because sometimes I might need to take ten or more readings before I get three that are within .03 of each other. Is this normal? I'm surprised the different readings can be that far off from each other.

 

My second question is how dark can the meter read? While at the Nebraska Star Party I took readings while it was cloudy and got 23.57. But in a completely dark environment it won't read. Just curious.



#2 City Kid

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Posted 16 August 2018 - 09:24 PM

Oops, I meant to  put this in the equipment forum. Could mods move this please?



#3 TOMDEY

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Posted 16 August 2018 - 09:39 PM

Howdy, Kid! Yes, that's the one that I use.

 

I discard the 1st couple of readings, because the 1st one always seems anomalous. After that, it settles down to quite consistent. I meter that two or three times in a night, just to see how things are going. The readings are generally consistent with how the Deep Sky observing session is going. With the 20-degree feathered field, I point it straight up and also sample where I happen to be observing with the scope. Other directions might be interesting, but not affectual to what I'm doing... Observing! I log those right with my Observing Journal.

 

So, you're taking 15 readings minimum, up to 50 or more, if variable, averaging consistent sets or three and then averaging those. None of my business, but that sure seems obsessive-compulsive... unless you are doing a study/thesis on sky conditions, rather than actually observing?! No kidding, I can see doing that and enjoying it! My personality is like that, too.

 

The sky, of course, varies a LOT... same place, different hours, even minutes, and certainly different directions. That could be some/much of the variability that you are getting? To research THAT (instrument variation vs object-space variation) you would have to hard-mount the gizmo and do structured data-collection.

 

For most of us, that make and model (cheap and nice) is intended for a disinterested, instrumental quick-check of what matters to our observing. Not intended to go beyond that.

 

They DO have much more sophisticated models that would possibly provide what you want... at higher cost. Go for it!  Tom



#4 City Kid

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Posted 16 August 2018 - 09:48 PM

Yes it is obsessive/ compulsive except I don’t want to take all those readings. 😂 That’s why I’m asking if this is normal. I want to make sure my meter is working properly. It’s aggravating to take two readings within .02 of each other and then a third .07 away. 



#5 TOMDEY

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Posted 16 August 2018 - 10:15 PM

Yeah, you are trying to tease almost 4 significant figures out of a hand-held photometer measuring a spatially and temporally variable object-space. That last digit Should be bouncing around. Actually, any properly-designed meter will provide display one digit beyond the last significant one.  Tom



#6 City Kid

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Posted 16 August 2018 - 10:19 PM

That’s good to know. Now I won’t be spending so much time trying to get consistent readings. 



#7 Jon Isaacs

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Posted 17 August 2018 - 08:18 AM

That’s good to know. Now I won’t be spending so much time trying to get consistent readings. 

 

I take enough readings overhead that I'm confident the meter has stabilized and then i measure at 45 degrees in the cardinal directions.  The meters are temperature compensated and i find if I let the meter thermally equilibate and use gloves when handling it,  it seems more stable. 

 

Jon



#8 Starman1

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Posted 17 August 2018 - 02:58 PM

I have an SQM-L and when I take readings at a site with clear horizons I take readings at zenith, north, south, east, west, and average them. For each direction I take three readings. When I take the readings they are regularly all over the place. For example, my first reading might be 21.65 followed by 21.72 and then 21.63. I don't like using three readings that are so far apart so I end up taking readings until I get three that are at least within .03 or .04 of each other. This can be very time consuming because sometimes I might need to take ten or more readings before I get three that are within .03 of each other. Is this normal? I'm surprised the different readings can be that far off from each other.

 

My second question is how dark can the meter read? While at the Nebraska Star Party I took readings while it was cloudy and got 23.57. But in a completely dark environment it won't read. Just curious.

It isn't temperature equilibrated.  My SQM-L reads +/- 0.03 over about 5 near-simultaneous readings.

 

Your second question is that the limit is a bit past 22.0--maybe 22.05 or so.

It'll read darker, but what a darker reading means is that something is blocking part of the field--a building, house, tree or something--in your case clouds.

Obviously, the darkest it can be is at a site far from cities under completely cloudy skies.

I don't know why you'd take a reading there, any more than you'd take a reading in a closet at home.

You'll get pretty good readings when the sky is clear of clouds, otherwise, not.

 

Notes: You'll get readings of 0.15-0.25 magnitude brighter when the Milky Way is in the field of the SQM-L

Careful about pointing too close to the horizon.  Your horizon is the height of trees or buildings or hills  in whatever direction you measure.  Try to stay 40-45° above the obstruction, not just above the true horizon.

 

I also use the SQM, and it gets far more consistent readings because I always point it at the zenith in a place where nothing can block the field.


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#9 City Kid

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Posted 17 August 2018 - 07:21 PM

 

 

 

Obviously, the darkest it can be is at a site far from cities under completely cloudy skies.

I don't know why you'd take a reading there, any more than you'd take a reading in a closet at home.

 

 

I was just curious.  I love it out there so much I stay out on the observing field all night long even when it’s cloudy. So since it was cloudy I was killing time. Yes it’s nuts, I know.



#10 noisejammer

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Posted 18 August 2018 - 06:58 PM

I'll echo Don's comment about the darkest real sky you'll see. I've used my SQM-L and measured 21.98 in a desert. Later that night, the December MW brought the brightness up to 21.8 or so. It was odd to experience the Milky Way as light pollution that was casting very clear shadows.

 

We were observing in an area with packed dark earth so I measured the brightness of light bouncing off the ground. if memory serves, it was in the mid-23's. I've also tried measurements in a cupboard with my hand over the lens. I think it reached 24.2 or so.



#11 Redbetter

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Posted 21 August 2018 - 07:48 PM

I find the Milky Way's influence considerably stronger on the SQM-L across multiple sites.  In June, early in the evening before the Milky Way is up very high I have seen 21.85 MPSAS at zenith, and a few hours later pointing at the MW overhead the result is 21.5 to 21.55.  I have seen this numerous times with starting conditions similar or as low as about 21.65 MPSAS, and the resultant hit is about 0.3 to 0.35 MPSAS.  Winter MW impact is more in the 0.25 MPSAS range from what I recall.  Same thing happens in reverse when the Milky Way begins to recede.

 

Things are more muted if one can measure the same portion of sky (e.g. closer to the empty areas around Draco/Ursa Minor/Polaris) rather than sampling different portions of sky that are inherently brighter.  Of course these far north areas tend to be somewhat lower in the sky to begin with, but if one has a dark northern horizon the resultant hit is typically only 0.15 MPSAS or so as the MW hits zenith.  I consider this more of the "light pollution" aspect of the MW on the rest of the sky.

 

My guess is that the lower values typically quoted for direct overhead with the narrower meter come from readings that are already getting a fairly large dose of MW since it is so wide and already partially in the field of view, especially in July/Aug/Sept.

 

The meter will read down to around 24 MPSAS before it gives the "out of range" code.  If it doesn't generate a reading in about a minute, the out of range code pops up.  I have seen this testing with fluorescent lights that have been off in a small utility room for several minutes with all other light sources blocked.  Had a reading in the high 23's the first time, but couldn't read subsequently even though the meter was pointing direct at the fixture and I could still see the glow.  The extra minute had made it substantially dimmer. 

 

SQM would be partially blocked even above 45 degrees when pointed at zenith at my dark sites.   


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