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Will there be a huge difference in my view if I go from SQM 19.7 to 21.2?

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#1 Rocklobster

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Posted 12 December 2019 - 11:18 PM

Sorry for this boring question. It's just that my wife and I were planning on a 4 hour drive at some point to a B2, SQM 21.7 site here in the UK. But I've been researching locations this evening and found a supposedly SQM 21.2 site about an hour away from our house.

I'm really just curious as to what to expect to see there. Will the difference in the view wow me? Obviously I should see the milky way clearly, but take for example M33..will that be visible in that location with my XT8? Or will the Veil be easier to see with my scope and a UHC filter? Im sure it will be worth going to and spending the night, I'm really just interested in what to expect from a SQM 21.2 site.

Sorry for the boring question that I'm sure has been asked a million times before.

Cheers


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#2 Venator

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Posted 12 December 2019 - 11:36 PM

At 21.2, you might be able to squeeze out seeing M33 with averted vision naked eye with some effort! M33 would be an easy binocular target at the very least and would be very nice in your XT8. Veil should look fairly detailed as well with a UHC filter. 


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#3 Redbetter

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Posted 13 December 2019 - 12:51 AM

Yes, it will make a huge difference.



#4 TOMDEY

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Posted 13 December 2019 - 01:32 AM

Yeah, 21.2 beats 19.7 by a lot. Of course, that 21.7 would be magnificent. My place is typically around 21.4 and I always bless the day I moved here (depressed rural upstate NY, USA) --- nearly 40 years ago, and the skies are still holding good... because population is leaving NY... high taxes and all...     Tom


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#5 ngc7319_20

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Posted 13 December 2019 - 02:14 AM

Yes, huge difference.



#6 Jon Isaacs

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Posted 13 December 2019 - 02:50 AM

My question:

 

Where are those numbers coming from?  The online maps I've seen are optimistic compared to what I and others actually measure.

 

19.7 is 1.5 magnitudes brighter than 21.2, it that's logarithmic scale so its 4 times brighter.  That's a big difference. But 21.2 on a map will probably be 20.7-20.9 measured so expectations need to be adjusted accordingly.

 

Jon



#7 bunyon

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Posted 13 December 2019 - 10:07 AM

Reading the first post, I suspect an error in the subject line. Yes, 19.7 to 21.2 is enormous. Unbelievable. 21.2 is an alien sky compared to 19.7.

 

However, I suspect you're asking about 21.2 to 21.7. I agree with Jon - where did you get those numbers?

 

But, let's assume they're correct. Now, the question is: do you drive an hour to 21.2 or four hours to 21.7?

 

If it were me, the 21.2 one hour drive would be my routine observing site. The place I go on a nice Tuesday night where I have to be at work the next day. Or a weekend night where I have other plans close to home.

 

A four hour drive is no joke and I would never, under any circumstances, do that as a one-nighter. But any chance I got, I would do a multi-night trip, especially if there were comfortable accommodations or other interesting things to see. 

 

I'm not great with the numbers but my guess is a 21.2 and 21.7 are discernibly different when the conditions are perfect at both places. But they're also close enough that conditions could level them or even make the 21.2 better (if it had better conditions). So you need a reliable weather map to tell. 

 

 

Again, 19.7 and 21.2 are different the way a gravel bed with ants and a nice modern bed are different. 21.7 and 21.2 are different the way a nice modern bed and a five year old bed are different. Put the five year old bed in a better house, with better heat, air and noise control, and it will be the choice. 


Edited by bunyon, 13 December 2019 - 10:08 AM.


#8 ngc7319_20

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Posted 13 December 2019 - 03:05 PM

My house is about SQM 20.2.  I will drive several hours to get darker SQM 21.5 skies.  Detail in galaxies, milkyway, etc., is greatly improved.  Of course, the SQM number is not the whole story.  Fog, haze, moon phase, etc., matter also.  And absence of local street lights, having astronomy-friendly neighbors, etc., helps too.


Edited by ngc7319_20, 13 December 2019 - 03:06 PM.


#9 Redbetter

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Posted 13 December 2019 - 05:39 PM

For what the OP is asking, going from the 19+ range to 21 range is the key.  Once you reach 21 you are in rural dark sky.  That doesn't mean light pollution free, but in the high mid 19's to 20.0 range is essentially suburbs or on the edge of suburbs.  19 to 20.0 isn't awful, but it there is so much missing by the time the background is this bright.  Getting into the 21.0 range is a big improvement.

 

It of course gets considerably better again if one can go from around 21.0 to 21.5+, but the biggest gain is just getting into that ~21 MPSAS dark range. 


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#10 Rocklobster

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Posted 13 December 2019 - 08:59 PM

My question:

Where are those numbers coming from? The online maps I've seen are optimistic compared to what I and others actually measure.

19.7 is 1.5 magnitudes brighter than 21.2, it that's logarithmic scale so its 4 times brighter. That's a big difference. But 21.2 on a map will probably be 20.7-20.9 measured so expectations need to be adjusted accordingly.

Jon

Hi Jon, I've got them from https://www.lightpollutionmap.info

I know that I should take these numbers with a bit of a pinch of salt and that they do not convey the whole picture so to speak.

Interesting that you say the online maps are optimistic, as that is what I assumed. I am aware that the SQM reading of my chosen location should probably be lowered a bit to align with reality.

Out of curiosity, have you found any particular online map to be more accurate?

Cheers

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#11 Rocklobster

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Posted 13 December 2019 - 09:01 PM

Reading the first post, I suspect an error in the subject line. Yes, 19.7 to 21.2 is enormous. Unbelievable. 21.2 is an alien sky compared to 19.7.

However, I suspect you're asking about 21.2 to 21.7. I agree with Jon - where did you get those numbers?

But, let's assume they're correct. Now, the question is: do you drive an hour to 21.2 or four hours to 21.7?

If it were me, the 21.2 one hour drive would be my routine observing site. The place I go on a nice Tuesday night where I have to be at work the next day. Or a weekend night where I have other plans close to home.

A four hour drive is no joke and I would never, under any circumstances, do that as a one-nighter. But any chance I got, I would do a multi-night trip, especially if there were comfortable accommodations or other interesting things to see.

I'm not great with the numbers but my guess is a 21.2 and 21.7 are discernibly different when the conditions are perfect at both places. But they're also close enough that conditions could level them or even make the 21.2 better (if it had better conditions). So you need a reliable weather map to tell.


Again, 19.7 and 21.2 are different the way a gravel bed with ants and a nice modern bed are different. 21.7 and 21.2 are different the way a nice modern bed and a five year old bed are different. Put the five year old bed in a better house, with better heat, air and noise control, and it will be the choice.

Awesome. That's exactly what u was hoping to hear. I think I will be a good first test...to see how long I spend outside, etc....rather 1 hour away than 4. Thanks for your opinion.



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#12 Rocklobster

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Posted 13 December 2019 - 09:02 PM

Yes, huge difference.

Thanks for the reply. I figured it would be fairly big difference and would rather travel one hour away instead of 4-5 for my first darker sky trip. ....can't wait!

Cheers

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#13 Rocklobster

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Posted 13 December 2019 - 09:03 PM

Yeah, 21.2 beats 19.7 by a lot. Of course, that 21.7 would be magnificent. My place is typically around 21.4 and I always bless the day I moved here (depressed rural upstate NY, USA) --- nearly 40 years ago, and the skies are still holding good... because population is leaving NY... high taxes and all... Tom

Thanks for replying. I was hoping to hear exactly what you said.

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#14 Rocklobster

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Posted 13 December 2019 - 09:04 PM

At 21.2, you might be able to squeeze out seeing M33 with averted vision naked eye with some effort! M33 would be an easy binocular target at the very least and would be very nice in your XT8. Veil should look fairly detailed as well with a UHC filter.

Interesting. So it seems that going to that location will definitely be a big step up in terms of darkness and viewing. Thanks for replying.

Cheers

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#15 Astro-Master

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Posted 14 December 2019 - 02:30 AM

Get a sheet of paper and write down everything you can think of you'll need for your trip so you don't forget anything.  Then make copies of the list for the next trip.

 

My first trip with my 18" Dob to a dark site was going great till I realized I'd left the truss polls at home, I was lucky I remembered when I was only 15 miles from home.

 

Lets us know how it went on your trip.

 

Clear Skies


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#16 Jon Isaacs

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Posted 14 December 2019 - 08:36 AM

Hi Jon, I've got them from https://www.lightpollutionmap.info

I know that I should take these numbers with a bit of a pinch of salt and that they do not convey the whole picture so to speak.

Interesting that you say the online maps are optimistic, as that is what I assumed. I am aware that the SQM reading of my chosen location should probably be lowered a bit to align with reality.

Out of curiosity, have you found any particular online map to be more accurate?

Cheers

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I have not found any that are better but I haven't looked carefully.  I have an SQM-L and I measure the sky brightness both overhead and at 45° in several directions several times a night. According to the chart, my high desert site is 21.64 mpsas. 

 

Typically I measure 21.3 overhead with a range from 21.0 to 21.5. 21.5 occurs when the cities along the coast and in Mexico are covered by clouds. The air here is typically dry and transparent.

 

Lots of good advice in this thread. Don't forget anything. 21.0 is way darker than 19.5. 

 

Jon


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#17 Tony Flanders

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Posted 14 December 2019 - 09:18 AM

I'm not great with the numbers but my guess is a 21.2 and 21.7 are discernibly different when the conditions are perfect at both places.\


Speaking as someone whose backyard (at my country home) reaches 21.2 fairly often, I can assure you that there's a very big difference indeed between 21.2 and 21.7. (These are actual readings with my own SQM, by the way, not taken from maps.)
 
At 21.2, the Milky Way is distinctly washed out; only the biggest and boldest dust lanes are obvious. Artificial light pollution is still stronger over the entire sky than natural sources such as the zodiacal band -- though not as strong overhead as the brightest parts of the Milky Way.
 
At 21.7, artificial light is weaker than natural sources over most of the sky, the exception being near the horizon in the direction of the one or two brightest sources of light pollution. The Milky Way really sparkles; it's crisscrossed with tiny dust lanes throughout.
 
But both at 21.2 and 21.7 the Milky Way is instantly obvious except possibly when it's hugging the horizon on spring evenings. You don't need to wait to get dark adapted to see it; it's the first thing that you notice in the sky after walking outside.
 

Again, 19.7 and 21.2 are different the way a gravel bed with ants and a nice modern bed are different. 21.7 and 21.2 are different the way a nice modern bed and a five year old bed are different. Put the five year old bed in a better house, with better heat, air and noise control, and it will be the choice.

 
Hmm. If 19.7 is a gravel bed with ants, then what shall we say about the 18.5 typical of suburban sites or the 17.5 typical of urban sites? When I'm at my city home, I frequently spend considerable time driving to 19.7 skies, and I'm very glad when I get there. At 19.7, the Milky Way should still be quite obvious, at least during the (Northern Hemisphere) summer.

 

Having said that, yes, the difference between 19.7 and 21.2 is vast.

 

The Bortle Scale has nine levels, and each level is different from the next in a major, qualitative, unmistakable way.


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#18 bunyon

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Posted 14 December 2019 - 11:23 AM

18.5 is flaming nails, I think.

 

(Edit to add: I don't own a SQM and am unfamiliar enough with the units that I probably shouldn't opine on threads like this. From my suburban home, with a streetlight on the property the city won't let me turn off, I don't do DSO observing. I observe planets, moon and the occasional double star. I know, because I've done it, that if I move the scope around back, put a hood over my head for half an hour and use averted vision, I can get a reasonable view of many DSOs. But I can also drive half an hour and get much better. I've traveled and seen much, much better. 

 

Tony has written some wonderful articles on urban and suburban observing but, for me, a lot of the appeal is the ability to lie back from the scope and take in the entire sky. That gets lost, for me, in a suburban setting. So, 18.5 isn't flaming nails. But it's a super soft bed with old, dirty sheets. Fine in a pinch if you're exhausted and, for some, a good night's sleep. But, for me, I'll pass it by more often than not). 


Edited by bunyon, 14 December 2019 - 11:36 AM.

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#19 Jon Isaacs

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Posted 14 December 2019 - 11:49 AM

Last night was an interesting experience with light pollution. Due to Christmas obligations, I will miss my normal ~10 nights in the high desert that typically begins around the third quarter moon.

 

I'm having to make do with a few nights with short periods before moon rise. Last night there was 10 minutes between astronomical sunset and moonrise and I wanted to make the most out of the time before astronomical twilight to view Phil Harrington's challenge objects NGC 1 and 2. 

 

These are mag 12.8 and 14.1 galaxies and normally would be straightforward with the 16 inch or 22 inch but with moon I had little time.

 

I did use the 22 inch and began with the region measuring about 20.0 mpsas on the SQM-L.  They were at 80° plus elevation with the sky rotating so it was a little trick but I picked NGC-1 up at about 20.7 and saw the fainter NGC2 almost immediately. As the sky darkened, they became more and more apparent.

 

The darkness peaked at 21.14 and for the period it was greater than 21.0, they were very obvious, quite easy. I did some other observing nearby and then returned to watch them as the moon began to brighten the sky before it rose.

 

As the sky brightened I was able to still see them until it was about 20.3. At that point, I said enough was enough. Later the brightness leveled off at about 19.7 and soon after I called it a night.

 

In San Diego, dark is 18.7 or so and I find plenty to look at. Our here in the high desert, the moon seemed so bright.

 

Jon


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#20 Starman1

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Posted 14 December 2019 - 02:35 PM

My backyard here in LA varies from 16.5-17.5, bright enough to read newspaper headlines.

OK for Moon, planets, double stars, carbon stars.

 

I have been to Mt. Wilson locally, where I've seen mag. 19.5-19.8.  Sorry, that's just not dark.

When you dark adapt, you never need a flashlight, even a red one.

I grew up in the midwest, a mile from the city center, and my backyard sky was a lot darker than that.

 

I have observed at a site about 30 miles away in the Santa Monica mountains and seen mag. 20.5 there.

Sorry, still not dark enough for any other than the brightest couple hundred deep sky objects.

 

My nearest dark site (21.1-21.3) is about a one hour drive, and the sky is HUGELY darker there and I can actually do 

a fair amount of deep sky observing of galaxies and nebulae there.  So getting past 21.0 makes a big, big difference.

 

My preferred dark site (21.4-21.7) is an hour and a half drive, and the sky there is another level darker.  Not much difference

on the SQM, but the sky is noticeably darker and a lot more stars are visible and fainter stars are visible in the scope and more detail in galaxies.

 

I have a decent desert site I drive to (4-5 hours drive) at mg.21.6-21.85, and that's as dark as it gets in SoCal.

It's a lower altitude than the slightly brighter sites, so the sky from 45° down isn't as transparent, though the zenith gets really dark

and some galaxies passing through that 90° cone centered on the zenith really display a lot more detail.

 

I use the following formula to convert SQM readings to Naked Eye Limiting Magnitude (NELM) figures:

NELM = [(SQM-8.89)/2] +0.5

Solving for SQM,   SQM = [(NELM-0.5)x2]+8.89


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#21 Jon Isaacs

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Posted 14 December 2019 - 08:57 PM

Just to add to Don's excellent post as well as other excellent posts, something basic:

 

The measurement unit for sky brightness is magnitudes per square arc second, mpsas.

 

Magnitudes are a logarithmic scale to the base 2.5. This means an urban 18.0 mpsas sky is 2.53 = 15.6 times brighter than a rural 21.0 mpsas sky.

 

1 magnitude is 2.5 x brighter, 2 is 6.25x brighter, 3 is 15.6x and 4 39 times brighter. These are big numbers. 

 

Sometimes people don't understand the scale is logarithmic. In an another forum, the organizer of an event seemed to think that 20.5 mpsas was only slightly brighter than 21.5 mpsas instead of being 2.5 times brighter.

 

Jon


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#22 Rocklobster

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Posted 14 December 2019 - 09:56 PM

I have not found any that are better but I haven't looked carefully. I have an SQM-L and I measure the sky brightness both overhead and at 45° in several directions several times a night. According to the chart, my high desert site is 21.64 mpsas.

Typically I measure 21.3 overhead with a range from 21.0 to 21.5. 21.5 occurs when the cities along the coast and in Mexico are covered by clouds. The air here is typically dry and transparent.

Lots of good advice in this thread. Don't forget anything. 21.0 is way darker than 19.5.

Jon

Thanks jon. I really appreciate your reply.

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#23 csrlice12

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Posted 14 December 2019 - 09:58 PM

So roughly, earthquakes and sky darkness are similar....



#24 Rocklobster

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Posted 14 December 2019 - 10:03 PM

My backyard here in LA varies from 16.5-17.5, bright enough to read newspaper headlines.
OK for Moon, planets, double stars, carbon stars.

I have been to Mt. Wilson locally, where I've seen mag. 19.5-19.8. Sorry, that's just not dark.
When you dark adapt, you never need a flashlight, even a red one.
I grew up in the midwest, a mile from the city center, and my backyard sky was a lot darker than that.

I have observed at a site about 30 miles away in the Santa Monica mountains and seen mag. 20.5 there.
Sorry, still not dark enough for any other than the brightest couple hundred deep sky objects.

My nearest dark site (21.1-21.3) is about a one hour drive, and the sky is HUGELY darker there and I can actually do
a fair amount of deep sky observing of galaxies and nebulae there. So getting past 21.0 makes a big, big difference.

My preferred dark site (21.4-21.7) is an hour and a half drive, and the sky there is another level darker. Not much difference
on the SQM, but the sky is noticeably darker and a lot more stars are visible and fainter stars are visible in the scope and more detail in galaxies.

I have a decent desert site I drive to (4-5 hours drive) at mg.21.6-21.85, and that's as dark as it gets in SoCal.
It's a lower altitude than the slightly brighter sites, so the sky from 45° down isn't as transparent, though the zenith gets really dark
and some galaxies passing through that 90° cone centered on the zenith really display a lot more detail.

I use the following formula to convert SQM readings to Naked Eye Limiting Magnitude (NELM) figures:
NELM = [(SQM-8.89)/2] +0.5
Solving for SQM, SQM = [(NELM-0.5)x2]+8.89

Very interesting. Thanks for sharing your experience and opinions.

It seems from all the replies so far that I will not be wasting time going to a 21 and above site, which is what I was hoping to hear.
Really can't wait to go. Sadly, it will have to be a last minute trip, when the weather forecast is almost certain to be clear skies..and of course, no full moon either.

Thanks again to everyone who replied so far.

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#25 Rocklobster

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Posted 14 December 2019 - 10:04 PM

Just to add to Don's excellent post as well as other excellent posts, something basic:

The measurement unit for sky brightness is magnitudes per square arc second, mpsas.

Magnitudes are a logarithmic scale to the base 2.5. This means an urban 18.0 mpsas sky is 2.53 = 15.6 times brighter than a rural 21.0 mpsas sky.

1 magnitude is 2.5 x brighter, 2 is 6.25x brighter, 3 is 15.6x and 4 39 times brighter. These are big numbers.

Sometimes people don't understand the scale is logarithmic. In an another forum, the organizer of an event seemed to think that 20.5 mpsas was only slightly brighter than 21.5 mpsas instead of being 2.5 times brighter.

Jon

I absolutely am aware that the scale is logarithmic. That sounds very defensive, it's not intended to be.

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