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Light Pollution and DSOs - Bortle 4 vs. Bortle 5

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#1 MellonLake

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Posted 25 March 2019 - 02:33 PM

To set the background a bit:

  • I am a noob, 4 months with my XT10.  I have used the telescope probably 15 times in this period (the weather in the Toronto area has been terrible)
  • I live in a Bortle 8-9 area.  I am lucky if I can see a dozen stars from home.  
  • To use my telescope more effectively, I go to either a Bortle 5 (40 minute drive) or Bortle 4 (90 minute drive) area.
  • I have generally gone to Bortle 5 area because it is so much closer 
  • I have been spending my time going through the Messier objects.  When Messier objects are not visible I generally hunt down a few interesting doubles, check for E and F in the trapezium, or see how close I can get to the Rayleigh limit, etc.
  • My process is to arrive at the site at sundown.  After setting up, collimating, and starting the scope to cool, I hunt down a couple of doubles I know during astronomical twilight just to make sure things were working well.  After cool down I do a star test to confirm my collimation and tweak if needed.    

The past few times I have gone to the Bortle 5 site, I have tried and tried to find Messier objects but it has been frustrating, once in a while I can find one but it is just not consistent.  I was kind of stuck at 20 or so Messier objects identified for about a month when going to the Bortle 5 site once a week. I had tried to find M109, and the galaxies in Leo 5 different times at the Bortle 5 site with no luck.    

 

This past Saturday, the weather finally cooperated again and I took a leap and headed out to the much further away Bortle 4 area. I got lucky; a nice dark night (at least until the moon came up) with good transparency and average to better seeing.  I started knocking off Messier objects at an amazing rate.  Within an hour and a half I found, M109, M106, M51, M63, M94, M64, M3, M53, M85, M98, M90, M89, M58, M60, M100, M99, M84, M86, M87, M95, M96, M105, Leo Triplet, M97, M108.  If not for the moon coming up, I figure I would have had huge percentage of a Messier marathon by the end of the night!  

 

So I guess my question is: What is everyone else's experience with DSOs in Bortle 5 vs. Bortle 4.  Was my experience this past Sat. just a result of better conditions or is the difference between Bortle 4 and 5 just that large that DSOs are not going to be visible in Bortle 5?   

 

 

Thanks  

 

 


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#2 treadmarks

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Posted 25 March 2019 - 07:50 PM

My experience closely matches your own. There is a huge difference between Bortle 4 and 5. Galaxies become much more attainable at Bortle 4. At Bortle 5 I can see only the brightest galaxies - M81, M82, M31, M101. At Bortle 4, I was able to see M51, its spiral arms, and its NGC companion galaxy. It was one of the few "satisfying" views of galaxies I have had. Pleiades also showed its fully glory when I was able to see the nebula surrounding it, something I have never been able to see at Bortle 5+.

 

Given this, I am questioning if Bortle 5 is even worth the drive. I suppose it makes the clusters look better. But I am also in the same situation in that getting to a Bortle 4 involves about twice as much driving due to urban sprawl. So while there are definite rewards to making that drive, there are also some significant costs.

 

One more note about Bortle 8-9... I live in a Bortle 8. When my eyes are not dark adapted and there's glare, I can only see a dozen stars too. But if I take steps to get dark adapted and block out the glare (very difficult where I am), I can see many more stars. I would also point out that star clusters respond to aperture very nicely, and there are filters that work well on nebulae. So I might also suggest that you don't give up on observing at home.



#3 MOwen

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Posted 25 March 2019 - 08:56 PM

I'm in a class 5 where the SQM value is 20.08 and it is <10 miles from an 8; our local club site is about 10 miles out from where I am and shows as a class 4 with an SQM value of 21.16.  I have also been to an even darker site about an hour's drive away, where I was blown away by how much I could see naked eye.  When I checked the LP map later, I found it difficult to believe that it was also a class 4, but the SQM shows 21.53.  Last October, I was at a class 2 site (SQM=21.93) with the scope; it was truly awesome but I ended up spending more time viewing naked eye.  So what I've learned from all this is that the Bortle number is a good 'coarse' indicator but the SQM value is the real target.  I've had amazing nights here in the driveway but I have to give the edge to the club site with its nice bump in SQM. 

 

If you haven't already done so, I would suggest you scan the LP map for other possible spots within both the 5 and 4 destinations and see if you can find something where the SQM peaks out.  That seemingly small difference of .37 mentioned above (21.16 to 21.53) represents a significant boost in darkness.

 

I'm extremely grateful to have a little 'black hole' over my head, being so close to urban sprawl.  The Milky Way comes through from time to time so a 5 is not so bad after all.  



#4 Tony Flanders

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Posted 26 March 2019 - 04:44 AM

I'm curious how you know that one site is Bortle 4 and the other is Bortle 5. Is that your own personal judgment based on the criteria outlined by John Bortle's criteria, or is it based on one of the light-pollution maps prevalent on the internet?

 

If the latter, beware of taking the map's rating as gospel. First of all, there are many light-pollution maps, and they often disagree on how bright any given spot is. Moreover, the correlation between the colors shown on those maps and the Bortle classes is often extrinsic to the map -- done by someone else -- and is even more questionable than the colors themselves.

 

For what it's worth, the main differences between classes 4 and 5, according to John Bortle, are:

 

In Class 5, only hints of the zodiacal light are visible. In Class 4, it is "evident". Treat this one with many grains of salt -- I for one disagree with John Bortle about this. Moreover, the visibility of the zodiacal light varies greatly depending on the season.

 

In Class 4, the "Milky Way well above the horizon is still impressive but lacks all but the most obvious structure." In Class 5, the Milky Way is "very weak or invisible near the horizon and looks rather washed out overhead." Does that correspond to your own experience of the two sites?

 

Anyway, getting back to your original question, each Bortle class actually spans a huge range of different conditions. The difference between the dark end of Bortle 5 and the bright end of Bortle 5 is instantly obvious. And if your two sites are in fact at the dark end of Bortle 4 and the bright end of Bortle 5, then there's actually almost three zones difference between them, not one zone. So yes, it's entirely plausible that one site that is legitimately Bortle 4 could be dramatically better than another site that is legitimately Bortle 5.

 

In addition, transparency has a huge effect on Bortle class. In the original article, John Bortle stated that the ratings applied to specific sites, but he has since clarified this to mean that any given site's rating can vary from one night to the next due to transparency. So the difference between your two sessions may be due as much to the weather at the time as to the difference between the sites.

 

For what it's worth, I find all the Messier objects instantly obvious through a 10-inch scope both in Bortle 4 and in Bortle 5. More precisely, if the Milky Way is reasonably clearly defined anywhere in the sky, then I feel confident that I can see all the Messier objects with ease through any reasonably large telescope -- say 4 inch aperture or more. However, I am a highly experienced deep-sky observer. Those same Messier objects at exactly those same sites would have been barely visible to me -- if at all -- during my first year of observing. Of the innumerable variables determining the visibility of deep-sky objects, the observer's skill level is arguably the most important of all.

 

To put that another way, keep up the good work! It gets easier -- much easier -- with practice.


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#5 MellonLake

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Posted 26 March 2019 - 12:15 PM

I really don't have a way to block out lights in my back yard so I am stuck not getting well dark adapted at home.  As such, I will probably still do DSO viewing at site remote to my home.   

 

As for the Bortle number I am quoting, it is from a map website.   In reviewing the criteria listed by Tony, I would say that my Bortle 5 site is high Bortle 5 as the Milky way is really not visible either at the horizon or even overhead.  At my Bortle 4 site, the Milky way is apparent overhead but not at the horizons so it is probably mid to lower Bortle 4.  I think Tony is right,  there is nearly a 3 Bortle zone difference between the two sites.   Further, given I really haven't seen the Milky way from the Bortle 5 site, it is probably not the right place to seek out Messier objects.   

 

Based on all of this I am going to spend more time at the Bortle 4 site. I will go to the Bortle 5 site on occasion, as my skill gets better, to see if I can find the DSOs even in more light polluted skies.  As for finding another "hole" in the light pollution closer to home, unfortunately living in the Toronto area, with nearly 7 million people within a 1 hour driving radius of my home, does not bode well for finding any better sites (I have looked).  I am lucky enough to have a 3 season cottage in a truly dark sky area and will be able to view there on weekends once the ice is out (late April).   

 

Thank you all for your help.  I find all the input I get here on CN to be of great benefit.  



#6 whizbang

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Posted 26 March 2019 - 06:09 PM

I live in a Bortle 6 and have visited three Bortle 4 sites.  Amazing difference. 



#7 emflocater

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Posted 26 March 2019 - 08:51 PM

Wow....I been crying about my Bortle 3 sky when I walk out my backyard and look up at the night sky. After reading these posts I am shutting up my moaning and groaning!

Cheers

Don


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#8 Akol47

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Posted 26 March 2019 - 11:22 PM

Agreed emflocator, I feel like I squandered all those nights on camp outs out in the middle of nowhere. I should be thankful that dark skies aren't very far away...



#9 jcj380

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Posted 27 March 2019 - 07:41 AM

I live in a Bortle 6 and have visited three Bortle 4 sites.  Amazing difference. 

Going from a 7-8 to a 4 is a real gobsmack.

 

Growing up in a 2-3 probably had a major positive effect on me getting interested in astro.


Edited by jcj380, 27 March 2019 - 07:43 AM.


#10 Tony Flanders

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Posted 28 March 2019 - 06:05 AM

Wow....I been crying about my Bortle 3 sky when I walk out my backyard and look up at the night sky. After reading these posts I am shutting up my moaning and groaning!

Cheers

Don

If you have bonafide Bortle 3 conditions in your backyard, you should definitely count your blessings. You're way ahead of 95% of all people living in industrialized countries. (Actually, that's probably closer to 99%.)


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#11 emflocater

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Posted 28 March 2019 - 11:54 AM

If you have bonafide Bortle 3 conditions in your backyard, you should definitely count your blessings. You're way ahead of 95% of all people living in industrialized countries. (Actually, that's probably closer to 99%.)

I apologize and made an error based on bad memory! I went and re-checked the maps and I have a Bortle 4 not 3 frown.gif

So I guess I can go back moaning and groaning just a little as I know it can be much, much worse. Sorry for the error.

Cheers

Don 



#12 jcj380

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Posted 28 March 2019 - 01:25 PM

To put it in perspective, my dark sites are 4s.  The nearest 3 AFAIK is a 4 hour drive away.


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#13 Redbetter

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Posted 28 March 2019 - 03:04 PM

My suburban backyard is on the margin between Bortle 5 and 6 depending on the night.  I only observe DSO's there as a way to challenge myself (often with small refractors) and to appreciate what others are trying to see in their yards with their gear. 

 

I do some outreach in Bortle 4 and decent Bortle 5 sites.  Both are usable for DSO's, but not great. It is hard to show anything but the brightest DSO's to complete novices in such conditions.   

 

I carry my gear to darker sites as many nights of the new Moon cycle as I can.  My primary site is Bortle 3 to Bortle 2 on the best nights.  My darker sites are Bortle 2 and probably Bortle 1 on the best nights.  I have been to pristine Bortle 1 sites before.



#14 Tony Flanders

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Posted 28 March 2019 - 07:39 PM

I apologize and made an error based on bad memory! I went and re-checked the maps and I have a Bortle 4 not 3 frown.gif

So I guess I can go back moaning and groaning just a little as I know it can be much, much worse. Sorry for the error.

Cheers

Don 

OK, that only puts you ahead of 90% of everybody, not 95%. But I sympathize with you; I'm pretty unhappy with the backyard of my country home, which is arguably Bortle 4 by most measures. That's based on my own observations, by the way; I don't pay much attention to the Bortle numbers given by maps.


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#15 Jon_Doh

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Posted 29 March 2019 - 09:45 AM

I envy anybody with a Bottle 4 or 5.  My yard with ugly gray skies is a Bottle 7.  It was maybe a 6 or 5.5 when I built my house and on moonless nights I could see quite a number of things.  But the city glow has gotten much worse and vacant land around me is now covered with a lot of houses adding to the light pollution.  Where our club meets is a 4 and it's quite a drive away on a mountaintop accessed by dirt roads.  So, you guys with a Bottle 4 should really count yourselves very lucky with the way things are today.



#16 Feidb

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Posted 29 March 2019 - 02:28 PM

I hate to mock,  but Bortlebee, Bortlebum. I don't pay attention to any of that stuff including NELM, or half a dozen other math problems.

 

I live on the east side of Las Vegas and when I go out side and look up, I can barely see the main stars of Orion, and barely the belt. To me, you can call it whatever math problem you want, Bortlebla bla bla, but to me, it's un-viewable. Period.

 

I have my usual observing site which I have to drive to and it's 36.5 miles door to parking lot. Someone once measured the sky there and I was totally unimpressed with the number. There's another site eight miles further along and they eked out a bit darker number. Oooh aaah. I not only don't remember what those numbers are, I don't care.

 

What I know is that this spot, while not pristine, is dark enough, especially looking north, south, up and East, certainly dark enough for me. I've seen the Horsehead unfiltered, so it's Bortle something.

 

Those numbers and formulas and readings don't make any difference to my observations. Maybe to many of you out there, but what I do is I check a site out first, if it's dark enough, I'll use it again. The rest is up to the weather, which is a whole different issue.

 

Best of luck with the math formulas.


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#17 Pelayo

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Posted 29 March 2019 - 03:20 PM

I apologize and made an error based on bad memory! I went and re-checked the maps and I have a Bortle 4 not 3 frown.gif

So I guess I can go back moaning and groaning just a little as I know it can be much, much worse. Sorry for the error.

Cheers

Don 

Bortle 4 are still not common skies to people living in cities, you are lucky. 

 

My house is in a village in the countryside not far away to a city, just a few kilometers. The sky is bortle 5. If I want darker skies I drive to the mountains, what I do quite often. After 45 min I get bortle 4 skies and the difference is massive. 


Edited by Pelayo, 29 March 2019 - 03:25 PM.


#18 Pelayo

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Posted 29 March 2019 - 03:24 PM

I hate to mock,  but Bortlebee, Bortlebum. I don't pay attention to any of that stuff including NELM, or half a dozen other math problems.

 

I live on the east side of Las Vegas and when I go out side and look up, I can barely see the main stars of Orion, and barely the belt. To me, you can call it whatever math problem you want, Bortlebla bla bla, but to me, it's un-viewable. Period.

 

I have my usual observing site which I have to drive to and it's 36.5 miles door to parking lot. Someone once measured the sky there and I was totally unimpressed with the number. There's another site eight miles further along and they eked out a bit darker number. Oooh aaah. I not only don't remember what those numbers are, I don't care.

 

What I know is that this spot, while not pristine, is dark enough, especially looking north, south, up and East, certainly dark enough for me. I've seen the Horsehead unfiltered, so it's Bortle something.

 

Those numbers and formulas and readings don't make any difference to my observations. Maybe to many of you out there, but what I do is I check a site out first, if it's dark enough, I'll use it again. The rest is up to the weather, which is a whole different issue.

 

Best of luck with the math formulas.

 

It's not a matter of formulas, but having a criteria to understand how dark our skies are in a forum. In any case, I agree with your message. 


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#19 Feidb

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Posted 29 March 2019 - 10:25 PM

Pelayo,

 

Where in Spain are you? I used to live in central Spain just outside Madrid in a place called Eurovillas, between Pozuelo del Rey and Nuevo Baztan.

 

Did some pretty decent observing back in the 80's with first an 8-inch f/9.44 reflector and then a 16-inch f/6.4 reflector, both home-built.

 

That was before Eurovillas became as built up as it is now. I bet the skies are way too bright now!


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#20 Pelayo

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Posted 31 March 2019 - 10:11 AM

Pelayo,

 

Where in Spain are you? I used to live in central Spain just outside Madrid in a place called Eurovillas, between Pozuelo del Rey and Nuevo Baztan.

 

Did some pretty decent observing back in the 80's with first an 8-inch f/9.44 reflector and then a 16-inch f/6.4 reflector, both home-built.

 

That was before Eurovillas became as built up as it is now. I bet the skies are way too bright now!

 

I hope you enjoyed your time in Spain!

 

I live in Asturias, north Spain. There's quite a lot of light pollution in the centre of the region, as that's where the cities and heavy industry are. But a drive of 45 min gives pretty decent skies.


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#21 jcj380

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Posted 01 April 2019 - 08:01 AM

I just got back from a couple days (nights) under Bortle 4 skies.  The number of stars visible was absolutely mind-boggling compared to the gray soup sky at home.  I went out early this morning for a quick look at home and man, was that a disappointment.  I'm going to have to rethink driving out to darker skies on a regular basis or maybe seriously look into EAA.



#22 Muddman97

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Posted 01 April 2019 - 01:21 PM

Myself, I'd just bypass the short drive to Bortle 5 for the Bortle 4 skies.

 

I lived under class 1 - 2 skies for 40 years before moving to class 5 skies 6 years ago.  Man, what I would give to be able to take my scope back home for a few weeks.

 

I can manage most Messiers from my current class 5 skies, minus the dimmer galaxies, but I am also only a few years into the astro game as well.  With more experience and a more trained eye, I'll probably do better.



#23 Scott Regener

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Posted 01 April 2019 - 05:32 PM

Back to the OP: I think you're doing something wrong. I did almost the entire Messier catalog in a Bortle 7 location using a 4.5" reflector. Now, that doesn't mean that when I jumped to a 10" things weren't better - they were - but almost every Messier is going to be visible in an 8x50 finder at a Bortle 5 site. Visible does not mean bright, and it may take averted vision and a little waiting for the atmosphere to steady. Some of those Virgo galaxies may be tough, too. One thing that matters is transparency - the more light pollution you deal with, the more transparency has to be pretty good to let those dimmer objects shine through.

 

What are you using for finders? Charts? I found my success rate really jumped with a RACI and a unit finder. So much easier to star-hop with that combination, where everything is right-side up and you're not breaking your back just trying to look through the finder. Good charts make a world of difference, too. I use the jumbo version of the Pocket Sky Atlas, but when my eyes were younger, the smaller version worked, too.



#24 Arthur NY

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Posted 01 April 2019 - 05:54 PM

Wow....I been crying about my Bortle 3 sky when I walk out my backyard and look up at the night sky. After reading these posts I am shutting up my moaning and groaning!

Cheers

Don

You're near Rochester...I'd be quite surprised if you are Bortle 3. 

 

I'll be driving 25 min to Bortle 5, so I'm sour....



#25 MellonLake

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Posted 01 April 2019 - 07:13 PM

I have an RA Finder and a Telrad. I can find clusters and very bright galaxies and nebula (M81 and M1) in Bortle 5-6 and many even in Bortle 7-8. However,In Bortle 5-6 skies I just can't seem to find galaxies or nebula where the surface brightness is closer to 10. I think in the long run, as I get better at finding DSO and using averted vision, I will be able to find more in increasingly more light polluted skies. For now going to the Bortle 4 site will be my goto.


Edited by MellonLake, 01 April 2019 - 07:20 PM.



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