September 28, 2018: The Bortle Scale, or Why Gas is Your Friend

In my second-to-last blog post, I talked a lot about the Bortle Scale.  What is it?  Why should you care?

Invented by John Bortle in a 2001 Sky & Telescope article, the Bortle Scale is an effective way for you to objectively assess the darkness – or, unfortunately, the brightness – of your sky, or any sky, with your naked eyes.  The Bortle scale is amply described not only in his S&T article, but also in the Wikipedia article, which is excellent, and is required reading for this post.  So stop reading this post, click on either link, go read either article, and then come back.  I’ll wait.

Ready?  Okay.

My point in getting you to read that article is to be able to explain quantify the impact of what the different Bortle levels mean, and by that, I mean what they mean to your aperture.

In conversing with someone on one of the Facebook astro groups, it occurred to me that, yeah, sure, everyone knows that if you go to a dark sky site, of course you’re gonna see more.  You’re gonna see more with your naked eyes, but you’re also gonna see more with your scope, no matter how small or big it is.  Duh.  Going to a dark site is like adding aperture onto your scope.  But how much?

Before we discuss that, going to dark skies means more than that.  Less light pollution means that you can see things that you ordinarily can’t ever see under light polluted skies.  This goes back to what I said in an earlier post about low-surface brightness objects.

The tl;dr version is:  M33, the Triangulum Galaxy, is the poster child for these types of DSOs.  Although it’s rated as being magnitude 5.7, which is pretty bright, it’s also huge, much, much larger than the full moon.  Larger than a few full moons, actually.  This means that the light is spread out over a very wide area.

That 5.7 magnitude rating is what’s called integrated magnitude – as if all the light coming from the entire, huge galaxy was shrunk down to a point, to the size of a star. It’s just not very much brighter than the black, or “black”, sky around it.  And low-surface brightness objects like this are the first things to go missing in light-polluted skies.  The light pollution, the gray sky, just swallows them up, so you can’t detect them as against the background sky.

However, my main point here is about aperture.  Looking at either article, Bortle 9 skies are the worst, most light-polluted skies, with a naked-eye limiting magnitude (NELM) of 4.0.  These are the skies deep inside a city, or perhaps not so deep if your city is large and the light pollution is bad.

As I discussed in that post about magnitude, to go a magnitude deeper (well, really 0.9 magnitudes, but close enough), which is a very significant jump, you have to add 50% more aperture.  That’s roughly going from 90mm (3.5 inches) to about 130mm (5.1 inches), going from 5 to 8, 6 to 9, 8 to 12, 10 to 14.  As you get to the upper end of those apertures, those jumps become pretty expensive.  To go 1.5 magnitudes deeper, a truly big jump, you have to double your aperture – and yeah, of course, that’s very expensive.

But what if, instead of spending all those hundreds, perhaps even thousands of dollars on a huge, gigantic scope, with all the inherent drawbacks of a large scope (weight, size, transport), what if you could wave a magic wand, and with a magic word, increase the size of your scope that way?  Well, you can.  The magic wand is your car, the magic word is gas, and the size of your scope “increases” because of much darker skies.

The Bortle scale ends at 9, but let’s extrapolate a bit.  Two years ago, back in Manhattan, my skies barely let me see Albireo, putting my NELM at magnitude 3.05.  Every two Bortle skies is a full magnitude.  That would put Manhattan at about a Bortle 11.  Last year, when I was living in Glendale (technically, I was just over the border into Denver, but again, close enough), about 4 1/2 miles from downtown Denver, I could almost get to 4th magnitude.  Almost Bortle 9.  This year, living in Aurora, 10 miles from downtown, I’m just barely past Bortle 9.

So, let me finally admit something here in this light-polluted astronomy blog after almost four years of writing it:  Observing from light-polluted skies sucksSUUUUUUUUCKS.  Okay, so maybe that’s a bit much.  First of all, I’m “just” talking about observing DSOs.  Second, it doesn’t really suck until you’ve gone out to darker skies and you know what you’re missing.  Kinda like going out west and skiing in powder, and then coming back east to ski on ice.  Light pollution makes things invisible, almost invisible, or washes them out.  I didn’t observe 40 Messiers and about 100 NGCs in two years of observing from Manhattan – I just looked at them.  Most of them, anyway.

My “local dark sky spot“, about 8 miles east of me, about 3 miles out away from the eastern edge of Aurora and out into nothingness, and 18 miles from downtown, puts Denver’s light dome firmly in the western half of the sky, and behind me as I observe in the east.  The Milky Way becomes barely visible overhead.  I can only get to a NELM of about magnitude 4.7, and that’s just for stars near the zenith; between Bortle 8 and Bortle 7.  However, that’s a very welcome improvement for only a 15-minute drive.

bortle scale

And taking that little 15-minute drive is like adding almost 2 inches onto my 5-inch Mak, or like going from my C9.25 to a C13 (which doesn’t exist).  For the cost of less than a gallon of gas, for about two bucks, I can add a coupla inches onto the size of my scopes.

Or, I can take an hour’s drive and go out to my club’s dark sky site.  From there, almost 60 miles away from downtown, I’m seeing stars deep into 5th magnitude, and the Milky Way stretches from horizon to horizon.  That’s Bortle 5.  (Or, as I discussed before, maybe it’s Bortle 4, because I’m nearsighted.  That means the faintest stars aren’t pinpoints, but blurred.  Spreading out their light makes them invisible to me, as they blend into the background.)  That’s about 1 1/2 magnitudes deeper than I get from where I live.  That much of a jump is equivalent to doubling the size of my scopes – a 10-inch Mak, an 18-inch SCT.

The cost to double the size of my scope:  4 gallons of gas, less than $12.  

I’m sure you get my point here – get thee to dark skies!

Now, this isn’t completely accurate.  One of the other benefits of actually having increased aperture – really having a larger scope – is increased resolution = detail.  You don’t get that just by moving your scope out to a darker sky.  So, sure, for example, globs are brighter, but you still don’t fully resolve them into individual stars with a smaller aperture scope at a darker site.  Still, they definitely do look better than they do in the city.

Also, and most importantly, I’m not suggesting that it’s possible for everyone reading this post to be able to just pick up and get out to a dark site, just like that.  In fact, one of the many reasons I moved out west here was precisely to be able to do that – to get to really dark skies without much hassle.  I know that many people living in eastern metropolises – myself included – don’t even have access to cars.

Many more people can’t get away from family or work commitments.   Or, because of the size of and development of the city they live in, they can’t drive an hour, or even 15 minutes, and be able to improve the darkness of their skies much.  Driving an hour from Manhattan gets you less than 40 miles away (traffic lights, traffic, reduced speed limits, no direct route).  Even when you drive an hour out of the City, you’re still well underneath the City’s ginormous light dome.  You need to drive almost two hours, each way, to finally get to some significantly darker skies.

So, yeah, not only do I get it, I also know it from first hand experience – darker skies aren’t easily accessible to everyone for a variety of reasons.  But if you can just get maybe 30 or 50 miles outside of where you live, you can probably improve your Bortle rating by about 2 notches.  That’s a full magnitude deeper.  That is the same as adding a shade more than 50% more aperture onto your scope – going from a 5 to an 8, or a 6 to a 10, or a 10 to a 16.  How do you know where those darker skies are, where you should go?  Well, by checking the dark sky map, of course!  The colors beneath each Bortle sky on the above chart roughly correspond to the equivalent colors on the dark sky map.

What you can do instead is to make observing part of some other trip.  Going out to a park to go picnicking, or hiking, or camping?  Bring the scope.  Going fishing or hunting?  Bring the scope.  Going on a road trip?  Bring the scope.  Going out to a National Park?  Bring the scope.

BRING THE SCOPE!!!  You’ll be glad you did.  It’s like breathing new life into it.  This is why my 5-inch Mak continues to perform for me – because it’s so tiny, it’s very easy to take places.

And this is why I AM BEYOND PSYCHED to be going to Okie-Tex in just less than 10 days.  It’s truly held in the middle of nowhere – Bortle 1 skies, baby!  That means the NELM will be deep into 7th magnitude.  Compared to using my scope at the local park half a mile from my house, my eyes, and more importantly, my C9.25, will be able to go fully THREE MAGNITUDES DEEPER.  That’s practically like leaving your house with a C9.25 and setting up with a THIRTY-SEVEN INCH scope!  WHOA!!!


3 thoughts on “September 28, 2018: The Bortle Scale, or Why Gas is Your Friend

    1. FYI, this isn’t my picture. I got it from a Google search. The credit in the upper right corner is from “”.


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