If you’ve watched some of my other videos, you’ll know that I emphasize the importance
of finding as dark a sky as you can for astrophotography, but when you’ve been doing this awhile,
You might start wondering.
How can I compare the sky quality at one location to another?
Well there are three main methods, and in this five minute friday, I’m going to run
through some of the pros and cons of these methods for gauging sky quality
The first method for gauging, which i’ve shown in several videos now is using a website
like light pollution map dot info which shows you satellite data of how bright the earth
is from space looking down.
I like this method because it’s easy, and it’s universal, you don’t have to speak
the same language as someone, you can just point them to the website and they can say
what color they are and you have some frame of reference.
What I don’t like about it, is I think people read too much into it.
The surveys from satellites are measuring the brightness of the earth looking down at
it.
The websites are then taking a best guess at how bright the sky will be for an observer
looking up, and if you click they even calculate the sky quality in magnitude per arcsecond
squared and the Bortle scale.
If I get anything across in this video, it will be take these guesses with a grain of
salt.
I find that in cases where it’s really obvious, like Downtown Boston is going to be Bortle
9, sure the website can predict that.
But I’ve seen it guess something will be a Borlte 4 when it’s actually a Bortle 6
and the other way around as well.
But I’m jumping ahead of myself.
Let’s actually jump in to the Bortle Scale because it’s probably the one people use
the most even more than the colors on this map, but I wonder how many people actually
measure their skies with the actual method in the article.
The Bortle Scale is a method devised by a legendary amateur astronomer named John Bortle
who published his scale 20 years ago now in this February 2001 issue of Sky And Telescope.
The article is also available online and I’ll put the link in the description.
I’d encourage anyone interested in this topic to go read the article if you never
have.
It’s not very long, it’ll just take a few minutes.
You could even pause the video and come back.
What the Bortle Scale is, is it’s a way for a visual observer with no equipment other
than their eyes to gauge the sky quality at any location.
And that’s a very useful thing.
It does require a little bit of night sky knowledge to use this scale, but I think picking
up that bit of observing know-how even if you are an astrophotographer is a good idea.
I also love that Bortle includes other little hints throughout the scale like in Bortle
7 skies the clouds will be brilliantly lit by the light pollution while in Bortle 4 skies
the clouds will be dark overhead.
Or another favorite is in Bortle 8 skies you can read newspaper headlines without any additional
light.
So, I like the Bortle Scale, but what are the cons of this Method?
Well I find its a little uneven I think Bortle 4 is a pretty big bucket, it describes, in
my experience, most rural dark sky locations on the east coast, while it’s much harder
for me to distinguish between Bortle 5 and 6 for instance, at least with just what he
wrote in the article.
That said, I hope everyone watching this actually reads the article, maybe bookmark it on your
phone and try it out, because it can be fun, and of course it’s very useful to have a
method anyone can use to compare locations.
Okay the last method, I’ve saved for last because while it’s by far the most accurate
method, It typically involves buying some equipment.
And that is to actually measure the brightness of the sky with a meter.
The most popular meters are made by Unihedron, and they have a bunch of different models
at different price points.
I have the Unihedron SQM-LU which you connect to a computer to make readings.
This measures the sky brightness in something called magnitude per arc second squared or
mpss for short, but to confuse things even more, you will often see people say SQM = 20.5,
Sqm in this case stands for sky quality meter, the device they are using, but for some reason
this has become sort of interchangeable with MPSS, but I prefer mpss as that’s what you
are actually measuring with these numbers.
A city sky is usually somewhere between 17 and 18 mpss while the darkest sky on earth
would be 22.0.
For me, here on the east coast anything darker than a 20.0 straight up at zenith I’d call
a dark site, but if you live in the Western US, I doubt you’d consider that a dark site,
so it’s all relative.
Last thing about dark sky meters, I recently special ordered this robotic, app-controlled
one called the TASS from a Spanish non-profit organization calle Stars4all.
This is a pricey little device, but I really enjoy it so it’s worth it to me.
What it can do is, is it measures all over the sky automatically and builds me a light
pollution map for the whole sky at each site I go to.
So I can quickly compare and know okay this site is better if I’m shooting to the west
and south, but this one is better for the north.
I should used to do this with my Unihedron meter too.
It was just a lot more manual work, so this new device is a huge time saver.
Okay, I fear I may be over 5 minutes, so let’s stop here.
Next week I’ll talk about horizontal vs. equatorial coordinate systems, and why we
use both in amateur astronomy.
Till then this has been Nico Carver from Nebula photos dot com.
Clear skies