 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 a while, you might start wondering, well, just how dark is this? 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 gonna run through what I think are some of the pros and the cons of each method for gauging sky quality. And the first method for gauging, which I've shown in several videos now, is using a website like lightpollutionmap.info, which shows you satellite data of how bright the earth is from space, looking down. I like this method because it's easy. I can just say, go to this website. We don't have enough to speak the same language. I can just go to the website, put in their location where they wanna shoot and tell me what color that location is. And then I have some frame of reference, like an orange zone or a green zone. What I don't like about it is, I think people read a little bit too much into it sometimes. 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 on any location, they'll even calculate the sky quality in magnitude per arcs second squared and the Bortl scale, both of which we're gonna talk about. But if I get anything across in this video, I want you to know that those are guesses and they're not super accurate. I find in cases where it's really obvious, like downtown Boston, like I'm showing now, it's gonna be Bortl nine. So sure, the website can predict that. But I've seen it guess something, in other cases, like a Bortl four when it's actually a Bortl six. And I've also seen it the other way around, like getting like two Bortl classes off. I'm jumping ahead of myself a little bit, I realized, but let's go ahead and just jump in to the Bortl scale because it's probably the one that people use the most, even more than telling me the colors on a map. They'll say, I'm in a Bortl four. But I don't think that people are actually using the method in the article when they're telling me that. The Bortl scale is a method devised by a legendary amateur astronomer and a comet observer, John Bortl, who publishes 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. And I encourage everyone interested in this topic to go read this article if you never have. It's not very long, it'll just take a few minutes. You can even pause the video right now and come back. Basically what the Bortl 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're an astrophotographer, is a good idea. 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 this method is to actually measure the brightness of the sky with an accurate meter. The most popular meters are made by a company called Unihidron and they have a bunch of different models at different price points. They have handheld models you can just push and it tells you I have this Unihidron SQM-LU which you connect to a computer by USB to make readings. What it does is it measures the sky brightness in something called magnitude per arc second squared or MPSS for short. To confuse things a little bit, you'll often see people say SQM equals 20.5. SQM in this case stands for Sky Quality Meter, the device they're using, but for some reason this has become sort of interchangeable with magnitude per arc second squared, but I prefer MPSS because that's what we're 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. Last thing about dark sky meters. I recently ordered a different one. This beauty, this is called TAS. It's from a Spanish nonprofit organization called Stars for All. It's a pricey little device, but I really enjoy it so it's worth it to me. What it can do is it measures all over the sky automatically and builds me a light pollution map right in the app on my phone for the whole sky. And so then I can compare each side to know, okay, this side is better for shooting to the west and the south, but this one is better for the north. I should note that I also have done this with my unihidron meter. It's just a lot more manual work of pointing it and then building the map myself. So this is really nice. All right, I think that's it. Next week I'm gonna be talking about horizontal versus equatorial coordinate systems and why we still use both for amateur astronomy. Until then, this has been Nico Carver from NebulaFotos.com. Clear skies.