Saturday, December 05, 2020

beware the airmass

The "airmass" value in astronomy is an indication of the amount of atmosphere or air you're looking through when you view objects in outer space.

If you look straight up, you're looking through the least amount of air possible. Looking to the the zenith means the atmosphere is at its thinnest. 

Darned air. We need it and yet it works against astronomers in many cases. It's filled with moisture and particulates. Sometimes that moisture is white and fluffy or crystallises. Our air is arranged in moving layers which are at different temperatures. Anyway, it is what it is. Until amateurs can get their own private telescopes parked in low Earth orbit, we're stuck with atmospheric effects and weather.

Still, you can improve the view or image by looking only straight up. Well, that's not practical! But as much as possible, you want to avoid viewing celestial targets at low elevations.

The problem is, that's where we often find planets and comets! At sunrise and sunset, some of the elusive planets, the inner planets like Mercury and Venus, or bright comets, are by their nature close to the Sun so then low down near the horizon when we want to look at them. Often astronomers describe the telescopic views of planets low in the sky as "swimming." It's like you're underwater. In some ways, we are.

And that means, you looking through more air or airmasses.

relative airmass graph

This graph shows the relative airmasses as we aim closer and closer to the horizon. The red line shows that the lower a target is, the thicker the atmosphere and the higher the airmass value. You can see clearly it spikes dramatically around 10 or 9 degrees.

The plot of the relative airmass was generated using the "plane-parallel atmosphere" formula which is a very simplified model where the value is equal to the secant of the zenith angle or distance (i.e. the angle from the zenith). In the graph, I inverted the zenith angle so we see the angle above a perfect horizon.

I first learned about the importance of airmass when I became a SkyTools user. This software emphasises the two airmass threshold. The author of the powerful planning application encourages observers to view objects above or higher than two atmospheres. A number of years ago I had heard that imagers prefer to photograph objects above 45 degrees altitude or elevation.

I added the blue vertical and orange horizontal lines for the 2 airmass limit. You can see that occurs at 30° elevation. I added the green line for the 45° limit where the airmass is around 1.4.

And what, in the end, does all this mean? Looking through more air or atmosphere has obvious detrimental effects. 

If all the additional air is dirty or turbulent or filled with clouds or smoke, you get a poor quality view. You're down in "the muck." 

The atmospheric "seeing" (the steadiness of the air) will be worsened or amplified. 

The fact that you're looking through more air means things simply get dimmer. That's called atmospheric extinction. A magnitude -2 planet will effectively appear with a magnitude of +1; that's 15 times dimmer. 

And then there's the refractive effect of the atmosphere where shapes will get distorted. 

You've seen all this first-hand during colourful sunrises or sunsets where the reddened Sun that you can look at directly without melting your eyeballs appears shaped like an oval or kidney or jelly-bean.

I raise all this now because we're all going to be trying to look at Jupiter and Saturn in their grand conjunction over the next few weeks and they are going to be low. As I showed on 27 Nov, the gas giants will be 17° or lower, at 6:00 PM EST. In fact, on the "big day" on 21 Dec, at 6 PM, they will be 9 degrees up. Check my graph: that's 6 atmospheres. Yuck-o. 

Again, this is not meant to discourage you from looking or for imaging. What I'm trying to say is that the view might be, will likely be, poor. So, go in eyes open. Go in expecting bad seeing, turbulence, distortion, red colouring, etc. In turn, that tells us to start early, pick up the planets as high above the horizon as you can. It tells us to try often. Every clear night, you must try.

Happy viewing!

You can learn more about airmass at the amazing wikipedia.

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