Lately, the weather gods have been conspiring against me in terms of my not being able to get out to see. It’s been a good couple of weeks since I’ve been able to get out at all. The last couple of times I was out earlier in the month were as sort of a warmup for the Jupiter triple shadow transit that occurred on the night of January 23-24. On January 9 there was a double shadow transit. And I couldn’t see it. On January 10, the Great Red Spot was rotating into view at a good time for me to view it. No dice.
On both nights, the sky was clear, the stars were bright – well, as bright as they can get with this light pollution, anyway. As I discussed in this post, I was able to see Europa’s shadow transit quite distinctly on the face of Jupiter a few weeks ago in December. But since then, I haven’t been able to see jack on the surface of Jupiter. Oh, sure, the two belts are there, hints of mottling, hints of more going on towards the poles, but nothing sharp and crisp. I was plenty patient, observing for at least an hour each session, and before observing, I let my scope cool down for over an hour beforehand, and Jupiter was well above 30 degrees high in the sky on both occasions.
I first thought there might be something wrong with the scope, that maybe I really did need that Bahtinov mask to get a better focus. But checking Clear Dark Sky, I’ve found that I’ve been getting skunked because of poor seeing. Or maybe because of poor transparency. I always thought that clear, cold nights would have excellent viewing for astronomy. I figure when it’s 15 degrees on the ground, all the water vapor, and dust, and other schmutz would just freeze right out of the sky. Obviously not. And what’s the difference between transparency and seeing, anyway?
Well, transparency is easy – it’s what we would colloquially refer to as a clear, dark night. No clouds, of course, but no haze either. And low humidity, because the water vapor suspended in the air also works to affect what detail you can observe.
Seeing is a little different. Seeing is how stable or turbulent the air is, particularly, that column of air, miles thick, that you have to look up through the entire atmosphere to get to your intended astronomical target. Seeing – or rather lack of seeing – is the turbulence that causes stars to twinkle. On a hot day, you can see the air shimmer and roil above a blacktopped road. That roiling of the air is always present, even when it’s not hot, and even though you can’t see it – it’s still there to a greater or lesser extent. And of course, whatever is there is magnified by the telescope.
Seeing can be affected by your telescope’s optics not being cooled down to the ambient outside temperature. A thin layer of warmer air forms over the optics that is roiling compared with the cooler air immediately above it. Seeing can also be affected by the air immediately around you. If you’re observing off of a roof, the roof could be radiating heat, either heat that it stored from being in the sunlight all day, or heat rising up through the roof from the apartment immediately below it. Seeing could be affected by something a little further away than that, too – if you’re observing through the path of a chimney, or through the heat rising off of a building across the street.
But, as I learned in bringing this up on Cloudy Nights, seeing can also be disturbed by factors very high in the atmosphere – particularly, the jet stream, miles above our heads. On the nights I couldn’t see the transits, or the GRS, the jet stream was whipping along at 200mph above my head.
Almost paradoxically, excellent transparency might mean that there’s poor seeing and vice versa. When there’s not a cloud in the sky, that’s probably because the winds in the upper atmosphere – including those in the jet stream – have blown them all far, far away. When there’s a haze hanging above your head, a thin layer of clouds, or maybe even smog, then the opposite is true – there could be excellent seeing on that night, because the atmosphere is incredibly still.
So, the seeing has been bad. What to do? Well, there’s nothing you can do about the seeing, so instead of observing planets, observe something that isn’t affected by seeing – like open clusters. And the winter sky’s got a ton of them. And because the winter nights are so long, I can still observe clusters in Cygnus in January right after sunset, and continue long into the night.
But then there’s still the issue of transparency. Or, in other words, just having a clear, or relatively clear, sky at night so as to be able to go out and observe. And here I keep on getting skunked as well. Because the hourly weather forecasts at weather.com, which is from no less a source than The Weather Channel itself, is just plain awful. The hourly forecast predicts out up to 48 hours in advance. So, if it’s cloudy on Friday night at 8pm, I can check to see if Saturday night and Sunday night are going to be clear.
Their forecasting ability even for something as simple as this is terrible. A forecast that says that it will be clear the next night – just 24 hours in the future – more often that not seems to just degrade into “Mostly Cloudy” by the time you get there. If they can’t predict whether the sky will even be clear 24 hours from now, well, that’s rather lame. Throughout January, what have been supposed good nights for observing constantly descend into nothingness.
Finally, earlier in the week, I was able to get out and conduct a survey of winter’s many open clusters. Even though the waxing moon was getting in the way, there was still plenty to see. But the most important part of the weather is still just how friggin’ cold it is. I could only take it for a couple of hours before packing it in. But at least I finally get the ST-80 aligned with the Mak. Or semi-aligned, anyway. It’s always been the case that if I had something in view at low-power in the Mak, that I’d also have it in view in low-power, i.e, 12.5x, in the ST-80. Now I can bump the power up a bit in the ST-80 and still have it in view in both. I’ve decided that perfect alignment is near impossible, but pretty good alignment is still fun.