Seeing v's Transparency

[Stu]

Our seeing (in Melb) is usually pretty good because we don't get the jet stream and it rains about one a day for 5mins...here and there, then gets sunny, then cloudy, then hot, then cold, then dark, then clear, then rainy, then clear, then freezing, then hot again...no wait, make that cold...

[ballaratdragons (Ken)]

LOL! That's Melbourne exactly.

Reminds me of that old Melbourne saying "if you don't like the weather, wait 5 minutes".

Not much different to up here in the hills except for the summer months when it's hot, hot & hot all day and hot & muggy all night.

[cahullian]

yeh Ken but how is the seeing and transparency on those nights??? lol

[RapidEye]

LOL You guys are overthinking it =-)

Go back to the begining of the thread: the transperancy and seeing (steadiness) are seperate measurements of the sky quality. Sort of like temperature and humidity of the air outside. They are related, but are independant measurements.

If it is cloudy, yes transperancy is nil; and seeing is irrelevant because there is no way to determine it 'cause you can't see past the bottom of the clouds.

Here in the SE US, it isn't uncommon to have very poor tansperancy from all of the humidity; however, usually when the air is like that, it tends to be very stable. I've had some great planetary viewing sessions when you could hardly see the planets naked eye from all of the gunk - but they are bright enough that it punched through the haze and the steady air made for very stable disks.

When transperancy is poor - DSO's viewing suffers
When seeing (steadiness) is poor - Planetary and Doubles viewing suffers
When both are poor - watch a good ball game on TV instead.

The real irony of it is - typically the conditions that make for great transperancy usually make for poor seeing and vice versa. Having a night of exceptional transperancy AND seeing is pretty uncommon, and if you are lucky enough to get that when there is no moon, don't go to bed: you've just hit the astronomy version of the Lottory At least that is the case here in the US...

[ving (David)]

Originally Posted by janoskiss

seeing = quantity
transparency = quality
(of light)



quantity = 0 implies quality is irrelevant.
But with quantity > 0, quality becomes important.

I'd have said the other way around...

transparency = quantity = cloud cover, dust, pollution, etc which will blot stars out

seeing = quality = atmospheric conditions that determine how well you can see the objects
?
no?
I too am making it up
makes sence tho. except bad seeing will also make fainter objs hard to see.

[janoskiss (Steve H)]

Of course you're right David. That's what I meant. Which is why I wrote the opposite. Makes perfect sense if you forget about logic.

[rumples riot]

Ok heres my take on both.

Seeing should actually be measure on a 1-10 scale. Seeing is most commonly vital for planetary work and close doubles. When seeing is at 1/10 little if any detail can be seen of any planet. In fact the image will dance around like it is in a washing machine. Little if any detail can be seen at all. When seeing is very good say from 7-10/10 the image of the planet will not move, it will be steady and lots of detail can be seen. This is even when transparency is around 5.

As to transparency this relates more to observations made of DSO's. Typically high cloud and dust particles will affect transparency. Fine detail of nebula and galaxies will be lost due to poor tranparency. Scale again is 1-10. You can still have good seeing even when transparency is very low. Light pollution will also be affected by transparency. In fact they are inter-related. If the sky has lots of dust particles in it, and you live in a city then the light pollution have a greater effect. The light bounces off all the crystals of dust or cloud and increases the light pollution. The best time for good transparency is usually after a big rain period. The sky will be crystal clear because the atmosphere has had a wash. This does not mean that seeing will be best either, that is because upper atmosphere winds can and do effect seeing.

A good example of what I mean is this. I live on the sound side of Adelaide. Most nights the light from the city will effect any viewing to the north. Being a city there is a lot of dust in the air, when the dust is really bad or there is high sirius cloud the transparency will be lousy typically around 4/10 and this will extend overhead and to the south. However even on nights like this I can sometimes get great seeing for planetary imaging. This is because the jet stream is calm overhead and local conditions are very clam. In addition even on nights when the sky is crytal clear the jet stream can make the sky twinkle and planetary work impossible.

So seeing is related to the air currents and transparency is related to things like dust, cloud and pollution. The two are not related to each other and are caused by different influences.

Hope that makes sense.

[ballaratdragons (Ken)]

From that Paul, a way I can remember which is which is to play 'word association' games in my head.

I will look up at the sky next time and if I am 'seeing' the sky shiver, distort or move in any way I will rate the 'seeing'.

I will then look at whether I am looking through 'Transparent', Translucent or Opaque air and rate the 'Transparency'.

Makes sense to me that way!

I will look at seeing if there is any movement and also find out if the sky is transparent enough to see through!