What Makes Good Seeing Conditions?

[Mowbs]

Hello.

Last night was my second proper night outside with my telescope and I was out for about 9 hours. I had decided to wait for Saturn but I gave up around 03:30 (I have some trees to my south so would have to wait until it was quite well above the horizon, mars had just come into view)

Anyway, the postman delivered my much anticipated Celestron X Cel eyepieces yesterday and I was keen to get outside and try them out. (7mm, 9mm and 18mm)

I had a great evening / night / morning and felt like I was getting to grips with setting up the GoTo star alignment, I was close enough to have Andromeda in my EP with a search. My night was mostly taken up with observing Jupiter, Orion Nebula and Andromeda. I'm still finding them mesmerising.

I decided to hunt for some more DSO's but I couldn't find a thing, I have very little light pollution where I am so that shouldn't be too much of an issue, but there is a bit of a glow on the horizon from Aberdeen which is around 20 miles away. I noticed after a while Jupiter looked like it was streaming with water (kind of like a heat haze) that really affected my view of it. I also noticed I couldn't see Jupiters moons with my 8x42 binoculars and I can often count 4 with averted vision.

I have read that Andromeda is 6 times the size of the moon in the sky, I haven't had any problem getting all I can see into my smallest EP, am I missing a lot of it's size because of the conditions?

So my questions are..... Can you tell when it's going to be a good night for seeing or is it a case of looking through the scope before you have any idea, what are the things that make good seeing? I thought a cold frosty night would have been the ideal but it was pretty cold last night, I can tell you! I couldn't really see the stars twinkling like I sometimes can.

And, how often have you experienced really good seeing, Is this a once in a year type thing, or once in your astronomy career?

Is this most likely the reason I couldn't see any other DSO's? I found Andromeda hard to find at first but now I can find it with the binoculars before pointing the scope at it, could it just be that they are there and I'm not seeing them?

And finally, any suggestions of any DSO's that you would recommend I hunt for first?

Thanks

[Twiggles]

From what I've found, conditions can change throughout the night. From what I've read on these forums it can depend on position of an object in the sky and how much atmosphere you are looking through, high altitude clouds, jet stream position...all sorts! As for finding DSO's, it depends what you are looking for I suppose. I managed to find the ring nebula last summer easy enough. Often I just take a look on stellarium before I go out, using it to work out what should be visible and where to find it, even working out a rough star hop to get to them. Don't forget that some DSO's are quite faint, so you could be staring right at it but if you have moved to it from a brighter object (moon or jupiter for example) then your eyes may need more time to pick it out.

We went to our local astro group last night (and joined!) and even though its not the darkest site I've viewed from I found the conditions quite good last night. Orions nebula looked fantastic and andromeda looked the best I've seen using the stock 25mm eyepiece. We also enjoy finding clusters so pleiades, hyades and the beehive cluster are common targets for us.

[acey]

"Seeing" is the steadiness of the air (which limits the smallest features you can resolve). "Transparency" is the clarity of the air (which limits the faintest things you can see). "Sky brightness" is mainly determined by the extent of light pollution, and also limits the faintest things you can see.

Good seeing is important for planetary views, good transparency is important for DSOs (e.g. M31). A hazy night has poor transparency but may have good seeing, so would be good for planets. A crystal clear night has good transparency (good for DSOs) but the air may be very turbulent (poor for planets).

Poor transparency (hazy air) not only blocks light from faint DSOs but also reflects ground light, giving increased sky brightness, and making the view even poorer. If you can see an orangey scum across large swathes of the sky then you can expect poor DSO views. On the other hand, you can sometimes have partly cloudy skies with excellent transparency in the clear patches.

Conditions can change in the course of a night - it can be very clear at sunset, then a haze forms in the still air as the temperature drops. You can have a heavy downpour followed by wonderfully clear and transparent sky as the front moves away. Cold and frosty nights can be great for DSO viewing - but not if mist forms. On a breezy night you won't get mist, but there could still be high, thin cloud. Even if the forecast looks bad, you can have brief periods of excellent transparency, and nights with a good forecast can turn out cloudy. Sometimes the satellite imagery is enough to predict how the night will turn out, but often it isn't.

The way to judge transparency is to check your limiting magnitude, i.e. what are the faintest stars you can see. Ursa Minor is a useful benchmark for this. Also you want the darkest sky you can find - preferably one where the Milky Way is visible to the naked eye. If you can't see our own galaxy then don't expect to see much of our nearest neighbour M31 -you'll only see its brightest central part, a small fuzzy blob. Best DSOs to start with are the Messiers - any that are high enough in your sky. Objects are always at their highest when due south. A planisphere is the simplest way to check what's up there at a given time.

Some of the best and easiest Messiers (when well placed): M42, M31, M13, M57, M81/82

[Astro Imp]

An excellent post by acey.

You mention what to look for, in addition to those mentioned by acey try:-   M36, M37, M38 open clusters in Auriga, M35 another open cluster in Gemini, if you want some double stars try Castor (needs good seeing), the belt stars in Orion, Mizar/Alcor in Ursa Major(easy).

HTH, and good luck.

[Mowbs]

That's all fascinating and valuable information, thanks Acey.

That just made me set the scope up outside again, just to see how the conditions are tonight! We've had a lot of sleet / snow / rain today.

If I'm still out there when Mars comes up again I'll hold you personally responsible Acey.

Thanks for the suggestions of Messiers too, I've got Stellarium open now while the scope cools.

[Mowbs]

That was a surprising difference from last night, M42 was much much clearer, in fact I could have done with a wider FOV to get the full contrast against the sky. I have a 82 degree 24mm MaxVision on order, I'm looking forward to getting out with that.

I didn't find much difference in M31.

Interestingly though, when an aeroplane passed above me I could see foggy mist surrounding it light up by its own lights, there must have been a misty haze although I couldn't see it and the stars looked clear.

Unfortunately I've got stuff to do tomorrow so I didn't have time for a search for other DSO's.

Lots and lots to learn, but the experience you all share on SGL saves a huge amount of trial and error.

Thanks again

[CumbrianGadgey]

You don't say where you are pointing the scope. If you are aiming it over your house, for example, that can make a huge difference because of the 'heat haze' rising from the walls and roof.

I noticed last night that I saw the shadow of Ganymede transiting Jupiter perfectly earlier on but when I had to aim over the house it simply disappeared.

Is this a possibility? Also, seeing conditions are affected not only by the sky but where you put the scope. It's better in winter with the scope over flags or concrete but it's still better if you set up on grass.

[AndyWB]

An excellent post by acey.

You mention what to look for, in addition to those mentioned by acey try:-   M36, M37, M38 open clusters in Auriga, M35 another open cluster in Gemini, if you want some double stars try Castor (needs good seeing), the belt stars in Orion, Mizar/Alcor in Ursa Major(easy).

HTH, and good luck.

On the double stars front, in Orion my favourite is Sigma Orionis (A lovely system with 4 components, with Struve 761 nearby) - so far, anyway!

[Mowbs]

Yeah, good point. I wasn't pointing over my roof when It became a bit hazy last night but I will have to bear that in mind, I only have to go over the house if I point north but with my wafer thin loft insulation and massive energy bills I'm pretty surprised i can't see the heat haze.

I am setting up on my tarred driveway though, if I go onto the grass I have more obstruction from trees. I have a burning ambition to build a observatory tree house though, maybe I should get the plans drawn up before summer.

[narrowbandpaul]

You will often find excellent transparency associated with low pressure and especially with cold fronts. The lifting of the lower layers removes a lot of the pollutants and disperses them high far and wide. In summer during high pressure the descending air collects and traps pollutants in the lower levels making for very hazy days. A good test for transparency is to look at a distant object during the day. If it has high clarity the transparency will be good. If you can't see it it's bad.

Seeing is caused by temp and pressure fluctuations in the atmosphere. This causes the refractive index of air to change on timescales of 10s of milliseconds up to a few seconds over patches a few cm across. Local affects count for much. Avoid looking over houses as others have mentioned. The heat given off will change the refractive index and disturb the path the photons travel. Concrete after a hot day is bad too as it retains heat for a while and then radiated it at night. Even your scope will have a residual heat if recently taken from indoors. You have control over a few things.

In terms of the atmosphere, ideally you want little change in wind speed with altitude. Look at aviation forecasts for the winds aloft. The met office have a free subscription for general Aviation weather forecasting. They have a spot wind chart from surface to 24000 for the UK and Europe.

To have such little change in wind speed you need high pressure where the isobars are widely spaced. This high pressure may trap pollutants though. Local conditions will determine the transparency. It depends in how high you are and how much crud is present in your local area.

Low pressure is associated with tightly spaced isobars and generally high wind speeds. This doesn't lend itself to good seeing. Position of the "jet stream" also would be something to avoid. A jet stream is any wind over 60kts and it is generated BY the polar front depressions that hit the UK every couple of days. The weatherman never explains this correctly. The met office also forecast the jet stream position.

So good seeing is normally encountered with high pressure and good transparency normally with low pressure or cold fronts. As with anything with so many variables only looking at the twinkling will tell you.

Paul

[Astro Imp]

On the double stars front, in Orion my favourite is Sigma Orionis (A lovely system with 4 components, with Struve 761 nearby) - so far, anyway!

Agree a fine sight on a good night.

[glappkaeft]

Good seeing is important for planetary views, good transparency is important for DSOs (e.g. M31). A hazy night has poor transparency but may have good seeing, so would be good for planets. A crystal clear night has good transparency (good for DSOs) but the air may be very turbulent (poor for planets).

 

Poor transparency (hazy air) not only blocks light from faint DSOs but also reflects ground light, giving increased sky brightness, and making the view even poorer. If you can see an orangey scum across large swathes of the sky then you can expect poor DSO views. On the other hand, you can sometimes have partly cloudy skies with excellent transparency in the clear patches.

 

Conditions can change in the course of a night - it can be very clear at sunset, then a haze forms in the still air as the temperature drops.

+1. This is the truth...