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What's limiting my vision?


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Hi all, 

Very basic beginner questions here but need help understanding...

Was viewing the moon tonight and it was very wobbly. Like heat haze on a hot tarmac road. 

What precisely is the issue here?

I'm guessing it's the seeing, which according to telescopius was "bad". The wind was also "bad", 20mph. Are the seeing and the wind directly linked?

Equipment is 150mm f8 skywatcher dobsonian, kept outside so at ambient temperature. 

I've attached the stargazing weather report and a video of the moon where you can see the shimmering taken through an 18mm 60deg eyepiece, taken at about 10pm so the moon was relatively low in the sky. 

I found looking at stars it wasn't so obvious that conditions were poor, although they didn't seem great. 

Many thanks!

Rob

Screenshot_20200229-230214_Chrome.jpg

Edited by RobH2020
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Lots of factors can give an unsteady image.....

Target at a low altitude

Scope not cooled properly - a scope stored outside in a garage/shed will still need a little time to reach true ambient temperature.

Observing over the roofs of houses giving off heat

Scope set up on concrete giving off heat - grass is best

High wind speeds

The Jet stream 

Your video is quite good given it was windy 👍

Edited by dweller25
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Hi Rob, I had exactly the same issue last night observing in Hertford, so not too far from you. 
conditions were far from ideal and the wind was very high, but with all the poor weather and continuous cloud cover we’ve had recently, just seeing a few points of light in the sky was enough to give it a go.

Your video was a good representation of what I was seeing as well.

Clear skies, Bryan

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Yep, as said, it can be caused by a few things.

Firstly tube currents due to the scope not being cooled properly but it sounds like this was not the case for your scope.

Secondly local low level turbulence caused by convection currents from warm buildings, concrete and tarmac that have heated up through the day and cool off during the evening, particularly on cold clear nights. Central heating flumes also cause havoc with the seeing. It's not always possible but try to avoid observing over houses, and set up on grass if you can. I have to look over my house to see south from my garden and get two noticeable better chances at the planets each night as they pass between the neighbours houses and mine. Normally these currents reduce after everything has cooled, so the early hours of the morning can be the best time to be out.

Last thing is the higher atmosphere generally, turbulent air can be caused by storms and weather fronts, but probably worst is the high level jetstream. These are the very fast winds up to 200mph or more in the upper atmosphere which run in an ever changing ribbon across the Atlantic, sometimes passing directly overhead. When this is the case, observing the moon and planets can be like looking at them under a stream of running water, no detail to be seen. Best to look at low power targets on nights like this.

So, the first two you can do something about, the last one is about checking the jetstream forecast, and observing as often and for as long as possible to catch those times when everything comes together. Even minute by minute seeing can change, so time spent at the eyepiece is what counts.

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Hi Rob,

When low in the sky, the telescope has to look through a greater depth of atmosphere, and unless the atmosphere is steady its going to struggle to focus the image. You may also have local heat sources or sources of turbulence such as motorways and town centres to consider. To be honest, I think your video is very good and it shows your scope is good too. Try observing when the moon is above 30° high, as below this any telescope is going to have a battle against the atmosphere unless its unusually steady. The best nights are yet to come for lunar observing as the spring moon will be high in the sky right through until May.

Edited by mikeDnight
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Thanks all!

Great you liked the video - it was just me holding my four year old phone camera up to the eyepiece! (Samsung galaxy s7).

The moon for me was right above some people's houses, so that could very well have been the primary issue. 

Interesting about the jet stream, i didn't realise that was the main thing for the high level seeing ability. 

Are there times of year when this is better than others? Feels like seeing has been rated bad by telescopia for my area for most of the year so far!

What's the issue with wind at ground level or lower than the jet stream anyway. Does that disturb the air significantly is that more about blowing your equipment around?

Cheers!

Rob

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2 hours ago, RobH2020 said:

Interesting about the jet stream, i didn't realise that was the main thing for the high level seeing ability. 

Are there times of year when this is better than others? Feels like seeing has been rated bad by telescopia for my area for most of the year so far!

I think because it is such high speed vs the winds at lower level it does have a significant impact. You can check the forecast position on various sites, one linked to here, and when it is not over head you will often read of successful planetary, lunar or double star sessions on SGL!

https://www.metcheck.com/WEATHER/jetstream.asp

You can see the impact of weather fronts too, often if there are clouds pushing through on the wind you see the seeing go wobbly then in the clear patches it stabilises again. I see that when solar observing quite alot.

That's not to mention the havoc that contrails play with both seeing and transparency!

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Well it's the absolute opposite today... no wobbly moon at all!! Absolute beaut of an evening.

Quite a bit of twinkle going on with Rigel i noticed though. Could see red and blue in it when moving the view near it. It almost looked like a double star - i know it is, but i thought it would be too hard to split in a small scope...

 

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7 hours ago, RobH2020 said:

Well it's the absolute opposite today... no wobbly moon at all!! Absolute beaut of an evening.

Quite a bit of twinkle going on with Rigel i noticed though. Could see red and blue in it when moving the view near it. It almost looked like a double star - i know it is, but i thought it would be too hard to split in a small scope...

 

Rigel is quite possible on a smallish scope, yours is more than capable of showing it. The secondary is a tiny pin point of light very close to it. Higher mag helps to separate them. Is that what you saw?

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At my dark site in SE France I find that poor seeing and wind are consistently linked. Indeed it takes more than a day of calm weather for the seeing to be restored. There are days of low perceptible wind when the seeing is also poor but that will be from high altitude winds.

Olly

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