What is "Seeing" - a beginners Guide

[davidpretorius]

Hi All,

Why do the stars twinkle??? Well they dont really, it is actually the atmosphere moving around.

It may be quite nice to look at with the naked eye, but try using your telescope to view any planet when the stars are twinkling violently, then the amount of detail you will see will be very little.

So how do we try and "predict" what will be a good "seeing" night??

Firstly, before we look at the 4 factors that determine seeing, lets try and "rate" seeing.

This is a great link -> http://www.damianpeach.com/pickering.htm (courtesy Damian Peach)

Find yourself a brightish star and then focus on it.... can you compare the amount of distortions from the animations in the link and rate the seeing???

Secondly, here are the 4 factors that I use to try and determine it is worth getting the scope out?

1. Uncontrollable External factors...Jetstream winds (aprox 30,000 feet) --> http://weather.unisys.com/gfs/6panel...panel_aus.html

The 300 mb chart gives us an idea of what is happening at the Jetstream level. Pink / Purple / Dark Blue are good conditions.

2. Uncontrollable External factors...Surface winds -->
http://weather.unisys.com/gfs/6panel...panel_aus.html

The 1000mb chart gives us an idea of what is happening at the surface level. Ideally slow / zero surface winds are very good

3. Uncontrollable External factors...local topography, ie what hills / mountains etc are near you? What direction of wind is best for your local conditions? Have a look at the island example in Damian's article (http://www.damianpeach.com/seeing1.htm). For me in Launceston Tasmania, the ocean is closest to me in the north and east, east of me are mountains..........so air moving off bass straight in a northerly direction is best........south west of me are mountains and south of me is the bulk of the land mass and wind from that direction never gives good seeing.

4. Controllable local factors...
- is your scope near a heat source ie asphalt or near a brick building?, are you looking over hot tin roofs?
- is your main mirror at ambient temperature? if it is at ambient temperature, do the temperatures fall drastically during the night. Remember a big chunk of glass can not loose heat as quick as the surrounding air. In a nutshell, you want your mirror within .5 degrees of the ambient temperature.


Good luck and practice makes perfect.......even if the conditions arent great, get out there, get a "feel" for the conditions. If it was a bad night, check out the Meteorological websites and try and work out why they were bad...................over time you will get very good at predicting what conditions will be like.

Further Reading

Planetary Imaging Everything on this website ---> http://www.damianpeach.com/ ...he is the best amateur in the world for planetary imaging.

Active Cooling of mirrors.....Newtonians have been cooled ( http://www.acquerra.com.au/astro/cooling/index.html and http://southcelestialpole.org.au/blogs/SDM.php and http://southcelestialpole.org.au/blo...d=21&blogid=11 ) ........and now SCT's have been actively cooled ( http://southcelestialpole.org.au/howto/coolingC14.php )

And I would really recommend star testing to see if your telescope is collimated ie aligned (http://legault.club.fr/collim.html )

[leon]

You have found some good links there David, thank you.

Leon

[DJVege]

Thanks for the lesson! Great read and great info. As soon as I see some CLEAR sky, I'll give the seeing star test a go.

[Ingo]

Good information. I think Iceman 1up's him though

[davidpretorius]

Originally Posted by Ingo

Good information. I think Iceman 1up's him though

Damian Peach's images or my beginners report? or Paul's and Anthony's cooling research?

[caleb]

most nights i dont see any twinkle, except when i have a look at a double star, i can see heaps.
im new to astronomy and im un-sure if this is normal, it can be blue, orange and red but no other stars do it. is it just this star?

[ballaratdragons (Ken)]

Originally Posted by caleb

most nights i dont see any twinkle, except when i have a look at a double star, i can see heaps.
im new to astronomy and im un-sure if this is normal, it can be blue, orange and red but no other stars do it. is it just this star?

Caleb,

if a star is turning blue, orange and red, that is the poor seeing. The stars that turn those colours are too close to the horizon or are sitting in a fog layer. If you try observing through a scope down at that level (when they change colour) it will look ghastly through a scope.

All stars twinkle even on a good night, but just a lot less on better nights. To get absolutely no twinkle at all, you would have to take away all the atmosphere.
Planets don't twinkle (except way down on the horizon or thru fog) which helps make it easy to find them among the stars when you are learning.