Twinkling Stars - good or bad seeing??

[RichieJarvis]

Hi All,

I've noticed in my time as an astronomer that I get much better images when the stars are twinkling. I thought that twinkling stars were caused by a turbulent atmosphere, which I thought made the seeing worse. Have I got this the wrong way round somehow?

All I know at the moment is that on clear nights when I can see the stars twinkling away, my FWHM values go down, I am better able to achieve focus, and my images are crisper - so whats going on??

Cheers,

Richie

[cygnusx1]

Read somwhere, twinkling means bad, no idea why, perhaps someone in the know could explain....??

[RobH]

Twinkling means bad for the reasons Richie says....turbulent atmosphere. This spreads your light around so you don't get pinpoint stars, the worse the turbulence/twinkling, the worse it is for observing, and especially for imaging.

I have absolutely no idea why you should find it better for imaging Richie...it makes no sense

Cheers

Rob

[timetraveller]

The actual scientific term for twinkling is called sintering and it is caused by the thermal gradient in the upper atmosphere forcing the precipitation of ice crystals which then descend downwards creating eddy currents as they do so.

So in theory sintering is bad news for seeing. so why you should be getting clearer imaging and sharper focus is a mystery to be perfectly honest

regards Pete

[timetraveller]

Richie I have given this observation of yours some greater thought and the only thing i can come up with is maybe the much colder air is firstly cooling down you camera and keeping noise to a minimum and maybe you have a slight abberation in the lenses of your scope which when cold are being corrected by minute changes in their shape under shrinkage also there should be far less low level atmospheric disturbance by thermals as the ground temperature is much lower now did you notice the same effect in the summer months to or is it mostly at this time of year?

dark skies and good seeing Pete

[Psychobilly]

Sintering or Scintillation...

[timetraveller]

I stand corrected scintillation apologies put it down to my age at 53 my powers of recall are diminishing rapidly.

Its all the fault of this little garden gnome inside my head who keeps pulling out the wires.

I have told him to go away and leave me alone but he said i would make him Gnomeless

Enough of this silliness if i choose the wrong term I am sorry guys

regards Pete

[timetraveller]

For anybody who is in the least bit interested Plasma Sintering is actually one of the new techniques used to fuse powder coatings of aluminium to glass to create the mirrors we use in our telescopes and other things The process actually combines the aluminium powder into the glass matrix giving them the ability to use ultra fine layers to produce highly polished surfaces with excellent reflective properties

regards Pete

[cygnusx1]

Now who said twinkle, winkle, little star... was just a nursery rhyme eh??

Plenty of technological meaning behind it!

[Tiny]

Scintillate as in the version of a well known nursery rhyme that starts...

Sintillate sintillate little orb...

Has someone got a link to the 10(?) levels of seeing and how to measure it using a star.

[James]

Last night was a strange one for me. It was very windy even in my most sheltered site, the stars were twinkling like crazy and it just didn't look that good. Resigned to failure I thought I'd have a look for the E & F stars in the Trapezium and much to my surprise the F star was clearly visible. I can only assume that twinkling stars aren't always a problem.

James

[michaelmorris]

Bare in mind that the weather conditions that tend to bring about atmospheric turbulence often also produce very good transparency (clear skies). Therefore, it could be that whilst you have poorer seeing your eyes are seeing fainter (not finer) detail.

Another explanation may lie in the fact that you are measuring seeing conditions by the amount of twinkling of stars. My understanding is that twinkling of stars visible to the naked eye is due to low frequency turbulence. Visual observing object blurring is most pronounced in high frequency oscillations in the light travelling to you then observers eye. The apparent frequency of oscillations created by convection cells in the atmosphere is proportional to how far they are from the telescope, those convection cells nearest the ground (and hence nearest the scope) will appear to have a low frequency, whilst the further away the convection cells are, the higher the apparent frequency of oscillation.

The net result is that if you are looking at star with turbulent air at ground level the star will appear to jump about, whilst if the turbulence is higher up in the atmospshere, the star will simply be blurred.

Now, there is a second issue here, the size of the convection cells and of your scope.

Most convection cells are only about 10cm to 20cm across. Therefore, with a small to medium-sized slow telecsope (<250mm aperature and f10 or above), the field of view is only about 1 convection cell wide. Thus, if there are convection cells at ground level, the star may jump about a lot but may be quite sharp if there is little turbulence higher in the atmosphere. However, with a large fast telescope (>250mm aperture <f10) the field of view may span signifcantly more than one ground level convection cell, causing both jumping and blurring.

Does this help or does it just make things more complicated?

[RichieJarvis]

Very very interesting!

So what your saying is that scintillation that the eye can see is good, whereas if you can't see any, then its high and bad, or not there at all?

Last night the scintillation was prominent on the bright stars, such as Sirius to the naked eye, but the images I was getting between clouds was pin-sharp.

[michaelmorris]

So what your saying is that scintillation that the eye can see is good, whereas if you can't see any, then its high and bad, or not there at all?

Yes and No!

Let me explain. Twinkling stars means the seeing is bad -full stop. If you can see twinkling with the naked eye then objects will appear either jumpy or most likely jumpy and blurred . This is because it its likely that turbulent conditions near ground level are associated with turbulent conditions at higher levels as well. However, just because stars are twinkling that doesn't automatically mean they will be blurred.

Of course, all of this only applies to visual observing or high frame rate webcam imaging. For normal astrophotography or low frame rate webcam imaging, any turbulence is usually bad news.

[David Galvin]

Scintillate as in the version of a well known nursery rhyme that starts...

Sintillate sintillate little orb...

Has someone got a link to the 10(?) levels of seeing and how to measure it using a star.

Is this one of use?

http://www.damianpeach.com/pickering.htm

[MartinB]

Often, very local factors have the greatest effect on poor seeing. The worst thing for seeing is an inadequately cooled scope. Surrounding obstructions such as trees, bushes and buildings will upset the air around the scope and this can be particularly bad on quite still evening. Wind, provided it isn't too turbulent around the scope generally greatly helps local seeing. Twinkling is caused by higher level turbulence which often isn't as big an issue as the ground level problems. Obviously, pointing at a twinkly star isn't going to be great but in some situations, especially when it is a bit breezy seeing can be pretty good high up in the sky.

Having said that, when I see twinkling short focal length imaging is order of the night

[Tim]

Everything last night pointed to Rubbish images on the way. The stars were very twinkly up to the zenith, and the sky was red. I was very surprised indeed when my subs of M42 showed the most nebulosity I had ever encountered. I can only put last night down to thin high cloud that made the stars LOOK bad, but actually werent too bad to a camera.

It darkened considerably at around 3am, but the images didnt improve as dramatically as I would have thought.

I suppose one thing was true, there was very little dew, and I wonder if the amount of moisture in the lower atmosphere would make a big difference? Almost acting like a lens?

[Insomniac]

I think that maybe Ritchie wanted to let us know that he has adaptive optics but is too modest to boast outright.

[cygnusx1]

I think that maybe Ritchie wanted to let us know that he has adaptive optics but is too modest to boast outright.

LOL