Seeing

[staffop]

I'm increasingly coming across this term but not sure i totally get it.

I understand it's to do with visual interference due to atmosphere which will impact on what one can actually see but is there a way of knowing in advance if conditions are good or bad? Some sites suggest a mark out of 5 but isn't it too subjective for that type of measurement?

[jetstream]

It has taken me a long time to wrap my head around this concept - but understanding seeing and transparency is one of the most important things for getting great views -and- knowing what objects to view under what conditions.

Basically good seeing allows excellent planetary/lunar and star splitting.

Those bright, clear nights with the Milky Way casting shadows on the ground are great for DSO- transparent skies rule here.

Unfortunately transparent skies don't usually correspond to good planetary/lunar views- good trans usually means challenging seeing IMHO.

I hope others will chip in here with a more in depth description.   For me picking the object to match the seeing conditions and cooling and collimating my scope is the secret to success.

[kalasinman]

Two factors combine , seeing which is boiling of the atmosphere, and transparency, which is the clarity (diminished by haze, humidity, smoke, etc.). The second one is easier, if you don't have good transparency, dimmer objects just won't be seen. Seeing , just look to see if the stars twinkle. Twinkling means poor seeing.

[bmc]

hi there, 

the recognized scale for seeing is called the Antoniadi scale.

The scale is a five-point system, with 1 being the best seeing conditions and 5 being the worst. The actual definitions are as follows:

(I.) Perfect seeing, without a quiver.

(II.) Slight quivering of the image with moments of calm lasting several seconds.

(III.) Moderate seeing with larger air tremors that blur the image.

(IV.) Poor seeing, constant troublesome undulations of the image.

(V.) Very bad seeing, hardly stable enough to allow a rough sketch to be made.

[staffop]

hi there, 

the recognized scale for seeing is called the Antoniadi scale.

The scale is a five-point system, with 1 being the best seeing conditions and 5 being the worst. The actual definitions are as follows:

(I.) Perfect seeing, without a quiver.

(II.) Slight quivering of the image with moments of calm lasting several seconds.

(III.) Moderate seeing with larger air tremors that blur the image.

(IV.) Poor seeing, constant troublesome undulations of the image.

(V.) Very bad seeing, hardly stable enough to allow a rough sketch to be made.

Excellent, another new term to add to my increasing vocabulary - loving this hobby more and more!

[jetstream]

I love this animation and still use this method sometimes. The mag you can use on the planets is also a good indicator ( cooling and collimation considered). For star testing collimation at least IV is required IMHO.

[staffop]

Ah, that's interesting. On the second day i had my first scope i looked at one of the stars in the plough and it was like the image in II - i assumed this was because of a poor pre-supplied EP but looks like it was due more to poor seeing !

[jetstream]

Ah, that's interesting. On the second day i had my first scope i looked at one of the stars in the plough and it was like the image in II - i assumed this was because of a poor pre-supplied sense but looks like it was due more to poor seeing !

What scope are you using? The biggest factors for good planetary are seeing conditions and a reasonable quality scope that is cooled and collimated.

When you saw "image II" in your scope how long did you cool it for?

In my newt an uncooled mirror can look similar to poor seeing- you can tell the difference as experience grows as can boundary layer issues.

[staffop]

It's a small fast refractor (4" f5) and had been outside for roughly 4 hours (it was about midnight) - so assumed cooling wouldn't be an issue (that said i never knew what issues it could actually cause!). I'd already been spoiled by seeing Jupiter and Venus better than ever possible through the naked eye so even this poorly resolved star still made me happy

[Steve Ward]

Some very useful bits from Mr Peach ... 

http://www.damianpeach.com/seeing1.htm

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

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

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

[Stu]

Excellent animation and great post bmc.

I tend to take a simplistic view of the importance of these two factors.

Seeing is important for planetary, lunar and double star observing, and other situations where high power and resolution are key to seeing your target. Atmospheric turbulence can play havoc with this.

Transparency is most important for larger, faint objects such as nebulae and galaxies, objects that tend to be viewed at lower powers and where resolution is of lesser significance, but contrast is important. Of course this also applies to the smaller galaxies and other small faint objects which need higher power to see them properly. Poor transparency robs the view of contrast and can make these faint objects invisible.

It makes sense to observe objects which are least affected by the conditions you find. As has been said, poor transparency often accompanies good seeing and bright planets cut through this no problem.

[Phil Fargaze]

I always log my observing sessions and first thing I do is take a note (or guess) of the seeing and transparency. For seeing I use the five point Antoniadi scale and for transparency it is either poor, med, med/good, good and excellent. Once you get used to doing this you can get a good guess on the conditions quite quickly and know what to expect when observing.

[staffop]

Thanks all, sooooooooo much to learn!

[jetstream]

Some very useful bits from Mr Peach ... 

http://www.damianpeach.com/seeing1.htm

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

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

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

Those are great links Steve, thanks for posting them.

[chiltonstar]

Although a write-off for the faint fuzzy hunters, these warm humid evenings can often have excellent seeing and therefore be the best times (as stated above) for high resolution targets like planets and doubles. The animation posted by Jetsream above is subjective, but so is the eye of course, and for most of us a scale of 5 is about as good as we can judge or need.

Chris

[Charic]

I love this animation and still use this method sometimes. The mag you can use on the planets is also a good indicator ( cooling and collimation considered). For star testing collimation at least IV is required IMHO.

Is it just late at night or am I missing something with the animation. I thought `I`  was perfect and no quiver?

[jetstream]

When I quote seeing as 4/5 I mean that seeing is very good and comparable to this scale- 5/5 would equal 100% etc. I think that this is a common method of describing the seeing, but it doesn't really matter as long as we all know what each other mean.  There are many scales and parameters around and maybe their systems are the other way round.

[Charic]

........Just seems that #5 and #6 are opposing each other in their format.

[jetstream]

Well I guess it depends if Antoniadi's format is preferred or Pickering's- I like Pickering's format and scale better...actually I usually let my VX10 tell me how the seeing is, 300x+ on the planets,400x on the moon=great seeing lol!

[Charic]

cheers jetstream....... I Like  Pickerings version better.........just had to check the web!

Its crazy and confusing having different formats?

* Always learning something new everyday!........its remembering, that's the hard part? *

[jetstream]

Yes it can be confusing, I like the notion that 10/10 or 5/5 equates to the best seeing conditions-as does Peach, his images are great representations of the conditions in his seeing scale for imagers, 3rd link down on Steve's post.

It seems there are more scales like Pickerings than Anton's.

[chiltonstar]

The Antoniadi scale appears to be reversed although it  normally uses Roman numerals (V, IV, etc) and was developed for planets.

There is a good summary of different seeing scales at:-

http://www.handprint.com/ASTRO/seeing2.html

Chris