Seeing and Transparency

[ZiHao]

Hi all,

Seeing describes how much the view through your telescope is disturbed by what's going in the atmosphere above you. Transparency describes how clear the sky is. 'Vibrating' moon and stars tend to be affected by the turbulence produced by the bad seeing in the atmosphere. DSOs like galaxies and nebulae are not really affected by bad seeing, but transparency, according to http://www.skyatnightmagazine.com/feature/general-guide/guide-seeing-and-atmospheric-transparency. Why? Why doesn't seeing have an influence on DSOs? I can't understand this.

Clear Skies and a Merry Christmas!

[Stu]

3 minutes ago, ZiHao said:

Hi all,

Seeing describes how much the view through your telescope is disturbed by what's going in the atmosphere above you. Transparency describes how clear the sky is. 'Vibrating' moon and stars tend to be affected by the turbulence produced by the bad seeing in the atmosphere. DSOs like galaxies and nebulae are not really affected by bad seeing, but transparency, according to http://www.skyatnightmagazine.com/feature/general-guide/guide-seeing-and-atmospheric-transparency. Why? Why doesn't seeing have an influence on DSOs? I can't understand this.

Clear Skies and a Merry Christmas!

It's a very good question. My answers below:

Firstly DSOs are often very faint, so the most important criteria is transparency because this will determine whether you can detect the contrast between the sky background and the target. With high haze (poor transparency), the light from the target is dimmed a little, and the sky background may be a little brighter (due to reflected LP). This can be the difference between seeing the object and not, or seeing detail in the object or not. It is all about contrast. Seeing does not affect contrast, transparency does.

Often DSOs are larger and are observed at low to mid powers, where the stability of the atmosphere is less of an issue. Planetary observing is a high power exercise, with a bright object (except Uranus or Neptune) so the transparency has minimal affect.

The one area where seeing may affect DSO observing, and I have limited/no experience of this, is seeing detail in faint galaxies at high power in a large dob. You might be observing at x200, so I imagine that both transparency and seeing are important for this type of viewing, although a member of the dob mob will need to confirm this.

[Moonshane]

I think of this as being similar to dark adaptation in that for faint objects you mainly use your rods which are low resolution but very sensitive. Therefore you see less detail (except for the brighter, larger objects which I think might sometimes activate the cones) but do 'detect' objects sometimes despite not even looking at them (averted vision). This requires good clean air free of moisture. This is often unstable so only low magnification is possible. Most people when observing faint objects are content to simply observe them as most are faint whiffs of vapour.

Bright things like the solar system objects certainly activate your cones which are higher resolution and detect colours. For e.g. planetary observing you are looking for high magnification and resolution to see fine detail which requires steady air and that often means air with some moisture. Most people would not be happy to just 'detect Jupiter and are looking for details.

Naturally nights when seeing and transparency are good to excellent together are scarce in the UK but on these nights all objects can be observed successfully assuming the skies are dark enough.

Really it's beneficial to try to select target objects and to some extent scope to suit conditions although if you are like most greedy astronomers (myself included) you will of course wish to see everything!

Hope this helps.

 

[mikeDnight]

Usually, seeing and transparency are at opposite poles, so that when its good seeing its often poor transparency. This is great for the planetary observer, as it can mean a steady atmosphere, and viewing a planet through a light mist will usually provide an exceptional view. It's not so good for dso observing!  Sometimes the sky can appear transparent but isn't really so. Looking for objects such as M1 can be a good indicator of transparency in small to moderate scopes. If you struggle to find it, then transparency isn't good. But when seeing and transparency are both excellent, then observing dso's is greatly enhanced and deep sky objects can reveal amazing detail. So poor seeing does actually harm deep sky observing, its just not as immediately obvious.

[JeremyS]

Dear ZiHao,

further to the above answers, you might find these articles helpful as they discuss transparency and seeing:

Observing and Drawing the Night Sky

Astronomical ‘Seeing’

Merry Christmas,

Jeremy

[Astro Imp]

An interesting question posed by the OP.

I've had an idea of the difference but found my understanding greatly enhanced by reading the views of experienced members @Moonshane , @mikeDnight and @Stu , thanks to you all.

[Mark at Beaufort]

2 hours ago, mikeDnight said:

Usually, seeing and transparency are at opposite poles, so that when its good seeing its often poor transparency. This is great for the planetary observer, as it can mean a steady atmosphere, and viewing a planet through a light mist will usually provide an exceptional view. It's not so good for dso observing!  Sometimes the sky can appear transparent but isn't really so. Looking for objects such as M1 can be a good indicator of transparency in small to moderate scopes. If you struggle to find it, then transparency isn't good. But when seeing and transparency are both excellent, then observing dso's is greatly enhanced and deep sky objects can reveal amazing detail. So poor seeing does actually harm deep sky observing, its just not as immediately obvious.

Mike that is a very good explanation which will be useful to many members - well done

[brantuk]

The more you magnify the object, the more you magnify adverse effects of the seeing. Dso's are mostly low power observing whereas planets are observed at higher magnifications. So you see atmospheric effects more when looking at planets. It's kind of analogous to movement in the eyepiece where low magnified dso's drift across the view slower than highly magnified planets. Hth

[Special K]

I’ve not observed enough to be certain but feel that globular clusters can suffer under poor seeing. Naturally you need good transparency to see it well to begin with but if you are trying to resolve the subject closer to the core, I think decent seeing will help. 2c worth!  

[Stu]

6 minutes ago, Special K said:

I’ve not observed enough to be certain but feel that globular clusters can suffer under poor seeing. Naturally you need good transparency to see it well to begin with but if you are trying to resolve the subject closer to the core, I think decent seeing will help. 2c worth!  

That would certainly make sense as they are higher power targets (as alluded to in my comment), and ultimately are a series of stellar point sources rather than extended objects. More like observing lots and lots of double stars I guess, so seeing would be important at high power.

[Victor Boesen]

Really interesting topic with some great answers! I can relate to some of these scenarios. For example, when viewing Jupiter through a thin layoff mist, the detail was actually clearer because it dimmed the brightness of the planet. And yes, I agree with @mikeDnight that when the seeing is good, the transparency is poor and reverse.

[rockystar]

My understanding of why they are not affected by seeing as much as stars, is because they are not points of light.  A star can be thought of as basically a single stream of photons that gets refracted through the atmosphere, in  bad seeing conditions this refraction effect is enhanced and will make the photon stream wobble. DSOs on the other hand are much bigger and thus many photon streams are hitting the objective, they will still be refracted but in different directions; as light is a wave, they cause interference and can cancel each other out, thus making a larger object less affected by wobbly streams of photons.