Does Interstellar Gas Affect Atmospheric Seeing in Ground-Based Observations?

[omthegreat]

This is probably a stupid question, but it has been bugging me.
We are severely limited in ground based observations due to atmospheric seeing effects distorting the image. Why then are we not concerned by the vast amounts of gas/planets/etc in the line of path to our source that should also introduce a seeing effect?
Is it just that the densities of these intervening gas clouds are so low that scattering is negligible and intervening planets are too small to have an impact?

 

[mfb]

• Stars, planets and all other dense objects are tiny compared to the empty space in between. If you look in a random direction in the sky, it is unlikely that any object is there (until you reach the CMB).
• The further away a gas cloud is, the larger the volume where "your" light goes through. That makes small-scale effects negligible - apart from the atmosphere of earth.
• With a typical density of something like 100 atoms/cm³, you need of the order of 100 000 to 1 million light years to get the same area density as our atmosphere.

 

[Tobychev]

We are concerned with the intermediary gas, but not with respect to seeing. The effect of interstellar gas basically distorting the spectrum we observe, calling the "reddening". I guess there could be seeing effects from the interstellar gas, but we are pretty far from that beeing a real concern, the turbulence in interstellar clouds is probably on much bigger scales than the apparent size of the objects we study.