[pgc hunter]

There is no doubt that there is a universal consensus out there that seeing plays an important role in planetary observing, which is of course true, but it is rarely regarded as a consequence when viewing deep sky objects. It is not hard to find books and articles that claim that seeing is not of importance to DSO observation, but in my years of observing, I've come to the conclusion that this is not the case.

While transparency is always important when dealing with DSOs, but is it really that preferable over good seeing when viewing certain classes of DSO? Let's consider small, bright planetaries and globular clusters. These are bright and of high surface brightness and will not suffer much from less than average transparency, certainly not to the extent that emission/reflection nebulae and galaxies do, but are 3 types of DSO that bear the brunt of poor seeing.



-Globular Clusters
-Small bright planetary nebulae

Why? These objects are bright, compact and are high surface brightness and will punch through poorer transparency. Many globular clusters are densely packed as seen through the eyepiece with stars sitting "shoulder to shoulder", often appearing like a mass of innumerable pinpricks. In good seeing, these is quite a site, seemingly a sparking carpet of pinpoints. But, in poor seeing, consider how a brighter single star appears. That's right, a bubbling, flaring fuzzy unfocusable mess. Well, that same thing is happening when observing globulars, causing those tiny pinpricks to smear and wash out, resulting in a somewhat subdued view that seems to "mush" through best focus.

Regarding small bright planetaries, often these objects feature tiny, fine detail. Poor seeing will smear these details, smudging it into the background glow of the planetary rendering it more subdued than otherwise would be or even invisible. Resolution of very tiny objects, those of about 5" diameter or less becomes impossible. In excellent seeing, stars will appear crisp, while the tiny planetary will appear diffuse and/or a tiny disk. However, in poor seeing, stars themselves will become bloated and messy, making it hard if not impossible to tell them apart from the planetary. The planetary itself will be smeared to some extent at best focus.

-Small Faint Galaxies

We're not talking about objects like M104 or M83, but small, faint galaxies at limit of the scope's and your eye's light grasp and resolution. While transparency here is critical, good seeing is important aswell. An unsteady atmosphere can smudge these delicate objects into complete invisibility simply by smearing their meagre light into the background. Faint clusters/groups such as the Hickson Galaxy Groups and the Abell clusters often are made up of very small, very faint objects tightly packed together requiring high magnification to resolve. High magnification helps the visibility of these faint objects by darkening the background sky increasing contrast, however, if seeing is very poor, using high powers as we all know will result in a poor image and difficult observation of these objects. To put in context the extent of the damage being done, view a planet at high power on a night of good seeing and one of poor seeing and translate that to a 14-15th magnitude, 0.5' size galaxy...