It is a tough task to put in writing "what we do" when we are at the telescope, in order to tease out that ultra-faint detail within objects; but I think Ron and Ray and Paddy and Rob K are doing a good job of describing the procedures that advanced visual observers actually do use.
Author's preliminary warning:
(regarding this post , and regarding my next post in this thread)
My comments reflect the approach that I have taken in
my own visual observations of diffuse (= extended)(= non-stellar) deep sky objects, such as emission nebulae and galaxies, which is based on my own
qualitative observations of what happens when the eye and brain are put behind the astronomical telescope.
In no way have I actually put any hard numbers to the various important factors that affect how well we see a diffuse object or feature which is near to the fringes of visibility (e.g. surface brightness, contrast, angular resolution at low light levels, feature recognition by the brain, etc.).
As such, everything that I say is very preliminary, and "disprovable".
The Human Brain is the Deep Sky observer's Camera :
Ron and Ray have made the important point (in their recent posts in this thread) that Deep Sky Observers - when they are viewing a very faint nebula or galaxy - are actually trying to gradually build up an image of the object in the human
brain. Which is why Ray's point about the utility of sketching (and/or a detailed prose description of what is seen) is so important; the process of sketching and describing an object implies that we are transferring the things that the human brain/mind sees at various times and moments.......onto a piece of paper, which serves as an “accumulator” for the things the brain perceives.
Putting together a mental image of an object :
I do agree with Ron that many and various magnifications should be tried on a diffuse object, because the final result of a deep sky observation is a sort of composite “mental image” of an object, made up of multiple individual views or glimpses of the object.
Therefore, a good approach is to put together, into a single "mental image" and/or a sketch and/or a prose description:
- the details that the observer sees over a prolonged period of viewing at a single magnification
- the details that she/he sees when viewing at various magnifications
- the details seen on various nights
As experienced observers will tell you, some of the
galaxy images which are very large in angular terms, such as M101, M31, M33,
NGC 4565, NGC 891, NGC 6744, NGC 5128, NGC 1365, NGC 1313, NGC 253, NGC 55, M83, etc., do contain substantial amounts of detail that is visible to the eye at the telescope, at least if these objects are observed in a dark sky. However, the details within these galaxy images tend to be fleeting, and these details are often seen haltingly because they may be near to the eye's limit. So this is why the approach of "gradually building up a mental image of the target object" can eventually result in the observer getting a really good idea of the structure of the image of an object.