Sketching is a curious item, and one I only just glanced over because there are more factors involved, though the core one is still ignorance.
Many, if not the vast majority of pre-photographic astronomers were not artists, and did not have the "imagination" on how to best portray what they saw through the eyepiece. This makes media selection even more difficult.
Another limiting factor was the printing technology of the time. Printing was a high contrast exercise, and one not suited the the subtleties of DSO's. Delicate pencil drawings were not possible to reproduce with the contemporary printing technology of the time. Below is an 19th century print of M31. Here you can see much of what I'm talking about. The ignorance of what the artist was looking at, the limitations of the medium being used, and the illustrative limitations of the astronomer (sorry I cannot remember the astronomer who's work this is):
Now, compare it to my sketch of M31. I do have an understanding of what M31 is, I've seen many photos of it so I come better prepared when I come to the eyepiece, I do have some artistic capacity, and I can use a medium (soft pastels) that allow for easy reproduction of the original (digital photo).
I also have the benefit of the advantage of having easy access to a very broad source of sketching inspiration:
Below is John Herschel's printed sketch of Omega Centauri:
Below is my rendition of Omega Centauri down from Sydney using a 17.5" dob:
Below is a collage of various sketches of galaxies from different early astronomers. Biggest difficulty for these astronomers is not knowing what they were portraying, optical equipment, and illustrative capability. While some galaxies below are recognisable, many are not. This is something less of a problem for current astronomers, amateur or otherwise:
image from Cambridge Uni Press.
Mind you, there were some VERY capable early astronomers who were quite accomplished artists. One of these was William Parsons. Below is his sketch of M51 from the mid 1840's. However, reproducing this quality of sketch with the printing technology of the time would have been impossible:
But now on a bit more reflection, I do guess that comparing early sketches to contemporary sketches helps demonstrate the difference that knowledge makes to current observing and sketching of DSO's.
Alex.