Dear DSF,

I don't know whether this is specific to my dob (which currently has a layer of stubborn dust on the mirror and on which I frequently use a large exit-pupil eyepiece), but when it comes to contrast, my 18" Obsession fails miserably compared to my friend Hemant's 5" Takahashi refractor.

Of course, when it comes to the Shakbazians, Abell galaxy clusters, or resolution of globular clusters, the 5" would be no match for the 18". So to give fair competition, we chose only those objects that were diffuse and whose observation would be limited by contrast rather than intensity or power. Here are the results -- I mention which telescope performed better on each observation. Comments are invited!

Veil nebula: 18"
Bright pillar in the "Pillars of creation" in M16: 5"
Dark pillars in the "Pillars of creation" in M16: 18"
Lagoon nebula: 5"
Helix nebula: 18"
Barnard E nebula: 5"
Andromeda galaxy overall: tie
Inner V-shaped dust lanes pointing towards the core of M31: 5"
Usual dust lanes of M 31: 18"
Triangulum galaxy: 18" by far
Maffei I: Tie
IC 342: The two observers disagree on the results. (I think the 18" was better, Hemant thinks the 5" was better)

(If I do get that re-coat I've been planning on, it'd be worth repeating this to see how much that dust degraded contrast)

Clear Skies

Regards
Akarsh