I don't want to beat a dead horse/angle for the big fish (insert your favorite metaphor here...) and I *have* read with interest the previous central obstruction threads. My question is, bearing in mind my average viewing conditions (East Coast humidity and light pollution) am I likely to notice much difference in practice if I replace the 4" secondary in my 16" Dob with one small enough to get below the 'magical' 20% central obstruction (CO) threshold?
I know the arguments (less vignetting, higher contrast, etc.) and physiological effect that supposed kicks in at 20% CO or less, but is this difference likely to be noticeable when starting from 25% CO? The 100% illuminated area will just about vanish (less than .1", compared to the .8" dia of the 4" secondary's fully-illuminated region), and I'll have to do major modifications (move the primary back in the tube, move the trunions, and bore a new hole for the 120mm high efficiency fan that blows across the mirror face) just to keep *any* fully illuminated area to the view with the smaller diagonal. Moving from 4" to a 3.5" doesn't require any scope mods, but it only reduces the obstruction from 25 to 22%, which I'm virtually certain is an imperceptable difference (except to my wallet-- those big secondaries aren't cheap even second-hand!).
I've read the theory and crunched the numbers, now I'd welcome opinions and views (informed or otherwise?!). I'd be especially interested in hearing from anyone who did move from one of the Meade/Coulter style 'oversized' secondaries to a smaller one falling below the 20% CO threshold. I'm inclined to think it's not gonna' be worth it for me to do this for those rare nights of perfect seeing, but I'm willing to be persuaded otherwise.
Thanks! Jim
I know the arguments (less vignetting, higher contrast, etc.) and physiological effect that supposed kicks in at 20% CO or less, but is this difference likely to be noticeable when starting from 25% CO? The 100% illuminated area will just about vanish (less than .1", compared to the .8" dia of the 4" secondary's fully-illuminated region), and I'll have to do major modifications (move the primary back in the tube, move the trunions, and bore a new hole for the 120mm high efficiency fan that blows across the mirror face) just to keep *any* fully illuminated area to the view with the smaller diagonal. Moving from 4" to a 3.5" doesn't require any scope mods, but it only reduces the obstruction from 25 to 22%, which I'm virtually certain is an imperceptable difference (except to my wallet-- those big secondaries aren't cheap even second-hand!).
I've read the theory and crunched the numbers, now I'd welcome opinions and views (informed or otherwise?!
Thanks! Jim
