Hallo Ralf und Toni,
zum "Riesenschritt" geb ich dem Toni Recht (d.h. es ist KEINER). Als ich (mit 50 Jahren) vom 8 Zoll Celestron auf einen 12 Zoll Dobson (ist ein spezieller, geb ich zu) umgestiegen bin, schrieb mir der Verkäufer (es gab ((8 Zoll und) 12 Zoll und 14.5 Zoll und ich wollte - natürlich - den 14.5 Zoll)) Folgendes:
"Just so you understand, I have no bias as to which scope you choose as I make about the same margin on all sizes. I'm merely trying to offer you what I consider a viable alternative. The difference in image brightness in the 14.5 vs. a 12.5 is visually insignificant as it is only ~1/3 a magnitude, while the cost for that extra little bit of aperture, and a lower f/ratio is significant. The reason for that fact is that the human brain's perception of brightness is not linear, but logarithmic. It takes close to a magnitude increase in brightness (which translates into a ~2.5 or more correctly, the 5th root of 100) to really notice a change. Starting with Galileo's 2" telescope, to get ~ a 1 magnitude increase in light gathering ability you'd develop the following series of sizes: 3", 4", 6", 8", 12.5", 20", 30", 1m, 2m, 3m., 4m, 6m, 8m, etc. If those look familiar -- that's why. Note that there is a two inch increase in the range most amateurs are familiar with, but that stops at 8", the next step is a 12.5" and from there to a 20" -- all the stuff in between, is simply playing to the male ego ("mine's bigger than yours!").
Rudi
