So lets say I have a AT102ED with a 2" focuser, but I only have a 1.25" diagonal and EPs. I could use a supplied adapter to mate the 1.25 diag with the visual back OR, lets say I only had a 2" diagonal. I could slip in a 2" diagonal and use an adapter into the visual back without any adapter and then use a 2" to 1.25" adapter on the EP side of the diagonal. Does it matter?
Where's the best place in the light path to place an adapter?
Posted 19 January 2021 - 07:14 PM
The light beam is a conus. Its wide when the light enters the scope, and gets narrower the closer you get to the focus.
If you place some stop with a small opening too far before the focus it might start cutting that light cone, causing vignetting (darkening of the outer areas of the FOV).
So if you have the choice, place an adapter that reduces the free diameter as late as possible in the beam....