Secondary size ??Posted By Mike Hosea |
Gary,
I'm sure you don't want a FULLY illuminated field in a 40mm Konig unless you're only going to do photography exclusively. Even a 2.6" diagonal will have some dropoff in illumination at the edge of a wide true field.
Selecting a secondary size unfortunately requires you to make compromises. Do you want excellent planetary performance or are you exclusively a DSO observer? I'm interested primarily in planetary targets and globular clusters, so I tend to prefer a secondary under 20% of the primary size, and I'm more than willing to accept a drop of 50% or even more in illumination at the edge of a wide true field to get it.
It depends on the focuser, specifically the distance from the axis to the focal plane, but I think I personally would go with 2.14" in this case. That's a 17% obstruction, and going lower is not likely to help planetary views much (and might hurt them if the edges of the secondary aren't as good as its center). It also probably gives about 60% or more illumination at the edge of the focal plane accessible to a 40mm UO MK-70 Konig. IMO, that's fine.
--
Mike
I'm sure you don't want a FULLY illuminated field in a 40mm Konig unless you're only going to do photography exclusively. Even a 2.6" diagonal will have some dropoff in illumination at the edge of a wide true field.
Selecting a secondary size unfortunately requires you to make compromises. Do you want excellent planetary performance or are you exclusively a DSO observer? I'm interested primarily in planetary targets and globular clusters, so I tend to prefer a secondary under 20% of the primary size, and I'm more than willing to accept a drop of 50% or even more in illumination at the edge of a wide true field to get it.
It depends on the focuser, specifically the distance from the axis to the focal plane, but I think I personally would go with 2.14" in this case. That's a 17% obstruction, and going lower is not likely to help planetary views much (and might hurt them if the edges of the secondary aren't as good as its center). It also probably gives about 60% or more illumination at the edge of the focal plane accessible to a 40mm UO MK-70 Konig. IMO, that's fine.
--
Mike