Revelation 12" Dob diagonal - offset or not

[Sponcom]

Hi All

Can anyone tell me if the diagonal in a Revelation 12" Newtonian (Dobsonian) needs to be offset with regard to the focuser and Primary or slap bang in the middle.

After spending a significant amount of time trying to collimate my 10" Meade SN-10 Schmidt Newtonian and ending up modding it extensively in order for it to hold collimation I'm ready for anything!

The Meade requires a 1/8" offset of the diagonal away from the focuser and towards the primary mirror. Is this because the primary is a spherical mirror rather than parabolic like the vanilla newt? I haven't discovered the reason for the offset and can't see any reason for it myself and there seems to be much debate around whether it is required or not.

Thanks

Paul

[Doc]

If you centre the secondary the offset is automatically taken care off.

[Merlin66]

Offsets in newtonians are only really required when using "optimum" sized secondaries ( ie not oversized) and on fast f ratio ie less than f5.(On my f3.6 Schmidt Newtonian the offset is VERY noticable!)

The axial offset will not be noticable when looking at the secondary through the focuser - the secondary should appear central and circular to the outer edge of the focuser tube.

The radial offset ie away from the focuser is required to centre the secondary in the cone of the returning light from the main mirror.

Roughly, the radial offset in a 12" f5 system is about 4mm. Most of the manufacturers design this into the secondary mount by gluing the secondary slightly off centre to the support stem.

http://stargazerslounge.com/diy-astronomer/77359-100-secondary-off-set-newtonians.html

If you want all the maths and numbers PM me.

[Sponcom]

Thanks for the replies.

I think that answers my queries for the moment!