Does any one know the optical path length and field stop diameter

[axle01]

Does any one know the optical path length and field stop diameter of a

Williams Optics 1.25 diagonal

Williams Optics 2" diagonal

Baader 1.25 diagonal

I am trying to work out what has the largest field stop and the shortest optical path for use with a bino

[cs1cjc]

I use the William Optics 1.25" carbon fibre for travel use and it is very nice and solid. The barrel is tight internally at less than 25mm (compared with a 27mm field stop on the widest 1.25" eyepieces). The optical path length is about 82mm.

I would always use a 2" diagonal unless weight or size or cost is a significant issue, though the Baader T2 also looks good. Tthis article by Roland Christen explains that the outer area of a diagonal mirror may not be as good as the rest: http://geogdata.csun.edu/~voltaire/roland/dielectric.html . The optical path length of my GSO 2" is about 107mm, but it comes with a zero height adaptor reducing the effective increase in length for 1.25" eyepiece to under 100mm. An issue with the zero height adaptor is that it is not really compatible with some fat 1.25" eyepieces, like the ES68 24mm.

[John]

For what it's worth the light path through a 2" William Optics mirror diagonal is 156mm from the top surface of the 1.25" adapter to the end of the 2" barrel.

[axle01]

Thanks Chris and John and thanks for the link Chris.

I have ordered a Baader Planetarium T-2 Star Diagonal Prism (Zeiss Prism) and it's got 41mm light path and the Denkmeier bino has 27mm field stop so I am hoping this will do.

I am going to mount a Denkmeier Binotron 27 to it but I won't be using the eyepiece adapter, I have a nose piece on the way that I am going to machine it to fit into the Baader diagonal then screw into the bottom of the bino to cut out as much light path as possible.

I am hoping the male T2 thread adapter on the Baader diagonal is removable cause I need a female T2 thread to do what I want.

I will post pictures of what and how I hope to achieve this but it won't be for a couple of weeks till the diagonal and nose piece turn up.