Some thoughts and considerations for finder reticule eyepieces:
- The virtue of the hard brass wire is that it does not require tensioning to be straight. This means it relatively easy to do eyepieces with recessed field stops.
- Most reticule eyepieces are simple designs and poorly corrected in a faster scope. Since finders are typically around F/4, this means a messy view. A year or two back, i purchased the standard 20 mm 70 degree illuminated reticule eyepiece for use in my finders, primarily an SV-50.
I was badly disappointed. The rericule was overly bright and bled into the field. The eyepiece was a mess off-axis. I considered it unusable and gave it away. I decided I could do better.
- I had an old TeleVue 24 mm Wide Field. I was my first really decent eyepiece but it had some damage from a previous owner and had seen a lot of use. It's not a 16 mm Type 5 Nagler but it's sharper by far than a normal finder eyepiece and it has the virtues that the field stop is accessible and the ring is wide enough to glue to.
- Works very well, it's reasonably sharp across the field and offers a 6.8 degree TFoV at 8.3x.
Cross hairs are an important part of my star hopping technique. I align the cross hairs with the vertical and horizontal axes of the scope. Since the finder focuser is at an angle, these axes are not intuitive. The aligned cross hairs allow me to match the horizontal and vertical star fields in Sky Safari. I need only one star in each axis for precise pointing..
- If I were doing it from scratch: a well corrected eyepiece that's affordable that's has a large field stop but it needs to be 25 mm or less so there's something to glue to.
20 mm Explore Scientific 68 degree..
- 0.004" hard brass. Thin is not good, it's not visible under dark skies.