Hi,
since nobody has reviewed the extender yet, I bought one and had a test drive with my old Kowas... for me it was a mixed bag, although this is not really the fault of the extender... some non-standard parts of my setup are plagued with backfocus problems with the extender...
I used two TSN-3 bodies (both are the old model without the protective glass above the prism) - one stock and one on which sb. has done some creative repair of a broken prism seat which results in some less backfocus and a slightly lower allowably outer diameter of the EPs.
The following EPs were used: Kowa TE 14WD aka the modern 30 wide, an old 60x fixed EP, the old 20-60x zoom from the 90s, my Opticron SDLv2 (8-24mm zoom or 18-52x on the Kowa) and an Opticron HDF 40861 (5mm or 84x on the Kowa - no direct fit but I'll print an adapter). I don't own the current Kowa 20-60x zoom, the TE-9Z - it would be interesting to try it with the extender... when I get the opportunity, I'll let you know.
- TE 14 WD: very nice at 45x, has 1/3 to 1/4 turn beyond infinity in the stock body but is quite short sighted in the modified one - there it doesn't focus beyond a few ten meters.
- the old 60x: is sharp at 96x and 1/3 to 1/4 turn beyond infinity in the stock body but is quite short sighted in the modified one - there it doesn't focus beyond a few ten meters
- the old 20-60x: ok at 96x and it has more than 1/2 a turn beyond infinity in the stock body, in the modified one it doesn't focus at low mag end but has more than 1/3 of a turn beyond infinity at 60x.
- SDLv2: nice and sharp in the stock body at 28-84x for normal birding ranges, almost reaches focus at infinity, no way at any range in the modifed one.
- 40861: sharp at 134x, more than 1 turn beyond infinity in the stock body, just comes to focus at infinity in the modified one
Top magnification was 134x with the Opticron 40861 and 96x with the old 60x fixed EP and the abominable 20-60x zoom from the 90s (strange enough the zoom is less terrible with the extender at max magnification than without - it seems the slower effective focal ratio of f8.8 of the body plus extender is more to its taste than the super fast f5.5 of the body alone).
- At around 90x the exit pupil is around 0.9mm - quite usable in good light. The 134x with an exit pupil of 0.6 is fine for bright astro objects like luna or planets but too dark for terrestial. Careful eye placement is needed for good results and the 134 will show all the floaters your eyes have...
- Backfocus is critical with the extender - Kowa EPs and bodies will probably work fine unless you have a very strong prescription and no glasses. If you use funny combinations like me, some luck is needed...
So will I keep it? That is an interesting question - I planned to use it either on the modified/repaired body with the 30 wide as a 45 wide (doesn't work beyond some ten meters) or with the SDLv2 in my stock body for 32-84x. The latter combination doesn't quite come to focus at infinity but is fine for birding distances. So it will probably stay on there.
If you have an ED or fluorite small body scope and use Kowa EPs, the extender will work great and give more reach as expected.
With normal glass bodies, it will turn the 25x LER into a 40x LER which should work well, the 30 wide at 45x will be borderline and nothing will be gained with the zoom as the plain glass bodies are hard stretched beyond 40-45x.
Joachim
