Rotherbirder,
Of your concerns, the fixed scope aperture is the significant one, transmission losses and internal reflections in this case are increased so marginally that their effect is negligible. A top-quality 88 mm scope is capable of showing increased useable resolution (more detail) up to roughly 0.9-1 mm of exit pupil diameter, or just about to the top magnification offered by a single extender. In planetary viewing, much higher magnifications are useful.
Whether the view is mediocre or excellent depends on your definitions of those terms. But, for practical perspective, in good viewing conditions the improvement in detail seen will with the Kowa 883/4 will be significant well beyond the 60x possible with the zoom alone, and from my experience with extended use of a Nikon ED 82 A capable of 75x and now the Swaro ATX 95 going up to 72x, I'd say that the point of no more return to your increased magnification with a good sample of the Kowa is somewhere in the 85-96X range, which makes it plenty useful for the money. With the Nikon I would not have been satisfied had the magnification range stopped earlier, and with the Swaro I often would need it to go higher, at least up to 85x, preferably closer to 100x. I have a booster (Zeiss 3x12) which has sometimes allowed me to ID a shorebird at about a mile distance at 115X that was simply impossible to ID at 72x without the booster. And the Zeiss booster really does have poor transmission and a whole bunch of glass to deteriorate the image, unlike the Kowa extender that has very few glass-to-air surfaces, no prisms, and much better coatings.
But not all birders use their scopes to their max, it depends on the kind of viewing you do, and if you have a Kowa and rarely use it at mags above 50X, you will have no need for the extender.
Kimmo
Of your concerns, the fixed scope aperture is the significant one, transmission losses and internal reflections in this case are increased so marginally that their effect is negligible. A top-quality 88 mm scope is capable of showing increased useable resolution (more detail) up to roughly 0.9-1 mm of exit pupil diameter, or just about to the top magnification offered by a single extender. In planetary viewing, much higher magnifications are useful.
Whether the view is mediocre or excellent depends on your definitions of those terms. But, for practical perspective, in good viewing conditions the improvement in detail seen will with the Kowa 883/4 will be significant well beyond the 60x possible with the zoom alone, and from my experience with extended use of a Nikon ED 82 A capable of 75x and now the Swaro ATX 95 going up to 72x, I'd say that the point of no more return to your increased magnification with a good sample of the Kowa is somewhere in the 85-96X range, which makes it plenty useful for the money. With the Nikon I would not have been satisfied had the magnification range stopped earlier, and with the Swaro I often would need it to go higher, at least up to 85x, preferably closer to 100x. I have a booster (Zeiss 3x12) which has sometimes allowed me to ID a shorebird at about a mile distance at 115X that was simply impossible to ID at 72x without the booster. And the Zeiss booster really does have poor transmission and a whole bunch of glass to deteriorate the image, unlike the Kowa extender that has very few glass-to-air surfaces, no prisms, and much better coatings.
But not all birders use their scopes to their max, it depends on the kind of viewing you do, and if you have a Kowa and rarely use it at mags above 50X, you will have no need for the extender.
Kimmo
