Apropos the vignetting issue, I removed the TE-11WZ from the scope and at the 40x marking the field stop started to blur. This increased up to the 60x setting, where there was a darkened periphery making up perhaps 20% of the radius. Viewing terrestrially with the eyepiece back on the scope this was no longer apparent except fo a somewhat fuzzy field stop.
In view of the otherwise excellend performance of the TE-11WZ with its wide AFOV (see: https://www.birdforum.net/showpost.php?p=3753347&postcount=58), flat field and negligible astigmatism, I don't regard this as a serious fault.
Btw, the 883/884 is assumed to have a focal length of 500 mm, so I found it a little confusing that the Kowa specs for the 500 mm telephoto lens show magnifications of 28-66x with the TE-11WZ. This would suggest that the FL of the 883/884 is actually shorter, around 455 mm. However, when I compared the same astro eyepiece on my Swaro ATM 65HD (460 mm FL) and the 883, the latter showed a significantly narrower FOV, so I think the 500 mm figure is about correct. Explanations anyone?
John
In view of the otherwise excellend performance of the TE-11WZ with its wide AFOV (see: https://www.birdforum.net/showpost.php?p=3753347&postcount=58), flat field and negligible astigmatism, I don't regard this as a serious fault.
Btw, the 883/884 is assumed to have a focal length of 500 mm, so I found it a little confusing that the Kowa specs for the 500 mm telephoto lens show magnifications of 28-66x with the TE-11WZ. This would suggest that the FL of the 883/884 is actually shorter, around 455 mm. However, when I compared the same astro eyepiece on my Swaro ATM 65HD (460 mm FL) and the 883, the latter showed a significantly narrower FOV, so I think the 500 mm figure is about correct. Explanations anyone?
John