Hi Frontier
The Denk 21's sound like they have some good features for binoviewers.
Quote:
Originally Posted by Frontieroptics
As you have said, Denkmeier binoviewers have a clear aperture of 26mm, The Denkmeier 21mm eyepieces have a field stop of 24mm. This allows the field to be fully illuminated to the edge without any distortions.
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Binoviewers have a conical light cone through them not a parallel one so there is always significant vignetting from the nosepiece, but if it can be kept at least 70% lighting at the edge of the eyepiece then cosmetically it is acceptable in the sense that the vignetting will not be obvious to the casual observer.
A quick `thumbnail' calculation suggests that putting for example an F10 cone ( either native F10 or an F5 scope with 2X OCS ) througha 26mm aperture binoviewer with a typical path length of 115mm yields a *fully illuminated field of around 15mm diameter* at the focal plane. Clearly the edge illumination must be better than 70% if you are not seeing any vignetting with a 20mm eyepiece.
Regards