I have stopped down my large telescope to f/10 (or 80 mm), but I can hardly make out any details. Do I need special eyepieces, or is my SunDancer defective?

The 3x telecentric system allows you to achieve very high magnifications or very small exit pupils very easily. A good guideline for the maximum magnification for H-alpha is an exit pupil of about 0.7-1mm, or a magnification that is not very much higher than the telescope aperture in mm. Due to the limitation to only one wavelength, the rules of thumb for observations in white light no longer apply! We were able to use a SunDancer on an 80mm refractor up to about 100x, and a few minutes later on a triband C8 at more than 200x – so before air turbulence became the limiting factor, magnification was limited only by the available aperture and not by the filter itself.

With telescopes with long focal lengths (especially when stopped down, resulting in slower focal ratios), you can quickly achieve exit pupils of less than 1mm due to the 3x telecentrics. Under certain circumstances, a 20-30mm eyepiece may already provide the highest usable magnification.

Fortunately, you don't need special "H-alpha eyepieces" - our Hyperion eyepieces reproduce the H-alpha line (which is also important for gas nebulae/deep-sky-observations) well and are just as suitable for deep sky as for solar observation. The scope of delivery of the 31mm and 36mm Hyperion Aspheric 2" eyepieces also includes 1.25" nosepieces (originally intended for use with a bino-viewer). The Hyperion eyepieces are not only recommended by us for H-alpha observing (and of course also for deep sky - unlike special "H-alpha eyepieces").

For more on eyepiece and magnification selection, see e.g. these threads:

https://www.cloudynights.com/topic/629655-h-alpha-eyepiece/?p=8772056
https://www.cloudynights.com/topic/808040-best-eyepiece-type-for-halpha-solar-observing/?p=11651756
https://www.cloudynights.com/topic/765356-getting-started-in-h-alpha-solar-limited-options-questions/?p=11017690
https://www.cloudynights.com/topic/265312-eyepieces-for-lunt-ls60tha-b1200/?p=3371594

and in German:
https://forum.astronomie.de/threads/hilfe-zum-baader-sundancer-ii-gesucht.324786/

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