Look guys (used in the genderless way), I really need to find a high quality 4" refractor of F10 through f15 for my h-alpha solar prominence equipment.
A Unitron would be perfect, or (?). You need an f30 cone of light passing through the etalon filter of an h-alpha system to get correct filter interference and resulting contrast, and a 2x telecentric is really as far as I want to go, maybe stretching it to 3x if I have to. I already have a Chinese Celestron C102 out in the shed that isn't being used, so I don't need one of those. And an APO or ED glass is of little more value in this project because everything is just in one wavelength of light, h-alpha, though they ARE generally of better quality overall.
Surely you have something you'll part with. We can talk trade or buy outright, whatever makes you happy. Please send me an email. I'm on the waiting list at a dozen places for a Unitron or something wlse of long focal length. I may also consider a 5" and retool my pre-filters and white light filter. Thanks, Fred