I purchased the svbony 3-8mm zoom to use in the OB bt 127mm sd but unfortunately they will only come to focus for short range observing. I know people have had success with the tv 3-6mm zooms in these particular bt’s so was hoping to save some $$ on the chance the sv215’s would work. Is there anything I can do or get so that that will come to focus for infinity observing, or should I just return them as I only received them today?
Svbony 3-8mm zoom focusing
Posted 22 March 2023 - 04:23 AM
Posted 22 March 2023 - 04:46 AM
IIUC the Nagler zoom has the field stop at F=6mm while the SvBony zoom has it at about F=-5mm. IIUC the Oberwerk can only handle in the range of F=6...-2mm, but this limited range could be shifted during purchase.
- Rich V. likes this
Posted 22 March 2023 - 08:04 AM
I want to say that I am an amateur. I have had similar issues with zoom eye pieces. What I have done that has worked for me is to get a fine focuser and use it along with the zoom and combined I was able to use the zoom. A spacer may be able to achieve the same result as I have done that successfully also.
- sevenofnine likes this
Posted 22 March 2023 - 02:02 PM
Like ihf points out, you don't have enough inward focus range to make up for that last millimeter you need for infinity focus. BTs don't have much backfocus, as the eyepieces need to be held close to the rear prism face for full illumination.