Attn BORG 7108 Users & Takahashi PM-1

BORG 7108 is well designed as multi 1.08x field flattener. But one of most common questions were how to set a filter with 7108. It was a bit challenging. Recently I has realized it has 40.5mm filter thread(!!) in front of the lens barrel. See below.


If you are looking for the filter solution with your 7108, get a 40.5 – 48 (or 52) step-up ring, so that you can set 48mm or 52mm filter in front of 7108.

Keep in mind, 52mm filter frame is a bit larger than 2″ focuser’s drawtube inside. You have to add an extension tube (if your system has extra inward room) to avoid interference. BORG 7835 – 2.7″ helical focuser or larger ones are OK.

As I announced, Takahashi PM series have been discontinued. No stock any more either. That’s sad since I receive very positive feedback from all of PM-1 users. Yes, I fully agree. I like PM-1.

Finally I completed PM-1 setup after adding 2nd counterweight. A cooled 6D was too heavy to use with. Probably this setup with 2 counter-weights should allow to use with 90FL F4 if a camera is not heavy.


This allows for the precision electric polar alignment and dithered imaging. Tak original polar scope was well set at the factory. But my old eye makes harder and harder. Since the polar alignment is precise, I don’t think I need DEC correction except dithering in this focal length.

One of my favorites is the mounting coupler, which allows me for quick set-n-release.





~ by tedishikawa on May 6, 2019.

One Response to “Attn BORG 7108 Users & Takahashi PM-1”

  1. Thanks Ted! This is excellent information. I bought two step up rings and they did not fit. Does anyone make one that size? We will see. Arthur

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