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Very interesting indeed.
If I understood correctly you split the all light in two paths. Transmission for your spectroscopy and reflection for guiding.
There is no splitting function of the wavelength (Visible, NIR for instance). It is not a dichroic BS, just a classical BS.
If so, with the IMHO 20/80, you use only 20% (reflection) of the incoming light (all wavelengths) for guiding, yet you can do so with a >10mag star.
Am I correct?
What is your guide star exposure time?
On the other hand the ONAG splits the light in function of the wavelength, acting as dichroic filter. It is another time of splitting.
>95% of the visible (<750nm) is reflected toward the imager, while >90% of NIR (>750nm) is transmitted toward the guider.
Unfiltered CCD/CMOS sensors are quite sensitive in NIR, since for most stars there is large NIR contribution and because we work with the same F number than the scope we can easily track with 11mag stars.
I do not have a good experience with pellicle standard BS, they do not handle well large temperature swings over time, nor dust. My equipment is always outdoor (under a dome). In summer the temperature can reach 40C, in winter -20C in common. Over time the membrane becomes lose and you can see some microscopic wrinkles.
Good dichroic pellicle BS are very expensive too.
Do you have some experience over a long time, large temperature range for those?
Maybe you do not need a very good star image for spectroscopy, the requirements are different than for imaging I would imagine.
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