It's hard to see anything other than the sun visually with the BF10 in place, however, it's dangerous to see the sun without the BF10. That makes it hard to measure its optical path length. Before the test today, I just use a mirror diagonal as a very rought approximation.
This morning, I attempted to use my DMK to do this job. Don't get me wrong, I will not use unfiltered sun for measurement. Instead, I use far away terrestial target with long exposure.
With the BF10, I set the exposure long enough to show the object on screen, guess what, with maximum gain, the exposure requirement would be like 1/4s. Without BF10, I use normal day time exposure.
So, the measured optical path length is around 7.3cm.