[avandonk]

Andy I have found for most targets that NII and HA are not much different. There are differences however as the NII seems to have more fine structure. Especially in the faint bits. I have seen differences in many but they are subtle.

There is a school of thought that there is overlap or leaking between even 3nm HA and NII filters.

The main advantage of 3nm filters is that the stars are about half the recorded flux of 5nm filters and the nebulosity is the same.

Even with 3nm filters the Moon can interfere when it is near full. It is far worse with OIII than NII.

My observatory is in Eltham with moderate light pollution so 3nm NB filters are a must. At a dark sky site 5nm are better for 'HA' as most targets are 50:50 HA and NII.

This is all about signal to noise. The fast F3 of my astrograph collects data about 2.7 times faster than F5. This means that thermal noise is down by a factor of 2.7.

A 32 minute exposure at F3 is roughly equivalent to 86 minutes at F5. The F5 image will have more noise.

If you consider the central obstruction, my optic is bit more than twice as fast as an unobstructed APO at F5 assuming it has 100% light transmission.

The results speak for themselves. Bert