? A wide field system will have a ray from the left side of the pupil making a much steeper angle to reach the right side of the sensor. Of course the field angle matters...
And because the field angle matters, if you are close to the edge of the bandwidth in the center of the field, the increasing loss of signal as you go out will be insidious and not directly noticeable - because the sky background is coming in at full strength - and flats won't correct for the loss of nebulosity signal.
I did not check the math in your equation - but it does need to include the field angle - especially for a fast and wide field system like hyperstar or rasa with a large sensor.
The main thing is - I don't see Astrodon providing a spec. on how accurately centered the passband is - and it should be spec'd for the entire filter area and not just the center. A typical spec. might be +/- 10% of the fwhm - which for 3nm would be 0.3nm. For 3nm fwhm, only the very center is at full transmission - so if you are already 0.3nm off center there would be some loss. And as you continue away from that the loss would increase much earlier than expected, compared to a perfectly centered passband and a rectangular rather than semi-Gaussian profile - and not including the field angle.
These things are hard to make accurately when the passband is narrow - and even harder with a large and square 50mm filter - if you want it consistent across the whole filter. That's why they cost a lot. So they are not expected to be an exact match to the spec - and more info is needed to know how well they would actually perform with a fast and wide field system.
Addendum - for a 280mm f/2.2 RASA with 43mm corrected image diameter, I get a shift of 7.2 nm in the center of the field and 9.4 nm at the edge of field. So - assuming a perfectly centered filter, 15nm bandpass would have full transmission in the center of the field but possibly significantly reduced transmission at the edge of the field, due to one side of the pupil shifting out of the passband.
But the Baader fast filters are designed with the nebula wavelength intentionally shifted to the right in the passband - which leaves more room on the shorter wavelength side to accommodate these steeper rays. So 15nm may be fine for RASA across the full field.
Edited by freestar8n, 28 September 2017 - 08:31 PM.