1) The pristine dark sky is as dark as 22 mpsas in the visual band. A B3 sky may be about 21.5.
2) Airglow in the IR is as bright as about 20 (and even worse if your NVD extends to very long wavelengths).
3) Objects are always the same (less extinction). For example NGC 6946 is 23.73.
Now, put everything together.
Case A) polluted sky, say SQM 18.5.
a.1) If you use NV+IR-pass then you have this contrast: 23.7 - 20 (NGC6946). i.e. it is 3.7 magnitudes darker.
a.2) If you NV and do not use any filter it is 23.7-18.5, i.e., 5.2 darker.
Case B) Be sky, say SQM 21.5.
b.1) If you use IR-cut then you have this contrast: 23.7-21.5, i.e., 2.2 magnitudes still darker.
b.2) If you use no filter the contrast is 23.7-20, i.e. 3.7 darker.
b.3) Visually the contrast is 23.7-21.5 = 2.2 (and NGC is faintly glowing)
Comments:
- a.2 has relatively better contrast than a.1. However, at 3.7 magnitudes NGC 6946 still not be visible anyway.
- b.3 is better than both (and in fact NGC6946 is faintly visible)
- b.2 is not visible
- b.1 is similar to b3. for contrast but gives higher resolution.
Conclusion: IR-pass under heavily polluted sky do not compare to dark sky, both visually and with NV+IR-cut. You can see more under dark sky.
Edited by Mauro Da Lio, 14 May 2024 - 01:14 PM.