Filter bandwidth and target velocity red/blueshift

[NickK]

I was thinking, here at SGL7, if the red/blue shift caused by targets moving away (redshift) or towards us (blueshift) would have impact on the amateur imager and their Ha filter. Some questions:

* Given a Baader Ha filter, for example, what velocity range is picked up?

The shift vs velocity is given by:

(λ' - λ)/λ = v/c

Where:

λ = wavelength

λ' = observed (or resulting) wavelength

v = velocity of the target (non-relativistic) in m.s-1 (meters per second)

c = speed of light (3.0 x 108 m.s-1)

Some interesting, commonly used lines:

Hydrogen (Alpha 656.28, beta 486.13nm)

Nitrogen ([N II] at 654.8 and 658.4 nm)

Sulphur ([s II] at 671.6 and 673.1 nm)

Oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm)

Iron (526.96 nm)

Ionised Calcium (396.85 and 393.37nm)

Carbon-Hydrogen bond (CH) (431.42nm)

So, if we take Baader's Hydrogen alpha, there's two filters - one 7nm and one 35.8nm, for fun lets take Astrodon's 4nm Ha too. Assuming an equal spread each side of the line:

(λ' - λ) becomes the difference (wavelength shift) where we now have, for the 7nm example, -3.5 or +3.5 nm. So for the 7nm, this means we see the following:

(-3.5/656.28)*299792458 = v. Where Ha atoms are travelling towards us (minus) at 1,598,820.02 meters/sec*

(+3.5/656.28)*299792458= v. Where Ha atoms are travelling away from us at 1,598,820.02 meters/sec*

* when I say towards/away - that's if we weren't moving too! So it's entirely possible for an object to be emitting Ha but the combination of blue/redshift caused by the target and the observer (earth) would mean it lies outside of the narrowband filter range (7nm)

I would love for someone to check my thinking on this.. but there could be an interesting difference in what an target looks like if a narrowband filter was selected for this.

M42 is moving at 28.9 km/s.. so no problem there!

Applying the same to the 4nm astrodon gives ±913,611.44m/s

Just looking at the estimate of M81's jets being 1000 km/s. So it would be interesting to see the difference between the 4nm and 7nm on those jets.