Filters - Basic understanding

[PKay]

Hi All

Most of you will find this a very basic question, but I wouldn't ask if I didn't know...

When you use for example a Ha filter (656 nm).

Does the filter stop that frequency, or only allow that frequency?

What is the difference between a 5nm and 7nm filter (aside from 2nm)?

[Merlin66]

Peter,
Each filter works by blocking certain wavelengths and transmitting others.
The Ha filter (DSO imaging) is designed to block all wavelengths other than those centred on the Ha wavelength.
If you look at the transmission curve of each filter the transmission width is usually given as the FWHM (Full Width Half Max) in Angstroms or nm. (1 nm =10A)
The narrower the FWHM the less background light (around the target wavelength) from the surrounding area is allowed through - highlighting the selected emission band (ie Ha).
Similar for the other DSO filters - Sii, OIII, H beta etc.
Hope this helps.

[Atmos]

I've got a 3nm Ha filter, it allows 656 +-1.5nm so it allows 654.5-657.5nm to pass through. A 12nm filter will pass 656 +-6nm so 650-662nm.

The tighter the filter the more contrast you get as you're blocking more of the unwanted light.

[sil]

Is it sort of a bell curve relationship? eg using Colin's example 656 +-1.5nm would 656nm wavelength be 100% transmitted (ignoring loss from filter glass etc) and 657.5nm be almost 0% transmitted, say 0.1%, 657.4nm maybe 0.23%. Not in a straight linear relationship, but more akin to a bell curve?

[bojan]

3nm means wavelength 1.5nm away from centre will be attenuated by 50% (3dB down, for RF people)

[JA]

That sort of filter is a bandpass filter (allows a certain frequency band to pass). In astro people often refer to it as a narrowband or line (spectral line) filter. In your example, the centre frequency for that filter has a wavelength of 656 nanometers (nm) and has a bandwidth (width of the range of wavelengths passed) of between 5 or 7 nm, and when specified fully, usually by the manufacturer, is based on a given signal intensity limit such as the 3dB, 6dB or other dB or possibly even %Signal Intensity bandwidth. The 3dB and 6dB bandwidths are commonly used in science, eng. , electronics etc... and for example:

* The 3dB bandwidth is the width of this band from the lower -3dB (~71% signal*) extreme to the upper -3dB (~71% signal*) extreme whereas,

* The 6dB bandwidth is the width of this band from the lower -6dB (~50% signal*) extreme to the upper -6dB (~50% signal*) extreme.

A pic is better and I'll post one if I can find it.

Best
JA

*Percentage quoted is for signal, NOT signal power

[Merlin66]

Here's a typical filter transmission curve ....

[ChrisV]

We often stretch the hell out of our pictures - well I do anyway.

Given that we are boosting small signals relative to larger ones I imagine that the "effective width" of our bandpass filters is a lot more than than the half width stated in the filter specs ? Or maybe the bandpass filters have a really steep slope at the cuff off frequencies so this is negligible ?

[Merlin66]

Google: astrodon filters transmission curves

The attached is pretty typical...

[RickS]

The "stray" signal is mixed with the "real" signal in pixels so they both get stretched together by the same amount. There's no mechanism that stretches the extreme ends of the pass band more than the middle.

Cheers,
Rick.