Filter Types

[technofetishism (Nick)]

Hi All,

I've been looking at filters lately (imaging focused, not visual work) and determining what they are used for and the catches with various types and want to make sure I've got them correct in my head.


RGB - Red Green Blue - Additive color imaging with monochrome CCD's within the visual spectrum. Keep an eye however on IR pass characteristics as they may need to be paired with a UV/IR cut filter.

CMY - Cyan Magenta Yellow - Subtractive color imaging with monochrome CCD's within the visual spectrum. Color reproduction accuracy is pretty meh however but they can be faster.

C - Clear - Used for capturing a full spectrum image while maintaining parfocus with other filters. Image may be used for a luminance channel.

L - Luminance (visual) - Clear filter used for capturing a luminance channel covering visible spectral range, Ideally matched to the RGB filter spectrum. IR and UV blocking.

LPS - Light Pollution Suppression - Bandpass filter designed to suppress the emission spectra of common sources of light pollution (Mercury and Sodium Lamps) while still passing Ha Hb SII NII OIII spectra. May or may not be color balanced across RGB and may also deal with IR and UV suppression.

Nebula - Nebula Filter - Similar to LPS, but focused around passing nebula emission spectra rather than just suppressing LPS. Can be used as a nebula luminance channel. May pass IR as well so LPS or IR cut may be needed,

Venus - UV Filter - UV pass filter suppresses visible and infrared light. Mostly used for planetary imaging.

NIR - Near Infrared - UV/Visible blocking filters passing light in the IR spectrum. Used for gathering detail obscured by dust or within bright emission nebula. Can be got at multiple different cutoffs for multi-channel imaging in the same fashion as LRGB.

Narrowband - Type of filter focused on the capturing the emission lines of various elements within nebula. A smaller bandpass increases the detail/SNR but also increases the amount of time taken and can have issues with fast scopes.

H-a - Hydrogen Alpha (656.3nm) - Primary emission line of Ionized Hydrogen. Main contributor to the Red of Nebula. Wider H-a filters will also pick up NII emission line. Also available in extreme narrow band in conjunction with an energy rejection filter for solar imaging.

H-b - Hydrogen Beta (486.1nm) - Secondary emission line of Hydrogen. Basically the same information as the H-a wavelength, but dimmer and Blue.

OIII - Oxygen-III (495.9 & 500.7nm) - Double Ionized Oxygen atoms. Blue-Green/cyan in color. Dominant in Planetary Nebula.

SII - Sulfur-II (672.4nm) - Single Ionized Sulfur atoms. Dark Red in color. Comparatively weak in brightness to H-a and OIII but often used in conjunction with them for the Hubble Palette.

NII - Nitrogen-II (658.4nm) - Single Ionized Nitrogen atoms. Also Red in color. Not common but used in Hubble images at times.

Red Continuum - Bandpass filter used in conjunction with H-a or SII to subtract stars from an emission Nebula image.

Photometric - Defined bandpasses covering set regions of the entire visual, NIR and UV. May or may not overlap.

Hubble Palette - R(SII) G(H-a) B(OIII)

Can anyone see any major errors with that or anything worthwhile to add?

[Terry B]

The only ones missing I can think of are UBVRI and "ugriz" filters. Photemetry filters. BVR filters are similar to RGB filters but with wider bandpass. They can certainly be used for imaging pretty pics.
ugriz filters are Sloan photometry filters that have very defined bandpasses that would probably make good colour filters for pretty pics as well.

[Poita (Peter)]

Maybe the extreme narrowband Ha filters used for solar photography, like the PST etalon for instance, and the energy rejection filters.

[technofetishism (Nick)]

Ahh knew there was another use for H-a that I was forgetting, cheers Poita.

Good point Terry, Would definatly be a false colour image, but still potentially pretty depending.