A narrowband filter is an optical filter used in astrophotography to isolate specific wavelengths of light emitted by certain astronomical objects. Unlike broad-spectrum filters that allow a wide range of wavelengths to pass through, narrowband filters are designed to transmit only a narrow range of wavelengths, typically associated with specific emission lines.

Narrowband filters are particularly useful for imaging nebulae, as they can isolate the light emitted by ionized gases, such as hydrogen (H-alpha), oxygen (OIII), and sulfur (SII). These emission lines are commonly found in nebulae and represent specific transitions in the energy levels of the ionized atoms.

By using narrowband filters, astrophotographers can capture and emphasize the details and structures of nebulae while minimizing the impact of light pollution and other unwanted sources of light. This is because narrowband filters selectively transmit the desired wavelengths while blocking other wavelengths, including those from artificial light sources or natural skyglow.

Typically, narrowband filters are used in conjunction with monochrome cameras, as they allow for precise isolation of specific wavelengths for each color channel. By acquiring separate images with different narrowband filters, astrophotographers can later combine and assign specific colors to each emission line during the image processing stage, resulting in vibrant and detailed representations of nebulae.

A duo band filter, also known as a dual-band filter or dual-bandpass filter, is an optical filter used in astrophotography to capture specific wavelengths of light emitted by certain astronomical objects. It is designed to transmit two narrow ranges of wavelengths simultaneously, allowing for the capture of multiple emission lines or features in a single exposure.

Unlike narrowband filters that isolate a single emission line, duo band filters are designed to capture two specific emission lines simultaneously. For example, a popular duo band filter is the H-alpha and OIII filter combination. It allows the transmission of light at the wavelengths corresponding to hydrogen-alpha (H-alpha) and oxygen III (OIII) emission lines. These two emission lines are commonly found in many nebulae, and capturing them simultaneously can reveal unique and detailed information about the nebular structures and processes.

Duo band filters are often used in astrophotography setups where color cameras or one-shot color (OSC) cameras are employed. They allow astrophotographers to capture detailed images of nebulae with a single exposure, providing the convenience of capturing multiple emission lines without the need for multiple filters or multiple exposures.

By using duo band filters, astrophotographers can capture the distinct features and structures of nebulae associated with the selected emission lines, resulting in stunning and detailed images with enhanced contrast and depth.