A Guide to Astronomy Filters, Sizes and Mounting Options

IC434 Horsehead Nebula Filter
IC434 Horsehead Nebula – Detail using a filter shows incredible detail
Image: Gerald Grech

If you have a telescope and want to enhance your night-time observations, there are tools and techniques you can use. One approach involves using Astronomy filters, which are important accessories that can isolate specific wavelengths of light. This helps to reveal fainter deep-sky objects with greater clarity.

Filters come in various sizes and mounting options and in this blog, I will explain the variations and give you an understanding of the purpose behind the diversity.

Planetary Filters

There are planetary filters available in various colours to assist in observing planets. For instance, using a Yellow #12 filter enhances the view of Mars by improving the contrast between light and dark areas. Similarly, a Blue #80A filter can be beneficial when observing Jupiter or Saturn, aiding in the visibility of Jupiter’s storm belts or the Great Red Spot.

Moon filter
Moon filter

Moon Filters

When observing the Moon, a Moon filter can be employed to mitigate the high luminosity reflected by the Moon. This reduces reflective light which can be harsh on your eyes, and improves contrast of craters and seas. Moon filters can come in a variety of strengths which control how much light is blocked. The greater the number, the dimmer the view will be.

Visual Light Pollution Filters

There are also visual light pollution / skyglow filters that minimise unwanted light interference from urban areas and other sources. Think of it as adjusting the screen brightness to enhance visibility. Astronomers can concentrate on their observations without distraction from surrounding light. Filters like SkyGlow, Oiii (Oxygen) or UHC prove beneficial in revealing the intricacies of nebulae.

What is light pollution and how does it affect astrophotography?

Bortle Scale of the Night Sky
Light Pollution – Bortle scale from 1 to 9
Photo: skyglowproject.com

Light pollution is the excessive and misdirected artificial light produced by human activities that brightens the night sky. This type of pollution has detrimental effects on astronomers and their ability to observe and photograph objects in the night sky.

Measurement of the night sky is measured using the Bortle scale – A nine-level numeric scale that measures the night sky’s brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. 

Light pollution significantly degrades the experience of visual observers by introducing artificial light into the night sky. As urban areas expand, the prevalence of stray light from street lamps, buildings, and other sources creates a luminous veil that obscures the night sky. This compromises the contrast between stars, galaxies, and nebulae and makes it challenging to appreciate the intricate details of what is above us.

Effects of light pollution
Overlighting is a huge problem!
Photo: Astrobackyard.com

The negative effects of light pollution extend to the visibility of the Milky Way, which is often rendered faint or invisible in areas with excessive artificial light. This not only diminishes the aesthetic experience for astronomers but also hampers educational and inspirational opportunities tied to observing the Universe.

Moreover, light pollution disrupts the natural behaviour of nocturnal wildlife. To counter these effects, promoting responsible outdoor lighting practices and advocating for dark sky preservation is essential. By addressing light pollution, we can ensure that the wonder and connection to the universe that a pristine night sky provides remain accessible to current and future generations of visual observers.

In areas with severe light pollution, astrophotographers are forced to use shorter exposure times. This reduces the amount of light captured and can lead to less detailed or dim photographs.

Light pollution also produces colour shifts as the dominant light sources, such as sodium vapour lamps, can cast an unwanted orange or yellow hue on the images.

Astrophotographers may raise the camera’s ISO settings, resulting in increased noise or grain in the images and in areas with severe light pollution, it becomes challenging to capture deep-sky objects like galaxies and nebulae. So the choice of subjects is often limited to the brightest stars and planets.

Light pollution not only impacts astrophotography but also diminishes the overall experience. I hear a lot of people don’t bother trying astrophotography, because the light pollution where they live is too heavy. 

Understanding astronomy filters and how they help with light pollution problems?

Astronomy filters are used with optical devices to aid with the transmission or block certain wavelengths of light, allowing observers to enhance their observations in different ways. 

These filters are instrumental in reducing light pollution, increasing contrast, and capturing specific features or details of deep-sky objects.

Variations in astronomy filter sizes

Filter sizes
There a few sizes of filters to consider.

Astronomy filters come in different sizes to accommodate various combinations of Astronomy Cameras and Telescopes 

Common filter sizes include:

1.25 Inches (Mounted)
Filters of this size are mounted and are suitable for cameras with a very small sensor size.

2 Inches (Mounted)
Larger cameras with much larger sensor sizes often require 2-inch filters.

31mm, 36mm and 50mm (Unmounted)
For advanced astrophotography. Filters come in various diameters, including 31mm, 36mm and 50mm to accommodate specific imaging systems that have larger sensor sizes.

Mounted vs. Unmounted filters

One of the key distinctions with astronomy filters is the choice between mounted and unmounted filters. Here’s a comparison of the two:

1. Mounted Filters:

Mounted filters come encased in a metal or plastic cell that ensures they fit snugly in filter wheels or filter drawers. These filter cells protect the filter and make handling easier and the choice you would make if you intend to swap out the filters often

Mounted filter
Mounted filters
Photo credit: Trevor Jones

Mounted filters are particularly popular in astrophotography setups, as they can be easily swapped out in filter wheels without the need to touch the delicate filter material directly.

They are available in various sizes to match the filter wheel’s specifications.

2. Unmounted Filters:

Unmounted filter
Unmounted filters
Photo credit: ZWO

Unmounted filters are individual glass filters without a protective mount. and are commonly used in filter wheels where you do not intend to manually handle the filters often. So to use them, you need to carefully handle the filter material and secure it in place. While this can be slightly more cumbersome than using mounted filters, they are easier to clean and less likely to cause vignetting

Unmounted filters are available in standard sizes, making them more universally compatible and enables a much lower profile than mounted filters, allowing you to use thinner filter wheels.

The purpose of different filter sizes

The choice of filter size depends on the equipment, the intended use, and personal preferences.
Here are some considerations:

1. Telescopes and Cameras: Matching filter sizes to the telescope’s or camera’s sensor size is crucial. This ensures a secure fit and prevents light from entering around the edges of the filter.

2. Field of View: Larger filter sizes are ideal for wide-field astrophotography and deep-sky imaging, as they reduce vignetting and maintain a broader field of view.

3. Larger filters are used with higher-end cameras and allow you to collect high-resolution data, critical for detailed astrophysical studies.

LRGB filters – which stand for Luminance, Red, Green, and Blue, are commonly used in astrophotography. Luminance filters enhance the overall detail and contrast of the image, while Red, Green, and Blue filters are used to capture colour information, allowing astronomers to create full-colour astrophotographs.

LRGB filters
Unmounted LRGB filters

The types of astrophotography filters 

Optolong L-Ultimate
Optolong L-Ultimate Filter

Multiband filters – are used with colour Astronomy Cameras to capture fine details in celestial objects. These filters are designed to allow specific wavelengths of light to pass through while blocking others. The filters are named based on the number of bands they allow to pass through. Duoband filters allow two bands of light to pass through, while triband filters allow three bands of light to pass through.

Multiband filters are used to capture images of nebulae, which emit light at specific wavelengths. The filters allow the light emitted by the nebulae to pass through while blocking other wavelengths of light that may be present in the sky. This results in images that show the nebulae in greater detail and with more contrast.

The choice of multiband filter depends on factors such as the level of light pollution in the area, the type of camera being used and personal preference.

Narrowband filters – are a specialised type of optical filter used with a mono astronomy camera to isolate and capture specific narrow ranges of wavelengths of emission. These filters are particularly valuable for astrophotography of deep-sky objects, such as nebulae, supernova remnants and galaxies. The key feature of narrowband filters is their ability to block a significant portion of light pollution and isolate the emission lines of certain elements, notably hydrogen-alpha (H-alpha), hydrogen-beta (H-beta), oxygen-III (OIII), and sulfur-II (SII).

Narrowband filter
Narrowband filters – Ha, Oiii and Sii filters
Photo credit: Baade

One of the most popular narrowband filters is the (Ha) Hydrogen-Alpha filter, which isolates the 656nm wavelength of light emitted by hydrogen. This filter is particularly useful for revealing the intricate structures within nebulae and star-forming regions, as it highlights the glowing hydrogen gas. 

The (OIII) Oxygen-III filter, on the other hand, isolates the 500.7nm and 496.0nm wavelengths emitted by doubly ionized oxygen atoms. This filter enhances the visibility of planetary nebulae and supernova remnants by highlighting the oxygen-rich regions within them.

nm filter graph
Typical transmission nanometres of each filter.
Ha, Oiii and Sii filters
Credit: ZWO

(SII) Sulphur-II filters isolate the 672.4nm wavelength of light emitted by ionised sulphur atoms, and they are especially useful for capturing the sulphur-rich regions in various deep-sky objects. These filters are often used in conjunction with hydrogen-alpha and oxygen-III filters to create what is known as the Hubble Palette, a colour mapping technique in astrophotography that assigns sulphur to the red channel, hydrogen-alpha to the green channel, and oxygen-III to the blue channel, resulting in stunning and detailed images of nebulae.


Astronomy filters are indispensable tools for both amateur and professional astronomers. There are different types of filters for both visual and astrophotography use. When capturing the objects in the night sky through astrophotography, choosing the right filter size and mounting option is crucial. By understanding the differences and purposes behind various filter sizes and mounting choices, you can enhance your astronomical observations and uncover the mysteries of the universe with greater precision and clarity.

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