Broadband & Narrowband Filters for Colour Cameras
Deep Space Astrophotography Filters for Colour Cameras:
In this article we'll look at some of the popular filter types available for use with colour cameras - both DSLR & Colour Astrocameras. Modern astro filters can be seen as real game changers particularly for astrophotographers based in the city and for most astrophotographers, building up the right filter set is a must have for any serious astro-imager.
Light Pollution Filters (Broadband Filters):
Light waves travel through the air at various frequencies just like radio waves and this frequency changes depending on the light source. The reason light pollution filters are known as broadband filters is because they allow light across a broad range of light frequencies to pass through the filter onto the camera sensor while blocking out just a small amount of unwanted light frequencies (ie. from street lights etc.)
There are many good light pollution filter manufacturers out there and one we recommend is the Optolong range. Optolong not only make great light pollution filters, but also dual narrowband filters, which we'll go into later below. The Optolong L-Pro is Optolong's light pollution filter that is available in both 1.25" and 2" mounted sizes or as a clip in type which clips in front of your camera sensor (Note: these are camera specific).
Tip: When buying filters, we would always recommend getting the mounted type filters because long term they make much more financial and practical sense. The clip in types are designed to clip just in front of your camera sensor on the camera body so if you have a Nikon, you need a Nikon one and likewise for Canon and other brands too. However the mounted types are designed to attach to your telescope, either on the corrector, focus tube or the T-Adapter, meaning you can use the same filter regardless of which camera you have, even if you change camera or upgrade later on to an Astrocamera.
Performance wise, the Optolong L-Pro filter is a really excellent filter for imaging from light polluted skies, and it also has very little impact on natural star colours, something which many other regular light pollution filters can struggle with.
In the image below we took two 150sec photographs of the Orion Nebula from Bortle 8 skies using a standard DSLR camera. In the first image we had no filter and the pink/orange glow from the city light pollution is very obvious, the stars are washed out and the nebula is just about visible. In the second image we took the same photo moments later with the same camera using the Optolong L-Pro filter and the difference is quite dramatic. A lot more stars are visible for one, and the background sky and stars are less washed out and the nebula itself shows a lot more detail, contrast and colour.
The image above will give you a good practical idea of how a good light pollution filter like the L-Pro can really help bring out the details in your astro images, even if you live in a large city like Dublin. The L-Pro filter is also perfectly fine to use on all deep space targets, including Galaxies, Reflection and Emission Nebula. Its also suitable for use with standard (unmodified) DSLR cameras as well as dedicated cooled OSC Astrocameras.
Optolong L-Pro suitability:
Camera Types: Colour Astrocameras and DSLR Cameras.
Galaxies: Yes
Emmision Nebula: Yes
Planetary Nebula: Yes
Reflection Nebula: Yes
Narrowband Filters:
Unlike broadband filters which allow a large range of light through, narrowband work the opposite way and only allow a small range of light frequencies through
while blocking out the majority of all other light sources.
So why is this important? Well most nebula in space (with the exception of reflection nebula) contain large amounts of Hydrogen gas as well as Sulphur and Oxygen - Hydrogen & Oxygen being the dominant ones. By allowing the light from these sources only to pass through, and blocking everything else you are going to be left with a much cleaner and sharper image, even on a moonlit night and from heavily light polluted cities. Its important to keep in mind that while these filters work exceptionally well on a wide range of targets eg. emission nebula, planetary nebula etc., you need to pick your target correctly as some nebula like dark nebula or reflection nebula will simply not be visible through these filters. These filters are also not suitable for Galaxies unless you are using HA data for example, to combine with a regular broadband image, to highlight the areas of nebulosity in the galaxy.
There are 2 popular Optolong narrowband filters in the range (also called Dual Narrowband Filters). These are the L-eNhance & the L-xTreme filters and both are designed for use with colour astrocameras. The main difference between the two is that the L-eNhance is less aggressive than the L-eXtreme filter and it allows HA, Oiii and H-beta light to pass through. Its very effective from light polluted areas and will make a considerable difference to your nebula photos, leaving them much cleaner with lots of detail and contrast.
The L-eXtreme is also a narrowband filter but works at a much tighter bandwidth than the L-eNhance and so its very restrictive as to what light passes through it. It allows only HA and Oiii light sources to pass through at a very small 7NM range and this results in outstanding clarity and detail even from the most light polluted locations. As the L-eXtreme allows such a small bandwidth of light to pass through, you will find the image is very dark when using it and so you will likely have to increase your camera gain and/or exposure times. You will also find the stars are smaller and sharper (which is great!) but you will loose the natural star colour unless you combine it with an RGB version to create a composite image.
The other thing to bare in mind is that these narrowband filters are not suitable for unmodified standard DSLR cameras, as many of these cameras have a built in filter which tends to block out the light that these filters are passing through, so we recommend only using dual narrowband filters with an Astromodified DSLR or a dedicated colour Astrocameras.
The image below is an example of the L-eXtreme filter in action. These images were taken from Dublin of the Rosette Nebula (a nebula rich in Hydrogen gas) and as you can se from the filtered version with the L-eXtreme, there is a lot less noise, much better detail and stronger colour.
Optolong L-eNhance/L-eXtreme suitability:
Camera Types: Colour Astrocameras and Astromodified DSLR Cameras only.
Galaxies: No (unless using as a layer with an RGB image)
Emission Nebula: Yes
Planetary Nebula: Yes
Reflection Nebula: No
So in summary if you are using a either an Astromodified DSLR or a colour astrocamera then we would definitely recommend both the L-Pro and the L eNhance or L-eXtreme filters as part of your astro imaging kit as these will give you lots of options across a very large range of deep space targets, even from cities. However, if you are using a standard DSLR then we would recommend just sticking with the L-Pro as this will work very well with immediate and noticeable improvements in your astro images.
If you would like to know more about these filters or would like some help choosing a filter for your specific setup, feel free to contact us here. or if you would like to view our range of filters Click here >
Happy Imaging & Clear Skies :)
