Choosing the Right Astrophotography Filters for Amazing Images

For any astrophotographer looking for the perfect shot of the night sky, selecting the right filters can often be a confusing or daunting task. By understanding which filters are best suited to various astronomical objects and situations, you can easily capture beautiful images of deep space with your camera. In this guide, we’ll discuss the basics of choosing astrophotography filters to help you get amazing astrophotos!

Why do we use Astrophotography Filters?

There are a number of reasons why we might want to use filters for astrophotography. These include:

• To help bring out contrast and detail in your photos, enabling you to capture the night sky in its truest form.
• Different filters can enhance various astronomical objects resulting in stunning images or improved observation.
• These filters are used to block out certain wavelengths of light, allowing you to bring out features that would otherwise be difficult or impossible to see.
• One of the most common reasons why filters are used for astrophotography is to minimize the effects of light pollution on your images.

Types of Astrophotography Filters Available

Astronomical filters come in many different types. Some of the most common astrophotography filters include light pollution filters, narrowband filters, and optical band-pass filters. 

Light pollution filters help reduce the amount of light entering your camera from artificial sources such as streetlights, restricting certain wavelengths to reveal hidden objects from your night sky photos. There are two main kinds of artificial light that affect astrophotography, traditional sodium street lights which are very orange in colour, and more modern LED lights. Therefore, light pollution filters must be effective at blocking both of these wavelengths of light.

If you’d like to know more, read this page I’ve written which explains light pollution and the effects it has on astrophotography.

Please note that no filter is perfect and so different filters that claim to do the same thing may give quite different results. I have also seen some, such as Cuiv the Lazy Geek, a popular astrophotographer on Youtube, claim that filters often vary in their quality and so two filters of the same type from the same manufacturer may have slightly different cut-offs for various wavelengths they are supposed to block.

Narrowband filters block out specific frequencies of light in order to enhance faint deep sky objects. How they do this is to pass one, two, or three wavelengths of light corresponding to those commonly seen in nebulae and other targets such as galaxies. 

Narrowband filters usually pass only Hydrogen Alpha, Oxygen III, and Sulphur. Note that these are names of gases present in the deep sky object. These gases create different wavelengths of light which is why one object appears red if it has Hydrogen Alpha, or Green/ Blue if it has Oxygen III. Sulfur is also red.  

Optical band-pass filters may enhance contrast for planetary photography.

There are also sky glow filters suitable for Moon glow and to reduce glare.

Another kind of filter is used to block Infrared or UV sources as cameras are often sensitive to these but they are not necessary for astrophotography usually.

Depending on the type of Astrophotography you’re doing, you may want to consider investing in adjustable astrophotography filters. These particular filters sit in a filter wheel and can easily be switched depending on what wavelength of light you need to be enhanced or reduced. 

Many serious astrophotographers use monochrome cameras with colour filters in a filter wheel. They do this because a mono camera is usually more sensitive because its photo cells are not divided into Red Green or Blue, however, to get a colour image it is necessary to image with at least three filters one for Red, Blue, and one Green, and then combine the three images together. A colour camera is much more convenient and needs no colour filters because they are integrated into the sensor and cover each photocell.

How to Select the Right Filter for Your Astrophotography Needs.

Choosing the right filter for astrophotography is important, but difficult. It depends on the type of objects you are looking to capture and the equipment you use. 

Consider your camera’s sensitivity, focal length, and field of view – these factors will determine which filter is best for your needs. 

Additionally, take into account the amount of light pollution in your area as well as basic knowledge about wavelength frequencies to narrow down which type of filter would be most suitable for taking beautiful photos of the night sky.

Here are some guidelines which I have learned doing deep sky astrophotography:

Emission Nebulae

The best filters to use for emission nebulae are either broadband RGB filters or Narrowband filters, especially Hydrogen Alpha. A good idea is to image in RGB and then add data captured with a Ha filter and then add this to the image to highlight the area of hydrogen gas in the nebula.

Narrowband filters with Oxygen iii and sulfur can also be used to capture these wavelengths that are also sometimes present. Particular targets may have more or less blue oxygen light than others. 

Reflection Nebulae

Narrowband filters are not a good idea for reflection nebulae because some of the reflected light will be blocked. Therefore, a broadband filter can be used so that a wider spectrum of light can be captured. This will make for a much better image.

Galaxies

These are broadband targets, so a broadband RGB filter should be used. However, I have imaged a number of galaxies, such as M81 and M82 in Narrowband and then added this to an RGB image.

M82 is the Cigar Galaxy and my image below shows it in Narrowband. There is quite a lot of hydrogen alpha in this galaxy and other galaxies are improved by some Ha imaging. At least this is what I have found adding Ha to my galaxy images. So it is well worth taking extra images specifically with a Hydrogen-alpha filter and this will highlight nebulae that are present in the galaxy you are imaging. 

It has taken me quite a bit of trial and error and experience to understand which filter will work best with different deep sky objects and this is why I can advise the above. Testing with your own equipment is always the best way to learn how to use filters as you cannot always trust the theory alone. Astrophotography is highly experimental and definitely creative!

With practice, understanding how different types of astrophotography filters work will help you capture breathtaking photos of the night sky!

Different Methods of Applying Filters in Astrophotography

There are several different ways to apply astrophotography filters in order to capture amazing night-sky photos. The method you use will depend on the type of filter, as some filters must be placed between the telescope’s eyepiece and camera lens while others need to be attached directly to the camera’s lens. Some astrophotographers even combine several types of filters for more detailed results. Be sure to have a reliable filter system and know how to use it properly before venturing out into the night sky with your camera!

To begin, make sure you have a set of filters that may range from simple nebulae filters to astrophotography pass-band filters and light pollution reduction filters—often times these sets can be very specific for different types of telescopes. Once you have sorted through your filters and identified which ones will be most useful for this particular shot, you’ll need to determine the proper method of implementation. 

Some filters, such as nebula or band-pass filters, can be inserted either on the camera lens or between the sensor and lens. I have found clip-in filters quite convenient for use with a DSLR. When I attached my DSLR to my telescope focuser I used a clip-in filter in front of the camera sensor or screwed a small filter to fit between the focuser and camera sensor. In this case, the telescope becomes the lens. See more here about how to do astrophotography with a telescope.

Now that I am using a dedicated astro camera, (ZWO ASI533 MC PRO), I screw a filter to a nosepiece that attaches in front of the camera sensor and then this fits into the telescope focuser. I use either of these two filters with this camera:

• An Optolong L Pro broadband filter
• A ZWO Duoband 15nm filter captures Ha and Oiii and has a bandpass of 15nm.
• I am now also experimenting with using no filter at all with reflection and galaxy targets.

Comparing Images I’ve Taken with Different Filters and the Lessons I’ve Learned

Like everyone else who ever tried astrophotography, I made my share of mistakes at the beginning. Several of these mistakes involved my use of filters. It was only by making errors and trying out different filters for different situations that I managed to learn how to choose the right filter.

So in this section, I’ll show you some of the results of imaging I have done on the same target with different filters and what the results mean, in my opinion. I hope that this will help you when you want to make your own decisions and choices in your own situation and with your own equipment which might be very different from mine.

Bode’s Galaxy Image in RGB 

From the above, you can see that using the RGB filter collected much more light and gave an overall satisfying image of the galaxy. The below image of Bode’s Galaxy, taken with a ZWO duo band filter gave a very different result. I think I also like the second narrowband image because there are highlights where nebulae glow with hydrogen-alpha and several details become apparent in the arms of the galaxy for example. There is also quite good colour here.

Bode’s Galaxy Image in NB 

Astrophotography is very subjective and each image produced carries the style and preferences of each individual creating it. Filters are another way that we can form the kind of image we want to see. I like subtle colours rather than over-saturated candy-coloured astropics as we often see online. I feel they are more natural and true in depicting the marvels that lie awaiting our discovery above.

So, remember, try different filters and experiment until you understand how to get the results you want. Good luck with your imaging adventures!