Astrophotography Filters

In this article we’ll be looking at the different astrophotography filters and how to properly use them.  These filters are mainly designed for monochrome camera users.  If you have a color astro camera or DSLR, then you won’t get the full benefits of using narrowband or LRGB filters.  You could go for some unique light pollution or sky enhancement filters though.

LRGB and Narrowband

First, let’s look at the two main types of filters – LRGB and Narrowband.  LRGB stands for Luminance, Red, Green, and Blue.  These filters will allow you to capture a full color image with a monochrome camera.  You will need to take a set of photos for each filter.  For example, 100 photos with Red, 100 photos with Green, 100 with Blue, and 100 with Luminance.  Once you have these different sets, you can stack each one.  Once all sets have been stacked to reduce noise, you can assign those stacked photos to specific color channels in Photoshop (or another image processer.)  As soon as the colors have been mapped correctly, your monochrome photo will turn into a full color image!  I’ll cover this process in a future tutorial.

Each one of these filters will only allow a specific range of wavelengths through to your camera.  The Blue filter will let 400nm – 500nm through.  The Green filter will let 500nm – 600nm through.  The Red Filter will let 600nm – 700nm through.  Finally, the Luminance filter will let the full range from 400nm – 700nm through.  If you are using a monochrome camera, all of these photos will be black and white.  You will not have a color image until you map the colors in post-processing.

Narrowband filters are much more precise, they only let a few nm of light through.  This has some great side-effects!  Narrowband Filters will allow you to photograph nebulae even during a full moon, or from a light polluted city!  That’s because these filters block out all the wavelengths of light that normally interfere with our astro photos.  Unlike RGB filters, which transmit about ~100nm of light, narrowband filters transmit between 3 - 12nm.  This very narrow range blocks most light, including light pollution and moonlight, while allowing only specific wavelengths through.

You can normally buy a set of Narrowband filters which includes Hydrogen Alpha (H-Alpha), Sulfur (SII), and Oxygen (OIII).  The Hydrogen Alpha filter will only allow light at 656nm through, which is the color red.  The Sulfur filter will only allow light at 672nm, which is also red.  Oxygen is 500nm, which is roughly blue/green.  These narrowband filters are usually marketed as 3nm, 5nm, or 7nm.  Usually, the smaller the better.  As the range gets smaller, only the specific wavelengths of light from Ha, SII, and OIII will be visible.  As the range gets larger, it’s possible that light pollution or moonlight will start to be picked up.

Keep in mind, these narrowband filters are only letting 3nm – 7nm of light through.  That’s not a lot of light!  Meanwhile, the RGB filters each let 100nm of light through.  Therefore, you will need to take longer exposures with the narrowband filters to make up for the smaller range of light.  Many people online take 10 – 20 minute exposures with narrowband filters!  Unfortunately, my SkyGuider Pro is not accurate enough to do that.  You may need to upgrade your mount to get the most out of your filters and camera.  If you don’t capture enough light, your images will have a lot more grain and other problems.

You might be thinking, how am I going to create a color photo with narrowband filters?  Well, that brings up an interesting point.  Narrowband Filters do not create “true color” images.  Therefore, you can assign these various filters to different color channels in post-processing.  The “Hubble Palette” maps Hydrogen Alpha to Green, Sulfur to Red, and Oxygen to Blue.  It might sound odd, putting H-Alpha to Green, when in reality it’s a red wavelength.  However, according to NASA this was done to better show structure and detail in the nebulae.  You are free to map the various narrowband images to any color channel you like! 

Picture saved with settings embedded. This photo of the Horsehead Nebula was captured using LRGB filters, as well as an H-Alpha filter.  You can watch my full post-processing tutorial here.

Brands

There are quite a few different filter brands to choose from – Optolong, ZWO, Baader, Astronomik, and Astrodon to name a few.  Like anything else, you get what you pay for.  Optolong and ZWO are considered “budget friendly” filters, Astronomik and Baader are usually considered “pretty good, a nice balance between cost and performance”, and Astrodon are regarded as the best filters available.

If you’ve got the money, your best bet is to invest in the Astrodon filters.  These should offer the best image quality.  If you are just getting started though, or don’t feel like spending over $1,000 on a single filter, then any of the other brands should do a good job.

Sizes

Regardless which brand you go with, you need to pick the correct size filters.  The most common sizes are 1.25”, 2”, 31mm, and 36mm.  The 1.25” and 2” filters usually have threads, so they can be easily installed to your filter wheel, camera, or telescope.  The 31mm and 36mm filters don’t have threads, so they must be installed a little differently.  Frankly, I found the installation process to be a major pain.  The threaded filters would be much simpler...

The 1.25” filters seem to be the most common, they are also usually the cheapest filter size available.  Next up we have the 31mm unmounted filters, these will cost a bit more than the 1.25” filters, even though they are basically the same size.  The 36mm unmounted filters will see another price increase.  Finally, the 2” filters will cost the most, since they are the largest in size.

The main reason you would get larger filters is to reduce vignette.  The smaller the filter, the greater the chance that it will cause a vignette in your photos.  This vignette can be removed with flat frames, but you will still be losing some light.  I ended up choosing the 36mm unmounted filters.  My thought process was – “I don’t want to worry about vignette, but I also don’t want to spend all that extra money getting the 2” filters.  I also need to make sure I can fit these filters inside a filter wheel”. 

Picture saved with settings embedded. This photo was taken with my ASI 1600MM Pro, WO SpaceCat telescope, SkyWatcher EQ6R-Pro mount, and ZWO filters (H-Alpha, Red, Blue, and Oxygen)

Mounted or Unmounted?

I want to stress the importance of choosing between "mounted" and "unmounted" filters.  I purchased "unmounted" filters, which don't have any threads.  This turned out to be a huge hassle!  Installing these filters into my filter wheel was incredibly frustrating and took me over an hour.  In the process, I accidentally smudged the filters a bit, and even hit them with my screw.  (I've got big, clumsy fingers...)  One of the hardest parts of the installation was determining which side had the anti-reflective coating.  One side of the filters has an anti-reflective coating, which should be facing the camera sensor.  Unfortunately, I could not tell the difference.  Therefore, I probably installed a few filters upside down.   

The mounted filters, with threads, are much easier to install!  You simply screw them into the filter wheel.  This makes it easy to swap filters, and you don't have to worry about which side has the anti-reflective coating.  The threads are only on one side of the filter, so you'll always install them correctly.  If I could go back in time, I would buy the 2" threaded filters, rather than the 36mm unmounted filters.

If you aren't good with small tools (like me), and you want a simple setup, then I'd highly recommend the "mounted" filters with threads.  This will even allow you to use a smaller filter wheel, like the ZWO EFW Mini.  There are only 5 filter slots in there.  You could pretty easily swap between the LRGB and Narrowband filters if you had threaded filters.  

Picture saved with settings embedded.

This photo of the Heart Nebula was captured with just H-Alpha and Oxygen data

Filter Wheel

Before you purchase any filters, you should also look into a filter wheel.  This is a really nice device that will allow you to quickly swap between your filters, without having to take apart your entire setup!  If you don’t have a filter wheel, you will be installing your filters directly to the telescope or camera.  Every time you want to swap filters, you'd need to take your gear apart and install a new one.  This creates a lot of opportunities for things to go wrong!

There are a lot of different filter wheels to choose from, so let’s break down the key differences.  ZWO makes quite a few different filter wheels, which you can see here.  The main difference between the various filter wheels is what size filters they hold (1.25", 2", 31mm, 36mm etc...), and how many filters they can hold at one time (5,6,7,8). 

Let’s start with the EFW Mini, as this was the filter wheel I originally considered.  The main reason I wanted this filter wheel was the small size, which would pair perfectly with my SpaceCat setup.  I soon realized this filter wheel would not work for me though.  It only holds 5 filters (either 1.25” or 31mm).  I need my filter wheel to hold at least 7 filters – LRGB + 3 Narrowband.  I also wanted to use larger filters, so there was less chance of a vignette.

This led me to the ZWO EFW 7 / 8 filter wheel.  You can choose either the 7 position filter wheel, which takes 36mm filters, or the 8 position filter wheel which takes 1.25” and 31mm filters.  This filter holder has plenty of space for LRGB and 3 Narrowband filters.  Since I only need 7 filters, and I want larger filters, I choose the 7 position filter wheel.  However, if you are using 1.25” filters, then the 8 position filter wheel is a great choice.  You’ll have one extra spot you can use for any other filter you want.

ZWO also makes a filter wheel which can hold the larger 2” filters.  If you want those big filters, this is your best bet.  If I could go back in time, I would probably buy this filter holder along with the 2" filters for LRGB and Narrowband.  It will be more expensive, but should provide great results.

My favorite part of the ZWO filter wheels is that they can all be controlled by the ASIAir app on your phone!  I’ve mentioned the ASIAir before, but this is a great product that should streamline your workflow.  Once you have an ASIAir, you can control your main camera, auto-guider, and filter wheel all from your smartphone!  Instead of going outside and manually changing the filters around, you just click a button in the smartphone app and it will change to whichever filter you want.  I love how this tech all works together!

You don’t need a ZWO brand filter wheel to have this functionality.  There are other manufacturers that allow you to electronically change filters around using your laptop and some software.  However, I like the ecosystem that ZWO has established for all of their products, so that’s what I use.

Picture saved with settings embedded. This photo of the Rosette Nebula was captured with narrowband filters - Hydrogen Alpha and Oxygen.  You can watch my full post-processing tutorial here.

Final Recap

The first thing to decide is what size filters you want to buy.  This will mainly come down to your budget and the gear you plan to use.  Many people start off with 1.25” filters and get great photos!  I personally went with the 36mm unmounted filters, to limit the chance of vignette in my photos.  After the incredibly frustrating installation process, I wish I would've spent more money and just gotten the 2" threaded filters... 

Once you decide the size of filters you want, you should consider which brand to go with.  Astrodon is the best choice, but also the most expensive.  Their narrowband filters usually cost over $1,000 per filter!  I went with the ZWO filters, which are potentially the lowest grade filters available from the list.  My thought process was “I’m a beginner, I don’t know how much I’m going to enjoy this.  I’d rather not spend $4,000 on a set of filters.  The ZWO filters should be good enough quality, and I don’t think they will hold me back for quite a while.  For the price, this is probably a good starting point.”  Astronomik and Baader also make good quality filters for a reasonable price.

After you’ve figured out which size filters you want, along with the brand, you can pick out a filter wheel.  You’ll need to make sure that the filter wheel can hold your specific filters, since they all take different sizes.  Also, make sure the filter wheel has enough spots.  If you want to shoot both LRGB and Narrowband, the filter wheel should have at least 7 spots.  If you only want to shoot LRGB, then a 5 slot filter wheel should be plenty.  

Before you purchase your filters and filter wheel, just do some research and verify that they will both work together.  You shouldn’t have any problems, provided you matched the sizes up correctly, but it’s worth double checking online.  Cloudynights forum is your best bet.