Astrophotography Filters part 1: Getting into filters

This is part 1 of a series of blogs about narrowband photography. In this introduction part, Bill talks about how – and why – he moved from taking astrophotographs with a plain DSLR to a monochrome CMOS camera with filters.

It’s all about how you want the hobby to develop

Astrophotographers aren’t normally wealthy people. In this hobby, we all have to start with cheaper equipment. Then, we upgrade what we have to what we can afford next. We’re always thinking about our next upgrade, and what it’ll get us for what we have to pay.

I’ve talked about this “upgrade path” at other times. I guess it’s all about using what you’ve got, doing the best you can do with it. At the same time you try to figure out what component is the part that’s holding you back the most. That’s the next target for upgrading!

Then it’s learn, struggle, research and upgrade all over again. I’ve done all of this process, of course. This is the story about my filter upgrade.

In later posts I’ll talk about how the filters work, and then try to explain narrowband and false colour images I’ve taken with these filters.

The start: mount, scope and camera

I started out with a small Schmidt-Cassegrain scope that I’d got for my wife’s science classes. It had an early go-to mount, and I quickly found that by adding a $30 adapter, I could hang my Pentax K3-II DSLR off the end and take photos of the sky. Cool!

Pretty soon, I swapped the mount of the scope to an NEQ6, which tracked for longer periods and gave me better photos. Here it is with the old Cassegrain and the DSLR.

A Meade Cassegrain on an NEQ6

A year or two after that I ditched the Cassegrain for a beautiful refractor. It doesn’t have much magnification, but boy is it a marvellous bit of glass. It’s my main scope today.

A couple of years ago, I upgraded my astrophotography camera from a DSLR to a monochrome CMOS sensor. This produces images with are much higher resolution than colour cameras. A slight downside of this is that now I have to use filters in order to get colour images.

And that’s where this story gets under way.

Filters: red, green, blue – and weird

A filter is a terribly simple thing. It’s just a bit of coloured glass that you put in front of your camera. Of course, manufacturing them is highly demanding – and expensive. Typically, you mount them in a wheel that sits in front of the camera. You can swap filters by turning the wheel, so you don’t have to pull the whole rig apart.

I’ll talk a bit about how filters work in the next part of this multipart blog.

I already had red, green and blue colour filters in a small manual filter wheel. I’d got this in a bundle a couple of years previously with a small monochrome camera (a ZWO ASI120MM). I’d intended to photograph planets right away, but I never got fantastic results. This was because my refractor, at 560mm, didn’t have the magnification I needed. It didn’t matter, in the longer term I’d planned to use it as a guide camera (see – there’s that upgrade cycle again).

Here is the new camera. It’s the large red soup tin at the back of the scope. The black thing that you can see right in front of the camera is the manual filter wheel.

A triplet refractor on the NEQ6 mount

Using this setup, I learned a bit about recombining these three channels on the computer. This gives a colour (“RGB”) photograph. This adds a lot to the complexity of taking astrophotographs. I don’t really mind, as I find it rewarding. I’m sufficiently geeky to enjoy learning these new techniques. Besides, the RGB images I was producing seemed to be a lot more vibrant than the ones from the DSLR.

I’ll talk about processing how photos taken through the filters in a future part of this multipart blog. Simply, you recombine them to make a colour photo.

It wasn’t long before I’d saved up a bit more money. My next upgrade was an electronic filter wheel with 36mm filters. It had enough room for seven different filters, and so I started with red, green and blue. I then got a “luminance” filter, which is nearly a plain bit of glass. Basically, it’s a technical term for “I’m not using a filter”.

Finally, I got three “narrowband” filters. These have weird sounding names, “hydrogen alpha”, “oxygen III” and “sulphur II”.

Using these new filters allows me to photograph in “false colour”.

So why go for narrowband?

Narrowband photos have several advantages over broadband (RGB) photos, and as far as I know, really only one disadvantage.

  • no unwanted light pollution
  • separating the colours increases sharpness
  • no chromatic aberration
  • low light means long exposures

Photos taken with narrowband filters tend to eliminate a lot of light you don’t want. This includes (thankfully) light pollution from cities. This is particularly the case with hydrogen. You can take a photo using a hydrogen filter near a full moon and still get a faint nebula. A DSLR would just give you a washed out mess.

Also, narrowband filters isolate light wavelengths so there’s a big empty gap between them. (I’ll explain this in the next part of the blog). This means that the eventual picture shows up all the interesting little filaments in nebulas. You can see some of this in the photo below. In true colour it can all be a blob of multi-coloured haze.

Another, more technical, advantage of using narrowband filters is that they eliminate chromatic aberration in refractors. This is when light of different wavelengths refracts in the lens by different amounts. Blue light bends more, so it comes to its focus closer to the lens than the red light. So nothing is in perfect focus, and bright stars have ugly blue halos around them. You can focus each narrowband filter differently. So because the light coming through the filter has about the same wavelength, it all focuses nicely.

This lack of chromatic aberration has an additional big advantage. If you’re using narrowband filters, you don’t need to spend thousands of dollars on a super high quality triplet refractor. A big doublet will do just as well. If only I’d known that when buying my 107mm triplet APO…

Here is a photo I took a short while back, of NGC 2736 (the Pencil Nebula). The background and the nebula itself is all narrowband. However, I’ve overlaid the stars from a separate set of broadband exposures because narrowband stars can have funny colours.

NGC 2736 - the Pencil Nebula

The downside of narrowband is that you’re cutting out most of the light hitting your scope. This is like photographing through a chunk of welder’s glass. Honestly, if you hold up a narrowband filter and look through it, it’s really dark. To deal with this, you need to take longer exposures, which is just another challenge. With a carefully set up NEQ6 and an autoguider, I’ve exposed for up to 20 minutes at a time. The real pros go over 30 minutes. I exposed the stars for only 30 seconds on each of the broadband filters. Even then, I think I overexposed them!

Oh, the other disadvantage of narrowband filters is that they’re expensive. Mine are Baader brand, which is relatively cheap, but a high quality Astrodon narrowband filter? That will set you back north of US$700 – for a bit of glass the size of a postage stamp!

Coming up…

In the next part of this series, I’ll explain the way filters work. I’ll also show the difference between broadband and narrowband.

I’ll also explain what photos through these filters look like and why I use a monochrome (black and white) camera.

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, or how to take photos of the sky. He can even help you to see some rare birds.

Share Button

1 Comment


  1. Thank you for the info. Just started learning about AP. Seems like this will be a hobby that will be greatly rewarded with time and patience. Clear Skies.

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *