Mono or Colour Camera?

By Jo on January 20th, 2015

When looking to buy an Atik CCD camera, this is one of the relatively fundamental things you need to consider. There’s a huge amount of debate surrounding the benefits of both mono and one-shot colour (OSC) cameras and there are a huge number of factors to be considered when deciding between the two. We’re going to try and keep things as simple as possible here to give you a basic overview of the considerations involved. Your eventual decision has to be based on your individual circumstances and what you personally want to get out of the camera – what may be right for one astroimager may not be right for you.

You can purchase all Atik cameras in both mono and colour variations, allowing you to enjoy our full range no matter what you decide. The exceptions are the new Atik 4120EX which is exclusively available as a colour camera, and the Atik One with its internal filter wheel for mono imaging.

Mono cameras

You can create colour images with a mono camera by using filters. There are two main ways of doing this. One is to use RGB (Red, Green, Blue) filters and the other is narrowband imaging, most commonly using Hydrogen alpha (Ha), Oxygen III (OIII) and Sulphur II (SII) filters, and sometimes also Hydrogen Beta (Hb) and Nitrogen II (NII). The two techniques may also be combined, for example you may see people using Ha with RGB. It is also common to add a Luminance (L) channel to RGB images to improve on the signal-to-noise ratio.

While OSC cameras also image in RGB (we’ll explain how in a bit), narrowband imaging is exclusive to mono cameras and comes with a number of benefits. Because you are only capturing a very small part of the light spectrum with each filter, you are able to detect more detail and fainter objects. It’s also very well suited to light polluted environments as the filters don’t pass a number of the wavelengths commonly associated with light pollution. Similarly, they don’t pass as much moonlight, effectively allowing you to image throughout more of the lunar cycle.

Narrowband imaging is particularly suitable for emission nebulae, as each filter can detect a different wavelength of light that the nebula is emitting. This is also the case with planetary nebulae and supernova remnants as these are also emitting their own light. However, this method is far less effective when imaging galaxies as they emit light in broadband rather than narrowband. Below is an example of an RGB galaxy and a narrowband nebula. If you have a look at the Atik forum, many of our users post details of the filters they are using alongside their images. This can be a good way to get a feel for the different kinds of images different filter combinations can produce.

: RGB galaxy by Joel Short

: Narrowband nebula by Angelos Kechagias

OSC and the Bayer Matrix

OSC cameras capture colour data in a very different way. They do it by placing red, green and blue filters over individual pixels in a pattern called a Bayer matrix. Processing software can then create a colour image through a process called ‘de-Bayering’.

A misconception often arises here. It’s easy to assume that creating colour images using a mono CCD and filters requires vast amounts of processing, while OSC processing is simple. This isn’t quite the case. While software does exist that will do most of the hard work for you, it’s still a long way from point-and-click. It’s fair to say that although it’s technically easier to capture colour image data from an OSC, to get top quality images from it can require just as much, if not more processing than with a mono camera and filters.

Time saver?

The big question tends to be whether OSC cameras save you time, and the answer is; not really. On the surface it’s easy to assume that because when using a mono CCD you need to take multiple exposures with each filter, you can multiply the time it will take by the number of filters you’re using. For example, to get an RGB image, you would need a set of red exposures, a set of green exposures and a set of blue exposures, which would surely take three times as long as if you could get your RGB data in one go. But it doesn’t quite work like that. Because an equivalent OSC camera will necessarily have a lower Quantum Efficiency, you would need an exposure three times as long to get the same amount of data.

It’s also worth noting that although OSC cameras are capable of binning pixels, this will cause them to lose all their colour data and produce monochrome images.

What you do gain, however, is convenience. Setting up and integrating an OSC camera into your equipment should generally be simpler than a mono with filters. This can make them a fairly popular choice if you’re primary goal is to use it for events such as star parties where simplicity of set up and ease of use can become important considerations.

: M42 and The Running Man by Sungryul Heo

OSC cameras are also much better suited to HyperStar and Fastar systems and this is for two key reasons. The first is logistical. The way the camera is fitted to the telescope leaves very little room to add a filter wheel, or even single filters. The second is more technical. Such systems are designed for fast focal ratios which can actually shift narrowband filters off band, meaning they don’t capture what they’re supposed to and a wider bandpass is needed for them to work effectively. The slim design of the Atik 4-Series makes them particularly suitable for this kind of setup.

Another factor to consider is, though we hate to say it, cost. Although there is little to no difference between a mono and OSC version of the same camera model, it’s worth noting that mono cameras do not come with filters. You’ll have to buy these separately and it’s worth researching this before you commit to purchasing the camera. You may also want to consider a buying a filter wheel such as the Atik EFW2, or perhaps a camera from our Atik One range which comes with a fully integrated internal filter wheel.

Advanced Applications

Mono cameras do have a distinct advantage for some people in that they have applications beyond astroimaging. These include spectroscopy, photometry and astrometry so, for example, the camera can be used to detect transiting exoplanets or to search for supernovae. The Atik 314L+ is well suited to this kind of usage and we expect its successor, the Atik 414EX, to follow suit.

This again highlights that your choice really does depend on what you want to get out of the camera. We hope this introductory guide has begun to give you a clearer overview of some of the key factors that should inform your decision. If forced to say it at its most simplistic, it largely becomes a choice between control and convenience.

Perhaps one of the best things to do is have a look at images taken with both systems and see if one stands out to you. Have a look through the colour images in our gallery – can you even say, with conviction, which are taken with a mono camera and which with an OSC?

Sometimes it’s obvious, but other times even experienced astronomers might struggle to know. This puts the focus firmly back on you – do you want to image in narrowband? What’s the light pollution like in your area? What might best suit your existing setup?