Choosing a CCD Camera

By Jo on

Astrophotography is an incredibly rewarding hobby, but the sheer volume of different cameras available can make it difficult to choose the right one. In the video, Stephen Chambers goes through some of the main considerations involved when finding your perfect camera. This includes how to match a camera to your telescope, factors influencing field of view and what some of the technical specifications really mean for your images.

 

Video Transcript

One of the questions we often get asked is ‘how do I choose which would be the right camera for me from the 20 or so in the Atik Cameras range?’

So what we’re going to do is look at some of the simple things that we can use to compare the different cameras to see how well they would suit your style of imaging and the telescope that you may have.

Image Scale

Probably the first thing to look at is image scale. This is simply the pixel size (um) divided by the focal length (mm) multiplied by the magic number 206 and the value you get there is the number of arcseconds per pixel. This value should be in the range of one to two, ideally.

(pixel size (um) / focal length (mm)) x 206 = arcseconds per pixel

If you find you have values of greater than two, that will indicate that you are undersampling an image and the stars may start to appear blocky and square. This can be a problem, or you can get round it by slightly smoothing the image afterwards, but in an ideal situation, you would either go for a smaller pixel size or a longer focal length.

The other situation that you may be more likely to come across is if the number comes out to be less than one. That’s indicating that we’re oversampling the image and the image will have large or bloated stars and tracking can become more difficult – that’s when you have a small pixel size and a long focal length. So to rectify the situation, you’d either need to consider a camera with a bigger pixel size, or introducing something like a focal reducer into the imaging train to decrease the focal length.

Field of View (FOV)

The next thing to look at is what field of view will a given telescope and camera combination give you. In other words, how much of the sky will you see? There are lots of good field of view calculators online where you can put in the size of a camera and the focal length of a telescope and it’ll give you an idea of what how much of the sky and what kind of images you can image.

If you have an iOS device, we have our own app available on the app store that will do just this, and it will help you to choose cameras and look at the kind of objects you’ll see well with a given camera and telescope.

So what I’ve got here is a picture of the Horsehead nebula and we’ve got an orange rectangle which is showing us the field of view here and the horsehead nebula’s nicely framed in this image. This is simulated with an Atik 420 which is a camera with a relatively small chip size and the telescope is an ED80 f/7.5 so it’s a relatively short focal length.

If we were to move and change to a much longer focal length telescope, something like the Celestron C11, which is a 2.7 meter focal length, though here used with a 0.6 reducer, then if we look again at what field of view we’d get, you can see we can just about get the Horsehead in this time, so that’s a field of view that’s probably slightly too small for imaging too many objects.

However, if we’ve got a Celestron C11 telescope and we want to try and match it with a camera we should look at choosing a bigger camera from Atik’s range, or a bigger sensor. So if we choose this time the Atik 4000, we look again and we can see the image scale there is much, much better and we’re nicely capturing both the bright star and the Horsehead itself.

So by choosing different focal lengths and different size of sensors we can optimise our setup for imaging different types of object.

Specifications

Next, I’d like to cover three other features you’ll often see listed under camera specifications.

Read Noise

We’ll start off with read noise. This is the amount of electronic noise that a camera will add to an image while it’s being captured. Obviously, we want this to be as small as possible and it’s really our job as camera designers and camera manufacturers to make sure that our cameras add the smallest amount possible, and this is what we do, so some of our cameras really are class leading and they have read noises of around 3 to 4 electrons.

Where this becomes particularly useful is when you’re imaging with very low sky backgrounds. So this is something like narrowband imaging, or imaging at longer focal lengths in very dark sky locations. Here, in particular, it becomes important to consider read noise and that will allow us to get the very best sensitivity from these very weak light sources that we’re imaging.

Cooling

After read noise, we have cooling. All CCD cameras for long-exposure astroimaging will require cooling in order to reduce the thermal noise and hot pixels. The amount of cooling a particular chip requires is very dependent on which actual chip it is. Chips like the older Kodak sensors need around ten degrees more cooling than the newer Sony sensors. Our cameras have cooling capacities from between 25 degrees all the way up to about 40 degrees cooling below ambient. All of our cameras have cooling that is what’s required for the type of sensor that’s being used for long-exposure astroimaging. So whether or not you see a cooling of 25 degrees or of 45 degrees, it’s been optimised for the type of chip that the camera has and optimised for astrophotography.

Quantum Efficiency

Finally, we have Quantum Efficiency. This is the ability of a sensor to convert a photon into an electron, and that’s really what the sensor is there to do. If we have a fairly average CCD, it may have a peak Quantum Efficiency of around 50% and one of the very best front-illuminated Sony sensors will have a Quantum Efficiency of nearly 80%. The thing to notice there is there isn’t a huge difference in those two numbers and the impact they’ll have an image if you compare things like pixel size and focal ratio of telescopes.

Where Quantum Efficiency does become particularly important is if you’re interested in imaging at the longer wavelengths, things like hydrogen alpha and sulphur, narrowband imaging, or imaging nebulae, then the new Sony EXview CCDs have very good Quantum Efficiencies in the longer wavelengths and will give a much stronger image than some of the non-EXview sensors.

The final attribute of a camera that I think we should consider before purchase would be the price of the camera. In the Atik range, you’ll find everything from entry-level cameras right up to more expensive cameras. It’s important to set a budget, and then look for a camera that fits that budget and also suits your style of imaging and the telescope(s) that you’d like to use the camera with.

And then?

Then you can enjoy astrophotography. It’s an absolutely fantastic hobby. It’s extremely rewarding, you’ll be able to take pictures of objects that are way fainter than you can ever see through an eyepiece and detail that you’ll never see through an eyepiece, and also you get to share those pictures with friends, family and online.

I hope you’ve found that useful, and thanks very much for watching (or in this case, reading).


View Products

Still unsure? You might also like to have a look at our introductory guide on choosing between a mono and a colour camera, or perhaps have a look through our gallery to see what camera and telescope combinations other Atik photographers are using.