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Cameras: Megapixels

Many video astronomy cameras are low megapixel in size. You might wonder why shouldn't I just get a 12mp monster like the ASI294MC Pro instead of a 2mp camera like the ASI224MC? Won't I get a better image with a 12mp camera? In most cases, no. Most objects in space are very small with a few exceptions. Most will easily fit in the frame of a 2mp camera. There are some exceptions like M31, the Andromeda Galaxy and some of the larger nebulae. But for the most part, the objects tend to be visually small.

Will you see more detail with a higher megapixel camera? Most likely not, you'll just see more of the area around the object. The primary determinant of how much detail you'll see in an object has to do with the pixel size of the camera and the telescope you combine it with.

The sky can be thought of a large sphere that circles 360°. Each degree is made up of 60 segments called arc-minutes and each minute is divided up into 60 arc-seconds. When shooting an image of an object, we want to strive for a particular ratio called arcseconds per pixel and we generally want it in the 1-2 arcsecond/pixel range. That is, how much area of space does each pixel in our image represent. To figure out what your scope/camera combination represents, we calculate it with the following formula:

arcseconds/pixel =

[(pixel size of the sensor in µm) * 206.3] /[focal length of the scope in mm]

So, for example, with my ZWO ASI224MC and my Orion ED80T-CF refractor, it works out to the following:

arcsec/pixel = (3.75µm*206.3) / 472mm = 1.63

So that means each pixel covers 1.63 arcseconds of space.

If I had an object that spanned 7 arcminutes in all directions, I could multiply that by 60 to get it in arcseconds (420 arcseconds) and divide that by 1.63 to arrive at 257.7 pixels. So that means the object will cover an area on my sensor of 257 pixels. So even if my sensor was an 18mp sensor, if the pixel size was the same, my image would be the same size (257x257).

A larger pixel will make the image look smaller (each pixel covers more space) and a smaller pixel will give more detail on a particular image. So, for example if I used the same scope with an ASI290MM (mono, 2.9µm pixel size) on the same object, the resulting image would be as follows:

(2.9x206.3)/472 = 1.26 arcseconds/pixel

420/1.26 = 333 pixels. So with the smaller pixel, the image is now 333x333.

Another reason for accepting a smaller sensor over a larger one is screen size. Assume we have a hi definition 1080p monitor which presents a screen of 1920 x 1080. With the ZWO ASI224MC (2mp), the sensor creates an image that's 1304x976. So the image fills nearly the entire screen. Even an UHD TV screen only presents 3840x2160 pixels. A camera like the ZWO ASI294MC Pro presents an image that's 4144x2822. As a result, it would have to be scaled down to see the entire image.

Lastly, small sensors present a small file size. As a result, it's easier for smaller, cheaper computers to handle (typically a couple of megabytes). A large 12mp sensor creates a large file that can tax systems with limited memory.

So why would you want a larger sensor? If your goto isn't that accurate, you can see a larger section of space to help find and center the object. Also, if you want to create a print, you can make a larger image with a 12mp image. Plus, there are objects that are really large that you may be able to fit in one frame. It's also possible to bin a camera sensor to make it more suitable for certain telescopes.

#videoastronomy #cameras


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