I’ve invested in some colour filters and a filter wheel for my astroimaging setup so I’m going to be posting some colour astroimages up over the coming months, I’ve already posted my first LRGB image of M27 into the gallery.
I thought I’d take a few words to explain how this works. The CCD camera I have is monochrome, these are generally better then singleshot colour cameras as they have a higher resolution and there are no filters in front of the chip. So to get monochrome images it’s just a case of capturing multiple exposures and stacking them in software to increase the signal to noise ratio. To get colour you need to take monochrome images through red, green and blue filters. The filters are very precisely made so they only pass through the correct wavelengths, they also block any infrared light which the cameras are sensitive to and can cause problems. They are also manufactured to ensure that they focus the light from the telescope to the same place, so you don’t have to refocus when you change filters.
The filters are held in a filterwheel, this is a mechanical device driven by batteries that rotates the filters into the lightpath at the push of a button. So there’s no requirement to dismantle the setup to put in the next filter.
So what is LRGB? An LRGB image is made up of Luminance data (monochrome), Red, Green and Blue data. What you do is capture a lot of high quality monochrome data. This provides all of the detail in the final image. You then capture some data through each of the coloured filters, this data can be with much shorter exposures and far lower quality. This colour data can then even be binned, i.e. each square of 4 pixels is summed together to make 1 pixel. You can also blur it with a Gaussian blur filter to reduce colour noise in the final image. Software is used to combine the three images taken through the coloured filters into what looks like a blurred, low resolution colour image of the object.
Now comes the clever bit, the human eye is really good at picking out detail in monochrome images, it’s rubbish with colour. So what you do is layer the colour behind the monochrome (luminance) data. Lo and behold you have a high resolution colour image!