Flats and CCD Imaging
It can be said that imaging with a telescope and a CCD detector is like searching for a needle in a haystack with the needle being the signal, the precious stream of ancient photons streaming towards your set up and the haystack being the noise which like the hay in the haystack will work to hide, obscure, distort, and dilute the signal.
You can also think of it as an archer shooting an arrow at a target. A good archer under good conditions will place his arrows in a small area in the center of the target. A poor archer will place his arrows with less certainty on the target and they will spread out to a larger area or perhaps miss the target altogether. Uncertainty is another name for the noise which haunts our images.
Replace the arrow with a stream of photons coming from a distant star. Ideally this stream of photons will come in a straight line, enter our optical system single file and strike our CCD detector on one pixel. Why is this almost never the case? We can all probably recite a litany of causes, from the diffraction, and absorption within our turbulent atmosphere to imperfections and absorption within the optics to tracking errors due to imperfections in our mountings and tracking systems.
Luckily solutions exist to help mitigate these problems and the one I would like to discuss now involves correcting distortion due to imperfections in the optical system. Ideally if you took an image of an evenly lit surface through your telescope the image would be a flat grey, however that is hardly ever the case. All telescopes produce a circle of relatively even illumination and outside of that circle the illumination begins to decrease causing vignetting. The problem comes in when the circle of even illumination is smaller than the CCD chip on your camera. In that case your images will be bright in the center with the brightness decreasing towards the edges.
The solution to this is a Flat Field or simply a Flat and it involves taking an image of an evenly illuminated surface with the camera and telescope set exactly as it was when taking the raw astro images. The Flat images are then loaded into an image processing program like Astroart, Maxim DL or Images Plus which uses them to "calibrate" or adjust the raw images to compensate for the uneven illumination. An added benefit to doing this is that dust on the optics and even the window covering the CCD chip in the camera will show up on the images as large "donuts" or dark spots. Since these will also show up on the Flat, calibration will also correct these.
I have found with my Starlight Express cameras that exposing the flat so that the brightest part is about 50% of the saturation level of the camera works best. On a 16 bit camera the saturation level is 65536 ADU. So half of that is around 32768. You don't need to be exact, a couple of thousand either way probably won't make much difference. It might be best to experiment with your camera to see what level brings the best results.
Since the Flat is going to be used to adjust you're RAW images, it is important to make it as noise free as possible. Therefore it is best to take multiple Flat images and then combine them in the image processing program before using them to calibrate your images. I think most of the popular Astro image processing programs will handle this step for you automatically if you point it to the folder with the Flat images. Since the Flat is also an image it should also have a dark and bias frame taken for it if darks are required for your camera.
There are many ways to take a Flat but the idea is to image as evenly illuminated a surface as possible. Some options are:
Sky Flats - The sky at dusk or dawn has a slight gradient but it can be used for a flat with pretty good results. It is best to move the camera around the sky or turn off tracking and then median combine the flats in order to remove bright stars which may appear in the Flat images.
Light Box Flats - This is how I do it. By building a light box you can do flats at the end of an imaging session without having to wait for dawn and also you can adjust the dimness of the light to achieve the proper saturation.
Tee-Shirt Flats - By stretching a tee shirt over the scope to act as a diffuser you can use a nearby wall or the ground as long as the illumination is relatively even. I have tried this and it works pretty well. Make sure the tee shirt is tight with no wrinkles; use a rubber band to hold it.
I have placed some images in a folder called Flats in the Image Processing Tips folder on the Yahoo Group site. They can be accessed here. Included in the images are a typical combined flat field image and then two images of M106 the only difference being one was calibrated with the flat and the other wasn't, so you can see the difference that flats can make. In next month's Night Times I will discuss making a light box.Published in the June 2005 issue of the NightTimes