Prelude to Correcting for Cameras and Filters


You have 3 images taken in Red, Green (visual) & Blue light. In theory, you should just be able to combine the images & get a true color image, but..its never that easy. The equipment used to take any image affects its appearance. In this case, the filters used & the camera used all make their own stamp on the image. We will correct for the camera sensitivity first and then filter sensitivity. This activity guides you to quantify what corrections need to be made; images will be corrected in the next activity.


Figure 3 shows a schematic of the path light takes from the telescope to the camera. When light hits the CCD camera, the camera records how much light hits each pixel as a count value. Referring to Figure 1 , this graph shows what percent of the light is actually captured by the camera for different wavelengths. (THIS IS ONLY FOR THE AP-7 USED AT THE YERKES 24" TELESCOPE, IF YOU HAVE A DIFFERENT CAMERA YOU WILL NEED THE MANUFACTURER'S GRAPH.) The X-axis shows wavelength in nanometers, with visible light covering about 400-700 nm. The Y-axis shows the percent of original light that the filter or camera actually passes on or captures. So, in order to create True Color images the 3 images must be corrected for light lost due to camera sensitivity.

Similarly, Figure 6 shows the percentage of light transmitted by different filters as a function of light wavelength. (THIS IS ONLY FOR THE FILTERS USED AT THE YERKES 24" TELESCOPE, IF YOU HAVE A DIFFERENT SET-UP YOU WILL NEED THE MANUFACTURER'S GRAPH.) As light passes through the filter, some of it is removed by the filter. A red filter passes mostly red wavelengths of light and subtracts out light of other wavelengths. Notice that no filter passes 100% of the light that reaches it. So besides correcting for the camera, it is necessary to correct for the filter as well. The following will guide the determination of values that must be used to correct each image. There are two main parts to determining correction factors: interpreting the graphs of filter transmission efficiency and camera sensitivity, secondly, determining correction factors for each image.

1. Interpreting the graphs of filter transmission efficiency and camera sensitivity. Consider just the red filter. According to Figure 6, what percent of the original light that hits the red filter actually passes through the filter? For the purposes of this activity, use the peak (high point on curve) wavelength transmitted by each filter to interpret the graph. What wavelength does the peak occurs at? For this peak wavelength, use Figure 6 to determine what % of light at this wavelength is captured by the camera.

2. Determining correction factors for each image: Now that you have determined the effects of the camera and filters on the light from your deep sky object, you have to use this information to correct the images. In mathematical terms, what could you do to 'correct' these images back to 100%? In other words, if the red filter only passes 50% of the light reaching it and a star in your image has a brightness count value of 100, what count value corresponds to 100% of the star's true brightness? What do you have to multiply the 50% count value by to get it to 100%?

Hint: One solution is by ratios, refer to Figure 2.

***So to correct our images we multiply our original images by 100% over the percentage of light transmitted or captured by the camera. This will bring the counts on the image back to their original values as if the filter or camera had no affect on the image. For the example given above, in order to get a correction factor for the red filter, 100% would be divided by 50% transmission by the red filter, so the correction factor would be 2. Therefore, we would multiply the red image by 2 to correct for the loss of light due to the red filter. This only an example, your correction factor will be different!


1. Organize a data table to record values for the red filter. It should include filter color, peak wavelength in nanometers, % transmitted by the red filter at peak wavelength, and % captured by the camera at peak wavelength.

2. Use Figures 1 and 6 to determine and fill in the values for the red filter. You will need to use the information in Explore from above to do this!

3. Expand your data table by adding space (rows or columns) for the comparable values for the blue and green(visual) filters. Complete your data table by using the graphs to determine these values in the same fashion as you did for the red filter.

4. Add additional space (columns or rows) to your data table to provide space to fill in correction factors for both the filter and the camera. Use the percentage values for filter transmission and camera sensitivity you obtained to get to determine correction factors for both the camera and the filters for each color. Fill in the expanded data table with your correction factors. You should have two different correction factors for each image.