Correcting Images 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 uses the data table of correction factors created in the previous activity to correct each image in preparation for combining them into a 'True Color' image.
Figure 3 in the Library of the Nexus Community shows a schematic of the path light takes from the telescope to the camera.
This schematic in the same from the previous activity, but is used here to indicate that since the camera is last in the chain, the images must FIRST be corrected for camera sensitivity, followed by correction for filter transmission. The corrections will bring the counts on each image back to the values that would have been recorded had the camera or filter not removed information from the images. Using the HOU-IDL software, each image will be multiplied by the correction factors for filter transmission and camera sensitivity determined in the previous activity.
Reminder: To proceed into the next steps of this process, you should have:
a. Measured and noted differences in your object between the images taken through different filters.
b. Subtracted the SKY count values from EACH image.
Procedure to Correct For Camera Limitations:
1. Multiply each image by its own camera correction factor you determined in the previous activity. USE THE IMAGES WITH THE SKY VALUE SUBTRACTED OUT!! Use the 'Data Manipulation: Multiply.' Tool in the same way you used the Subtraction tool. Multiply the image by number of counts equal to your correction factor and display as new image.
2. Save your corrected images with a file name appropriate for the work done (example: 'NGC 2419 Red Sky Camera.fts') and then use these corrected images in the next part.
Procedure to Correct For Filter Limitations:
1. Multiply each image by its filter correction factor you determined in the previous activity. Use the 'Data Manipulation: Multiply' tool in the same way as for the camera correction. Save your now fully corrected images with a file name appropriate for the work done (example: 'NGC 2419 Red Sky Camera Filter.fts') and then use these corrected images to combine into your 'True Color' image.