Aligning Corrected Images


The 3 images obtained through the different filters are now about ready to be combined into one 'True Color' image. One obstacle is still to be overcome: the fact that the images will not be in precise alignment. The HOU-IDL software will be used in this activity to align the red, green (visual), and Blue images. This is the final step before combining the 3 images into one 'True Color' image.


Due to slight differences in the pointing of the telescope/camera system during acquisition of the images, the 3 filtered images will not be precisely in alignment. Astronomical images such as the images in this activity span only a small fraction of the sky. Images taken with the HOU Yerkes 24" telescope cover less than 1/10th of a degree from side to side! Very tiny changes throughout the telescope/camera system lead to small variations in pointing accuracy. These changes can be caused by factors such as imperfections in the telescope dive gears, vibrations within the observatory due to air-conditioning, even heavy equipment being used nearby! This can be confirmed by opening two images you have been working on in this project. Use the HOU-IDL software tool 'Image Comparison: Blink' to blink these two images. They will appear to 'jump' as they are blinked due to problems with the alignment. The following activity aligns your images so they can be combined into one 'True Color' image.


Reminder: To proceed into the next steps of this process, you should have:

a. Measured and noted differences between the 3 images of your object taken through different filters.

b. Subtracted the SKY count values from EACH image.

c. Corrected EACH image for camera sensitivity and filter transmission.

1. Open HOU-IDL Level 3, as well as the three corrected images from the previous activity. Choose one image, such as the red-filtered image, to leave the same. The other two images will be aligned to this one.

2. Process the images for the best view.

3. It is helpful to organize your on-screen work area. Move the red image to the right side. Move the other two images on top of each other to the left. Make sure one of the left side images is the current image by clicking on it.

4. Choose 'Image Comparison: Align' from the main HOU-IDL toolbar. You will get a window with the current image displayed and a request to choose what image you want to align it to. Choose your red-filtered image from the drop down menu. As directed by the on-screen instructions, choose a star from the image on the left, then the same star from the image on the right. Repeat for a second star, as far from the first as possible. NOTE: When the HOU-IDL software blinks the images, LOOK CAREFULLY. Accurate alignments make better end results. Do them over if the images are not well aligned. When you are satisfied with your alignment, confirm it by clicking OK and make sure you leave checked 'Display Result In New Window.'

5. Your new image will be either your blue or green (visual) image ALIGNED to the red image. Save it with a new name indicating this, for example: 'NGC 2419 Blue Sky Camera Filter Aligned.fts'.

6. Repeat by aligning your other (blue or green) image to the red as above. Save the new image generated. NOTE: One image, the red image in this example, will be unchanged!!