Background

Subtracting is one way of manipulating images. Subtracting involves taking two images with the same astronomical coordinates, and then "canceling" all the objects that appear at the same coordinates in both images. This process eliminates stationary and non-changing objects such as stars and galaxies. The objects that remain could be several things, from supernovae to asteroids to cosmic rays to CCD chip defects. In the following tutorial, we will subtract a set of images to "find" an asteroid.

This is a step-by-step tutorial which can be used by novice searchers or as a refresher for those who have not used the software recently. For novice users, we recommend doing every step in order. Advanced users can proceed at any pace they find comfortable. The following tutorial is based on images for which the results or "answers" are shown here. You may use the procedure to process other images at your own risk.

Note that errors in subtractions do not usually make themselves apparent until the four steps have all been carried out and the subtraction has been made. For this reason, it is recommended that you note all numbers involved with your data, so that if you need to start over you won't have to do everything over again.

The subtraction procedure utilizes the following two images, f1911534.fts and f1911538.fts. Image f1911534.fts was taken at an earlier time than the other and is called the "reference" image. If you come to a point in your subtraction process where you need to save the images you are working with, it is generally best to save them under a new name. This way you will always have a copy of the original images. You NEVER want to alter your original data (image) permanently. Obtain the two required images from the location indicated by your instructor and OPEN the two images using the HOU Image Processing software. Keep your web browser open and move back and forth between the HOU software and the browser as you go through the tutorial. Use a logbook to keep a written record of your work as you do it!

Below are links to explanations of the four main steps involved in the image analysis process.

1. Subtract the sky value from both images

2. Shift the new (later) image to match the reference (earlier) image.

3. Normalize the brightness values of the two images

4.Subtract the reference image from new image

Note: Subtractions come out the best if they are done in the above order.

What do asteroids look like in the Deep Search Images? (Check it out!)

Revised 8 November, 1998. This tutorial is the result of the collaborative efforts of Hughes Pack, Drew Trainor, Will Dane, Tim Spuck, Jodi Asbell-Clarke, John Roop and Edward Guice.