CCD Lab Report

CCD Lab Report

Joe Regan

10-02-2015

Introduction

In this lab, we used CCD cameras to obtain bias images as well as dark and flat images at different exposure times in order to understand processing the images later. A CCD, which stands for Charge Coupled Device, measures photons that strike a detector in certain areas, exciting an electron in that area via the Photoelectric Effect. However, these areas, or pixels, are not absolutely perfect detectors of photons, as they can vary in photon count due to imperfections in the detectors themselves. There are, however, ways to compensate for these imperfections. Bias images are closed-shutter zero-second exposure images used to detect the noise generated by the detector itself, dark images are closed-shutter images taken at various exposure times to detect the buildup of noise on the detector over time, and flat images are taken to evenly distribute the light in a final image. Darks are normally taken at a constant exposure time, but for the sake of this lab we have taken two of them at five different exposure times in order to demonstrate buildup of light over time.

Procedure

For this lab, we were given a computer with Maxim DL installed, and an ST-I camera used to take the images. No telescope was used for this lab, as it was only for testing our ability to take the bias, darks, and flats themselves. The camera included a USB connector cord to connect it to the computer for use in the Maxim DL program. To connect the camera, first start up the Maxim DL program, then connect the USB cord to the camera and the computer. The computer will install the necessary software, and once that is done, check the camera model by clicking 'set up camera' in the settings tab and connect the camera to the program by clicking 'connect'. 

To take images, first go to the expose tab and set the exposure time to the desired value. For now, set the image setting to 'single' to test the detector. After we make sure that the camera is operational, it is time to take bias images.

We need to take a total of ten bias images, so set the exposure time to 0 and select the autosave option on the right. Keep the lens cap on the camera to block any light that may make it in, and then take the ten images.

Next, we take a total of ten dark images at various exposure times. For this lab, I chose to take two darks each at 1, 2, 5, 10, and 20 second exposure times. You can take each photo individually, but there is an easier way to do it. First, click on the autosave button to bring up the autosave settings menu. From here you can script the images to be taken a certain way. At the top left, in the name box, name your image set (in this case, just type "Dark"). On the bottom half of the menu, there are a number of lines for setting up images. Activate the first five by clicking on the numbers to the left. We are using five of these because we are taking two of each exposure time for a total of ten images. In the menu, set the image type, exposure time, and number of images (2 in each) to their respective values. You may also choose to set the suffix of each set, in this case I chose the exposure times. Finally, click 'apply' to finish these settings. Back in the camera menu, go ahead and take the images, keeping the lens cap on the camera. The images should go according to the script that was set up.

For the flat images, do the same setup as the dark images, but in the autosave menu replace all instances of Dark with Flat, and change the image type. Now, find an evenly illuminated object, be it a piece of paper, the wall, or even the floor. The exposure times should match the dark image exposure times. Remove the lens cap and take the images, keeping the camera very steady.

Figure 1: Bias image

Figure 2: Dark image at 20s

Figure 3: Flat image at 20s

Results and Discussion

For this lab, we calculated the average pixel value for each image, using the READFITS() function in IDL. I used a short program to do this, named ccdlab.pro and located on exo at /home/regan/LabData along with all the images taken.

Figure 4: Excel chart of pixel values

The principle of dark images is to measure the buildup of dark current over time, and one purpose of this lab was to show this buildup with respect to exposure time.

Figure 5: Dark Current vs Exposure Time

As shown in the graph, there is a positive correlation between dark current and exposure time, in this case having a slope of 0.637, showing a rate of change of dark current of roughly 0.637 counts per second. Showing this, along with the bias and flat images, gives us a better understanding of how CCD cameras work, along with a better understanding of image reduction.