CCD Lab

CCD Detector Lab

CCD Detectors

September 30, 2015

Ryan Hall

Introduction

            In this lab we obtained darks, flats, and bias images from CCD cameras. A CCD, charge-coupled device, camera takes in incoming photons that excite electrons in the pixels. These excited electrons are what is eventually used to build an image. Along with this, these cameras have other qualities that can work against the actual image you are trying to build. Darks, flats, and bias images are taken to help rid the scientific images of this background noise. Darks are images taken with the camera’s shutter closed to account for the noise generated by the electronics of the camera. This lab took a variety of exposure times while taking out dark images to show a relationship between the counts and exposure time. Normally when dark images are taken the exposure time is consistent. Bias images are taken to account for the base noise that will be generated for the camera, so it will have an exposure time of zero. Flats are images of a relatively equally lit screen. This finds which pixels seem to be more or less sensitive to incoming photons. These exposure times are not required to be the same as darks; however, if they are, it will make the work easier when the obtained data is being reduced.

Procedure

            This lab begins by opening up Maxim DL on your computer. In the Maxim DL window go to view and click camera control window. This brings up the window that allows images to be taken and saved by the CCD camera. Once your CCD camera has been plugged into the computer with a USB cable click connect under the settings tab of the window. These cameras are designed to be used for taking astronomical images; however, this lab is designed to understand how to account for the background noise of the cameras. Given this, these cameras are not needed to be connected to a telescope to accomplish the goals of this lab.

                At this point images can start being obtained. All images must be saved individually after they are taken and 10 images of each type are required. For bias images, the exposure time is set to zero and ten images are taken and saved. The shutter on the camera should remain closed for these images, if it does not, the lens cap can be put on to prevent light from entering. Our flat fields were taken by pointing the camera at a blank, white piece of paper. A few exposures were taken at first to make sure the counts were at appropriate levels. An appropriate level meaning an unsaturated image while having counts that are higher than the darks or bias. The flats we obtained ended up having .001s exposure times. Darks were taken with an exposure .05s and had the lens cap on. We also took dark images at exposure times of 1, 2, 5, 10, and 20 seconds and saved one image for each integration time.

Results and Discussion

                This lab saved 10 images for each type of image, darks, flats, and bias.  Using IDL we can use the command READFITS to allow the image to be viewed as a 648 by 486 array. At this point the tvscl command in IDL can be used to show the image in Xming. An example of each image type is shown in the figures below.

Figure 1: Bias Image

Figure 2: Dark Image

Figure 3: Flat Field

The flat field image shows darkening in lower corners of the image; however, the counts are fairly consistent throughout.

A mean function can be used on each image to find the average number of counts for each image. By taking the average of each of the 10 images of each type, the average counts for the darks, flats, and bias is found as shown in the table below.

Type	Exposure time (s)	Avg. Counts
Bias	0	1923
Flats	.001	6241
Darks	.05	1114

                This table shows that the counts in darks are lower than the counts in the bias. This is not supposed to be the case. This error seems most likely to have been caused by the fluctuating temperature of the CCD camera. These averages also include bad pixels within their counts. A find command in IDL was used to find a rough estimate of the number of these bad pixels. (as shown in image below)

Figure 4: Bad Pixel Count

This image shows there to be 27 bad pixels, assuming that any pixel with a count over 10,000 is defined as bad.

                We obtained our dark current rate by taking dark images at varying exposure times. It is expected that with increasing exposure time yields an increase in counts in a linear relationship. The slope of this relationship would then be the dark current rate while the y-intercept would be the average bias level. The results from our data are shown in the graph below.

Since we already discussed that there was error in our bias count, it is not surprising that the y-intercept of this graph is not around what our bias level is. Even with that, the y-intercept here has a value of 1021.8 which would be an appropriate value since our average dark counts was found to be 1114. It should also be noted that since there are only six data points on this graph, and that they are scattered rather heavily, that this trend line cannot be taken to be very accurate.

Conclusions

                This lab had us take flat field, dark, and bias images to find how much background noise is created by the camera. Our data yielded an average dark count of 1114, flat field count of 6241, and bias count of 1923. We obtained a dark current rate of a little over one, 1.17, and the bias level calculated from this graph was found to be around a more appropriate 1021.8. The approximate count of bad pixels was found to be 27. This data can be used to make a more accurate reduction of data obtained from future observations.

Shannon Dulz

AST 311

September 30, 2015

Detector Lab Report

Abstract

            In this lab, we obtained flat, dark and bias images on the CCD cameras in order to characterize background levels of these CCDs for later processing of images. We calculated the dark current rate and average bias levels.

Introduction

            As part of every astronomical image obtained with a CCD, there are background counts of electrons resulting from the detector itself. To account for these background levels, three types of calibration images are taken: bias, darks and flats. Bias images account for noise introduced from the readout of the detector. Biases are taken as zero-second exposures with a closed shutter. Dark images account for the accumulation of noise due to the electronics of the detector and it tends to grow linearly over the integration time. Dark images are taken at the same exposure length as flat images (or scaled to the same length) and with the shutter closed. Flat images account for pixel-to pixel variations in the detection of light. Flat images are taken of an illuminated screen (or the sky at twilight for sky-flats) with the shutter open at an exposure length results in the image being evenly illuminated but not yet saturated. During processing, the bias frames are averaged into a master bias and subtracted from all the other images including darks and flats. Then, the darks are averaged into a master dark, which is subtracted from the science images and the flats. Lastly the flats are averaged into a master flat, which is divided from the science image pixel-by-pixel.

Procedures

            For this lab, we tested the CCD cameras without a telescope. The cameras were connected to our laptops via USB cables. Then the program Maxim DL was used to collect images. To connect the camera in the program, under the settings tab on the program, we clicked set-up camera and insured the correct camera model is shown. Also insure, swap chips is off, guide chips is internal, and ext trigger is off. Under the options tab, turn off rotation orientation and auto dark subframe extraction. Finally, click connect.

            To begin taking images, under the expose tab, adjust the exposure time to required time. Images must be saved after each exposure unless Auto Save is used. We began by taking 10 bias images of exposure time zero seconds with the cap on the detector to block light. One of the bias images is below.

Next, we obtained flat fields with the camera pointed at an illuminated piece of paper. We took several test images at various exposure times to insure counts would be higher than bias levels but not saturate the detector. We choose an integration time of 0.01 seconds and took 10 flat images, an example of which is shown below.

While this image, appears identical to the bias image, the counts are in fact much higher and a slight grating in counts from top to bottom is noticeable. Using this same integration time of 0.001 seconds we obtained 10 dark images with the cap on the camera, an example of which is shown below.  

Lastly, in order to calculate dark current levels we obtained 1 image each at 1, 5, 10 and 20 second exposure times with the cap on the camera.

Results and Discussion

For this lab, we obtained multiple flats, darks, and bias (shown in the table below).

Image Type	Integration time (seconds)	Number of Images
Bias	0	10
Flat	0.01	10
Darks	0.01	10
Darks	1, 5, 10, 20	1 each

           

            Using IDL, the biases were averaged together by summing the counts in each pixel of each image and dividing that number by the total number of images. This produced a master bias frame the median of which was 2006, giving us the average bias count. The darks of exposure time 0.01 were averaged in the same way leading to an average dark count for those images of 1789. Lastly the flat fields were averaged into a master flat, the median of which was 4299. The average counts of the biases should not normally be higher than that of the darks. This variable bias level could be due to temperature and we will recalculate the bias level from the dark current rate discussed below. While bad pixels were evident in individual images by vastly different count levels in single pixels, these few dozen pixels averaged out when the images were summed together. The flat fields we obtained were very evenly illuminated with only a slight grating of counts from top to bottom of the image. To obtain the dark current rate, we obtained darks at 1, 5, 10 and 20 seconds. We took the median of these images individually to obtain the table below.

Exposure time (seconds)	Average counts
1	1026
5	1031
10	1037
20	1036

We expect to see the dark counts increase linearly over time, the slope of which would give the dark current. While the linear fit is far from perfect due to the few number of images taken, we obtained a dark current of 0.5 counts/sec. The y-axis intersection of this linear fit is the bias level for these sets of images. For this we obtain 1028 counts as the bias level.  This calculation is a much more reasonable number than that obtained from the bias images themselves of 2006. This discrepancy could be due to temperature, especially since the bias images were the first ones taken as the CCD was still warming up. The graph of this linear fit is below.

Conclusions

            In this lab, we took flat, dark and bias images to begin to characterize the CCD detectors. We found an average bias level over 10 images of 2006 counts. When recalculated using the linear fit intersection of the dark current, we obtained a much more reasonable 1028 count bias level. We obtained a very low dark current rate of 0.5 counts/sec. Lastly, we took very even flat field images with average counts of 4299. The low background levels of these detections especially with regard to the low dark current will allow for high signal to noise ratios when they are used for later labs.

CCD Lab Report

CCD Camera

9/30/15

Joshua Kern

Theory

Charge-coupled devices (CCD’s) are the primary devices astronomers use to image the sky. CCD’s have arrays of pixels where each pixel can be thought of as a potential well. When photons impinge upon the surface of the pixel electrons are released that, when biased by an applied voltage, create a measurable current. However, imperfections in the individual pixels cause errors in the measured count of electrons produced by the incident light.

These errors in measurement have multiple sources. Even if an image is taken with zero exposure time, the CCD will read a small signal known as the bias level. Also, the ‘hum’ of the electronics is responsible for creating a signal that is known as the dark current. The dark current accumulates with time at a constant rate and can be accounted for by taking images with a closed shutter. The third source for signal error is from pixel to pixel sensitivity. In other words, some pixels accumulate counts easier than others or a spec of dust might block light from entering certain pixels. This can be accounted for by taking images of uniformly lit screens which are known as flat-field images. Finally, stray light from other objects in the night sky can contribute to the signal as well. All of these sources of additional signal must be accounted for and subtracted off your data in order to leave only the data from your object of interest.

Experiment

In this experiment we characterized the bias level, dark current, and pixel sensitivity of the ST-I model CCD camera. We took 10 images with an exposure time of zero seconds to determine the bias level, 10 images with an exposure time of 0.001 seconds with the shutter closed to determine the dark current, and 10 flat-field images with the same exposure time the same as our dark current images to determine the pixel sensitivity. We also took images with the shutter closed at exposure times of 1, 2, 5, 10, and 20 seconds in order to determine the rate at which the counts increase due to the dark current.

Results & Discussion

After obtaining the images, we used READFITS in IDL to reduce the data. By taking the MEAN of each image, we obtained an average bias level and dark current of 1021.4427 and 1066.6979, respectively. The table below gives all the average values for each bias, dark, and flat image which we used to obtain the previously stated values. The flat-field images were uniform with variations in counts increasing to 50,000 towards the top-left portion of the image, and decreasing to 40,000 in the bottom-right portion.

Image	Bias	Dark	Flat
1	998.4084	1096.4714	38974.117
2	1015.5076	1099.4374	48256.285
3	1035.1007	1101.3552	47070.859
4	1019.3893	1086.2279	49321.992
5	1021.3006	1094.8556	47982.125
6	996.5331	1055.5481	47343.168
7	1027.8583	1039.1448	47978.512
8	1031.2988	1029.1586	47562.883
9	1033.0098	1030.3505	47111.668
10	1035.9933	1034.4302	46365.031

To determine the rate at which the dark current changes with time, we took images with the shutter closed with increasing exposure times in order to plot the average counts in each image versus the time of exposure. The data we obtained are plotted in the graph below.

As you can see, the data that was taken does not show strict linearity however a linear trend is seen. After taking the line of best fit through the data, we obtain a rate of change of dark current equal to 0.70 ± 0.005 counts per second. The values we obtained for the bias level, dark current, and the rate of change of dark current as well as the characterization of the flat field images allow us to better reduce data obtained with the ST-I camera in the future.