Lab #2 INTRODUCING ASTRONOMICAL IMAGING

2-weeks 9/12 & 9/19: Lab due 9/26, 6pm NO EXTENSIONS

   Note Observatory Field trip is 9/26

   9/12: CCDs and non-ideal detectors, statistics of distributions
   9/19: Show & tell. Introduction to error propagation

Background reading

   Handbook of CCD Astronomy, Steve Howell, especially Ch. 3.

INTRODUCTION

This experiment introduces array detectors. Unlike the PMT which has a
single sensitive element, a charge coupled device or "CCD" can have
millions of picture elements or "pixels". Pixels are like the rods and
cones in the retina at the back your eye. When placed behind a camera
lens the CCD can record digital images.

The goals are to:

   Set up the CCD in lab and acquire images;

   Use IDL to display and manipulate images;

   Understand the nature of internal store in computers---bits, bytes,
   integers and floating point;

   Measure the characteristics of the CCD: dark current, gain, read
   noise and saturation level;

   Make a personal web page on UGASTRO with images from the CCD.

CCD PROPERTIES 

To use the CCD you need to sign up for an "observing period" on the
gridded white board. 

1) Take a picture

   Learn how to take a picture with the APOGEE CCD. The camera is very
   sensitive and you'll only need a short exposure (<< 1 second). Copy
   the FITS file from the ECHELLE, the control computer and read the
   file into IDL using READFITS. Learn to display the image in IDL
   using the TV command

   On-line documentation is at: 

   http://www.ugastro.berkeley.edu/class/ay122/ccd/index.html

   Try sitting in front of the camera and take a self-portrait. Learn
   how to make your own personal web page to display your pciture.


2) Measure the saturation level

   Light detectors are not perfect, but introduce additive and
   multiplicative noise. How perfect is the CCD?

   A CCD's response to light is linear up to some light
   intensity. Above a certain level, the CCD saturates and the data
   value measured in DN (data numbers) no longer increases. Adjust the
   exposure time and the level of illumination in the lab to discover
   the saturation level. A graph of mean DN value vs. exposure time
   demonstrates the onset of saturation.


3) Measure the mean "dark current" of a CCD pixel in data numbers per
   second

   Eliminate all light sources and measure count rate at room
   temperature. Repeat when the CCD is cooled by its thermoelectric
   cooler. Plot a histogram for an ensemble of pixels. Use the methods
   from the last lab to estimate the mean and variance of the pixel
   dark current distribution.

4) What is the "gain" of the CCD?

   The CCD control software returns DN, whereas the physical process at
   work in each CCD pixel generates photoelectrons. What is the
   conversion factor, or gain, between DN and photoelectrons?

   Illuminate the CCD uniformly, and calculate the variance in each
   frame as a function of light level.  Plot variance vs. mean signal
   (in DN).  Use the technique of least squares to fit a straight
   line.  Interpret the values of the gradient and the intercept in
   terms of the gain and noise properties of the CCD.

Express the previously measured dark current and saturation level in
physical units.

Equipment - 

APOGEE CCD, camera lens, and control computer.  Dim light source of
controllable uniform diffuse illumination (reading lamp & variable
transformer).  Darkened box to eliminate room lights.