Aim: Generate stars with gaussian profiles, using random positions and fluxes.

'Basic' simulation program (04/10/2016): Simulation_test1.py.txt

- Simulation generates stars with random positions.
- Canvas size and no. of stars are variable.
- No gaussian profile for stars or grey values (pixel value is either 0 or 1).
- Example output: (100x100 pixel canvas, 50 stars)

- Generated stars now have 2d gaussian profiles:

- Maximum grey values (gaussian peak values, 'A' in above equation) are randomized - can result in star not appearing in output.
- Example output: (100x100 pixel canvas, 0-100 grey value range, 10 stars)
- Example 2: (1530x1020 pixels, 0-65535 grey value range, 50 stars) (Same size of images taken by telescope system)

- Change working directory
- 'import [filename]' (without '.py')
- '[filename].[functionname](variables)'

To show an image produced by a function:

- 'a=[filename][functionname](variables)'
- 'imshow(a, cmap='Grey_r)' (use 'imshow(a)' for colourmap image instead of greyscale image)

Aims:

- For each pixel, determine mean value via integration of gaussian function over pixel area.
- Use mean value to gain Poisson distributed value for each cell.

- Integration must be done via numerical method - multiple methods available within python (e.g 'scipy.integrate.quad()'). No indefinite integral exists for the gaussian function.
- x and y components of gaussian can be integrated seperately. Limits of integration are +/- 0.5 of pixel position (midpoint):

- To demonstrate the importance of integrating to gain the pixel value instead of inputting the pixel coordinates to the gaussian, I have created the following plot simulating an 8x1 pixel grid with a star at x=3.75 (A=100, Sigma=1):
- Important things to note: This is a projection for the 2d gaussian at y=0 (green line shows the 'maximum' profile on the x axis). Bin values are calculated using integration over both x and y axis, resulting in decreased bin heights, particularly near the peak.
- Near the peak, measured values from integration is less than value of gaussian.
- Position of integrated peak bin offset from actual gaussian peak position - could estimate position using relative heights of adjacent bins.

- Each pixel value consists of a summation of
*n*random variables (e.g. y=x1+...+xn). Each random variable corresponds to the contribution of a particular star (out of*n*stars in the image). - Each random variable follows a Poisson distribution, with a mean value (E[x]) equal to the integrated value of a star's gaussian profile over the pixel area.
- We can assume that these random variables are not correlated (correlation coefficient P=0), therefore the total mean value will be the sum of individual mean values (E[y]=E[x1]+...+E[xn]).
- We can also assume that the summed pixel values (from star contributions only) also follow a Poisson distribution.
- Therefore, we can simulate te 'counting' of photons in each pixel by using the total intgrated pixel value as the mean to gain a random poisson distributed value.
- Example: projection (y=0) of a 16x1 pixel grid with a star generated at x=7.75 (A=100, sigma=2):
- Test 1: 100x100 pixels, A=100, 10 stars
- Small pixel value range results in very obvious variations in signal data.
- Test 2: 100x100 pixels, A=65535 (pixel value range for camera), 10 stars
- Larger pixel value range results in less obvious variations - almost indistinguishable from earlier gaussian simulation.

Aims:

- Implement correlated 2d gaussian function
- Implement way to save simulated star fields as FITS files

- Implemented a true 2D Gaussian with correlation between x and y (with correlation coefficient
*P*(rho)):

- (Note: normalisation factor is ignored so far for programming purposes; maximum pixel value is being used in its place)
- Integration of 2D Gaussian in this form is done numerically.
- Tested for
*P*=0,*P*=0.5,*P*=0.999 (sigma_x=sigma_y=3 in each case) - (Note - image is rotated 90 degrees clockwise)
- Also - found a way to save images as FITS files, similar to images gained from observations.

I | Attachment | History | Action | Size | Date | Who | Comment |
---|---|---|---|---|---|---|---|

png | GaussianSimTest1.png | r1 | manage | 27.3 K | 09 Oct 2016 - 10:15 | AaronAndrews | gaussian profile test 1 (100x100, 10 stars) |

png | GaussianSimTest2.png | r1 | manage | 38.8 K | 09 Oct 2016 - 10:15 | AaronAndrews | gaussian profile test 2 (1530x1020, 50 stars) |

png | PoissonExample1.png | r1 | manage | 50.3 K | 13 Oct 2016 - 11:33 | AaronAndrews | Example of poisson distributed pixel values on a 16x1 pixel grid |

png | PoissonSimTest1.png | r1 | manage | 39.4 K | 14 Oct 2016 - 12:54 | AaronAndrews | Poisson simulation test 1: 100x100 pixels, A=100, 10 stars |

png | PoissonSimTest2.png | r1 | manage | 29.4 K | 14 Oct 2016 - 12:54 | AaronAndrews | Poisson simulation test 2: 100x100 pixels, A=65535, 10 stars |

png | PositionSimTest2.png | r1 | manage | 19.0 K | 09 Oct 2016 - 10:14 | AaronAndrews | random position test (100x100, 50 stars) |

txt | Simulation_test1.py.txt | r1 | manage | 1.5 K | 09 Oct 2016 - 10:15 | AaronAndrews | simulation test program 1 |

txt | Simulation_test2.py.txt | r1 | manage | 2.4 K | 09 Oct 2016 - 10:16 | AaronAndrews | simulation test program 2 |

txt | Simulation_test3.py.txt | r1 | manage | 4.7 K | 11 Oct 2016 - 22:55 | AaronAndrews | Simulation test program 3 - implemented integration |

txt | Simulation_test4.py.txt | r2 r1 | manage | 5.5 K | 14 Oct 2016 - 12:53 | AaronAndrews | Simulation test 4 - added poisson variations |

txt | Simulation_test5.py.txt | r1 | manage | 7.6 K | 19 Oct 2016 - 20:40 | AaronAndrews | simulation test 5 |

png | SkewTest2.png | r1 | manage | 18.6 K | 19 Oct 2016 - 20:41 | AaronAndrews | Testing x and y correlation (P) for P=0, P=0.5, P=0.999 |

png | ValueComparison1.png | r1 | manage | 50.8 K | 11 Oct 2016 - 22:46 | AaronAndrews | Demonstration of integrated pixel values for an 8x1 pixel grid with 1 star at x=3.75 |

This topic: Public > UserList > StewartBoogert > StewartBoogertAstronomy > StewartBoogertMSciProjects > StewartBoogertPhotometry2016 > Photometry2016_Simulation

Topic revision: r7 - 19 Oct 2016 - AaronAndrews

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