Images

  • We chose the Alpha Persei cluster (Cr 39) to look at, but as it is a large cluster, we took images of two different sections of it, example images of which can be seen below in the visual range.
  • Location 1 (Centred on HIP16001) image at 10s exposure.
  • Location 2 (RA = 2h29m29s, DEC = 49deg44'16'') image at 60s exposure.
alpha_persei_visual.png

alpha_persei_2_visual.png

Analysis of Obervation 1

  • First set about investigating the background levels for the images in order to be able to form thresholds in order to identify stars,
  • Checked for both locations investigated, formed in tables below.
Location 1 (Centred on HIP16001)
Filter Typical Value of Mode(pixels) Exposure Time (s)
Visual 196 10
Blue 200 90
Green 210 40
Red 230 40

Location 2 (RA = 2h29m29s, DEC = 49deg44'16'')

Filter Typical Value of Mode(pixels) Exposure Time (s)
Visual 590 60
Green 355 120
Red 411 120
  • Note here that due to time constraints blue filtered exposures were not taken for the second location.
  • It can be seen that for location 2, typical values for the mode are 2-3 times higher than for the first. This is perhaps due to the larger amount of stars in the image causing a brighter background, and perhaps due to the exposure times, although this does not appear to have a 1 to 1 correspondance with the mode.

Finding average dark and bias frames

  • During our observations we took 5 120s dark frames.
  • These were averaged in order to create a .fit file of these averaged values, which can be used to account for dark current effects etc in these images. The average dark frame is attached to this page.
  • The 120s image can then be scaled down and used for the other exposure images seen above.

  • 10 additional bias frames were also taken, and an average frame of these generated, for similar reasons as above.

Detecting pixels above threshold

  • A threshold was set such that
\begin{equation} \rm{threshold} = a \times \sqrt{\rm{mode}} + \rm{mode} \end{equation}<br />
  • where a is some multiplicative factor, such that if we want something within 3 standard deviations, $a=3$.
  • The following images are of one of the visual images for Location 1 at threshold where $a=3$ and $a=10$ respectively. White represent the pixels or the background removed.
threshold_initial_3.png

threshold_initial_10.png

  • Wrote an algorithm to detect the stars in our images in the following way:

  1. Threshold: Found the mode over the entire image, set threshold as described above.
  2. Image Scanning: Wrote a loop which runs over each element in the image, and compares the pixel value to its 8 nearest neighbours. If the central pixel has a larger pixel value than its neighbours, we define this as the centre of the star, and note the x and y coordinates.
  • This was performed on one of the location 1 visual images for a equal to 5 and 10, see figures below for comparison overlays showing the original image and the identified points.
whole_5std.png
  • When $a=5$ we observe detections around the brightest star. This is not accurate and a zoomed in version can be seen below.
star_ring_5std.png
  • It appears as if there is a luminous ring around the star and is separated from the star.
  • At 10 standard deviation threshold the following is seen.
whole_10std.png
  • So now we have x and y positions of each star which depend on the threshold value.

Gaussian Fitting

  • Next, Gaussians were fitted to the found stars:
  1. Using x and y positions of each star, each star was cropped around within a arbitrary range ($\pm$20 pixels).
  2. Then a Gaussian function:
    \begin{equation} f(x,y) = Ae^{-\left((x-x_{c})^{2}/2\sigma_{x}^{2}) + ((y-y_{c})^2/2\sigma_{y}^{2})\right)}+d \end{equation}
  • Some of the fitted stars can be seen below. The contours are the contours of the fitting values.
one.png

two.png

three.png

-- DavidHadden - 17 Nov 2015

Attachments Attachments
Topic attachments
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PNGpng alpha_persei_2_visual.png r1 manage 684.8 K 17 Nov 2015 - 11:22 DavidHadden  
PNGpng alpha_persei_visual.png r1 manage 704.6 K 17 Nov 2015 - 11:22 DavidHadden  
Unknown file formatfit biasaverage.fit r1 manage 6100.3 K 17 Nov 2015 - 15:00 DavidHadden  
Unknown file formatfit darkaverage_120.fit r1 manage 6100.3 K 17 Nov 2015 - 14:24 DavidHadden  
PNGpng four.png r1 manage 124.7 K 17 Nov 2015 - 16:48 ElenaCukanovaite  
PNGpng one.png r1 manage 101.1 K 17 Nov 2015 - 16:47 ElenaCukanovaite  
PNGpng star_ring_5std.png r1 manage 220.6 K 17 Nov 2015 - 15:18 ElenaCukanovaite  
PNGpng three.png r1 manage 124.7 K 17 Nov 2015 - 16:48 ElenaCukanovaite  
PNGpng threshold_initial_10.png r1 manage 47.5 K 17 Nov 2015 - 12:58 ElenaCukanovaite  
PNGpng threshold_initial_3.png r1 manage 66.3 K 17 Nov 2015 - 12:58 ElenaCukanovaite  
PNGpng two.png r1 manage 186.8 K 17 Nov 2015 - 16:48 ElenaCukanovaite  
PNGpng whole_10std.png r1 manage 572.8 K 17 Nov 2015 - 15:18 ElenaCukanovaite  
PNGpng whole_5std.png r1 manage 579.2 K 17 Nov 2015 - 15:18 ElenaCukanovaite  
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Topic revision: r8 - 17 Nov 2015 - ElenaCukanovaite

 
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