Difference: 20151117MSciPhotometryAnalysisOb1 (4 vs. 5)

Revision 517 Nov 2015 - DavidHadden

Line: 1 to 1
META TOPICPARENT name="StewartBoogertPhotometry2015"


  • 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.
Line: 24 to 24
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 />
Line: 32 to 40


  • 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.
  -- DavidHadden - 17 Nov 2015
Line: 42 to 54
META FILEATTACHMENT attachment="threshold_initial_3.png" attr="" comment="" date="1447765090" name="threshold_initial_3.png" path="threshold_initial_3.png" size="67917" user="zxap014" version="1"
META FILEATTACHMENT attachment="threshold_initial_10.png" attr="" comment="" date="1447765096" name="threshold_initial_10.png" path="threshold_initial_10.png" size="48653" user="zxap014" version="1"
META FILEATTACHMENT attachment="latex9235a31198320fdc055698d0b87c5b33.png" attr="h" comment="" date="1447765199" name="latex9235a31198320fdc055698d0b87c5b33.png" user="zxap014" version="1"
META FILEATTACHMENT attachment="darkaverage_120.fit" attr="" comment="" date="1447770268" name="darkaverage_120.fit" path="Y:\Major Project\20151113_DH_EC\dark_frame\darkaverage_120.fit" size="6246720" user="zxap012" version="1"
META FILEATTACHMENT attachment="biasaverage.fit" attr="" comment="" date="1447772446" name="biasaverage.fit" path="Y:\Major Project\20151113_DH_EC\bias_frame\biasaverage.fit" size="6246720" user="zxap012" version="1"
This site is powered by the TWiki collaboration platformCopyright © 2008-2019 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding RHUL Physics Department TWiki? Send feedback