Engineering Recipes
These recipes are devoted to calibrate the EMIR detector, a Rockwell HAWAII-2 unit.
Cosmetics
- Mode:
Engineering
- Recipe class:
- Input class:
CosmeticsRecipeInput- Result class:
CosmeticsRecipeResult
Detector cosmetics include: dead and hot pixels, inhomogenities in pixel-to-pixel response, stripes pattern and bias in the least significative bit.
Dead and hot pixels
Dead pixels have low response independently of the brightness of the incident light. Hot pixels have high response even in low brightness conditions. Both types of pixels are related with problem in detector electronics.
Dead and hot pixels are detected using two flat illuminated images of different exposure time. The ratio of these images would be a constant, except in dead or hot pixels, which will deviate significantly.
Asumming a normal distribution for the vast majority of the pixels in the ratio image, we flag as hot pixels those that lie a given number of standard deviations over the center of the distribution. Dead pixels are those that lie a given number of pixels below the center of the distribution.
The center of the distribution is estimated as the median of the full image. The standard deviation of the distribution of pixels is computed obtaining the percentiles nearest the pixel values corresponding to nsig in the normal CDF. The standar deviation is then the distance between the pixel values divided by two times nsig. The ratio image is then normalized with this standard deviation.
In the image below we plot the histogram of the ratio image (with median subtracted). The red curve represents a normal distribution with mean 0 and sigma computed from 1-sigma percentiles. The normal distribution is plotted upto 4-sigma. Overplotted in green are the regions at 6-sigma level. These points would be flagged as bad pixels in the final output mask.
Note
The procedure is similar to the algorithm of the IRAF task ccdmask
Requeriments
Name |
Type |
Default |
Meaning |
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Parameter |
4.0 |
Values below this sigma level are flagged as dead pixels |
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Parameter |
4.0 |
Values above this sigma level are flagged as hot pixels |
Products
'ratio' contains the normalized ratio of the two flat images.
'mask' contains a frame with zero for valid pixels and non zero for invalid.
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