Merge pull request #49 from fact-project/fix_relative_pixel_angle_naming

Fix relative pixel angle naming

fact/instrument/camera.py

@@ -4,7 +4,7 @@
 import pandas as pd
 
 from .constants import (
-    FOCAL_LENGTH_MM, DISTORTION_SLOPE,
+    FOCAL_LENGTH_MM, PINCUSHION_DISTORTION_SLOPE,
     PIXEL_SPACING_IN_MM, FOV_PER_PIXEL_DEG
 )
 
@@ -73,18 +73,20 @@ def get_pixel_dataframe():
     pm['x'] = -pm.pos_Y.values * PIXEL_SPACING_IN_MM
     pm['y'] = pm.pos_X.values * PIXEL_SPACING_IN_MM
 
-    pm['azimuth'] = np.rad2deg(
-        np.arctan(pm.x / FOCAL_LENGTH_MM) * (1 + DISTORTION_SLOPE)
+    pm['x_angle'] = np.rad2deg(
+        np.arctan(pm.x / FOCAL_LENGTH_MM) * 
+        (1 + PINCUSHION_DISTORTION_SLOPE)
     )
-    pm['zenith'] = np.rad2deg(
-        np.arctan(pm.y / FOCAL_LENGTH_MM) * (1 + DISTORTION_SLOPE)
+    pm['y_angle'] = np.rad2deg(
+        np.arctan(pm.y / FOCAL_LENGTH_MM) * 
+        (1 + PINCUSHION_DISTORTION_SLOPE)
     )
 
     return pm
 
 
 FOV_RADIUS = np.hypot(
-    get_pixel_dataframe().azimuth, get_pixel_dataframe().zenith
+    get_pixel_dataframe().x_angle, get_pixel_dataframe().y_angle
 ).max()
 
 

fact/instrument/constants.py

@@ -3,30 +3,39 @@
 '''
 import numpy as np
 
-LATITUDE_DEC_DEG = 28.7616  #: FACT's latitude in decimal degrees
-LONGITUDE_DEC_DEG = -17.8911   #: FACT's longitude in decimal degrees
-ALTITUDE_ASL_M = 2200  #: FACT's altitude above sea level in meters
-
-position = {
-    'latitude': {
-        'dir': 'N',
-        'deg': 28,
-        'min': 45,
-        'sec': 41.9
-    },
-    'longitude': {
-        'dir': 'W',
-        'deg': 17,
-        'min': 53,
-        'sec': 28.0
-    },
-    'altitude_above_sea_level': 2200
-}
 #: The inner diameter of the hexagonal pixels in mm.
 #: This is also the grid constant of the hex grid.
 PIXEL_SPACING_IN_MM = 9.5
+FOCAL_LENGTH_MM = 4889
+#:The segmented imaging reflector of FACT is well described using the thin lens
+#:equation. However, the most prominent deviation from the thin lens, is the 
+#:imaging reflectors pincushin distortion. The pincushin distortion projects 
+#:incomning light further away from the optical axis as it is expected from the 
+#:thin lens equation. The additional outward pin-cushin distortion gets stronger
+#:the further the distance to the optical axis is.
+#:
+#:``actual_projection_angle = (1 + PINCUSHION_DISTORTION_SLOPE) * thin_lens_prediction_angle``
+#:    
+#:Example:
+#:According to the tĥin lens model one expects incoming light of 1.5 deg 
+#:incident angle relative to the optical axis to be projected in:
+#:
+#:``tan(1.5deg)*focal_length = 128.02mm``
+#:
+#:distance to the optical axis on the image sensor screen.
+#:But the reflector actually projects this incoming light further to the 
+#:outside to:
+#:
+#:``128.02mm * (1 + PINCUSHION_DISTORTION_SLOPE) = 130.67mm``
+#:
+#:As one can see, the correction is minor. It is only half a pixel at the outer
+#:rim of the field of view.
+#:
+#:.. image:: figures/pincushin_distortion_slope.png
+PINCUSHION_DISTORTION_SLOPE = 0.031/1.5
+LATITUDE_DEC_DEG = 28.7616  #: FACT's latitude in decimal degrees
+LONGITUDE_DEC_DEG = -17.8911   #: FACT's longitude in decimal degrees
+ALTITUDE_ASL_M = 2200  #: FACT's altitude above sea level in meters
 FOCAL_LENGTH_MM = 4889  #: FACT's reflector focal length in mm.
-DISTORTION_SLOPE = 0.031/1.5
-
 #: Field of view of a single pixel in decimal degrees
 FOV_PER_PIXEL_DEG = np.rad2deg(2 * np.arctan(0.5 * PIXEL_SPACING_IN_MM / FOCAL_LENGTH_MM))