Cornell University
Memo:
Subject:
Distribution:
Author:
TM-FOR11-103
Distortion Correction Algorithm
Performance
T. Herter, J. De Buizer
J. Adams, J. Wang
FORCAST
Date:
Version:
Orig. Date:
Posted:
11/7/11
1.0
11/7/11
----4
Summary
The anamorphic magnification and nonlinear distortion of the FORCAST optics is documented
in TM-FOR04-071. This memo presents an analysis of post-rectification data which includes
both internal extended pinhole grid images and in-flight images of points sources in OMC 2. The
pinhole grid data were rectified to within 0.2 pixels (including inherent mechanical tolerances).
Positions of in-flight point sources were rectified to better than 1.
Algorithms Overview
Both the pipeline and MIRA use the raw pinhole positions in images of the extended grid as
input. The distortion is fit to a 2-D polynomial function. The distorted data is then transformed
and rebinned to an undistorted grid according to this polynomial function.
Performance
1. Pinhole Grid Rectification
Images of the pinhole grid were acquired during CD 43. These images were processed with the
MIRA pipeline. For the SWC, the pinhole grid spacing was rectified to <x> = 21.11  0.04
pixels and <y> = 21.07  0.07 pixels (Fig. 1). There were no x,y dependencies on the spacing
(Fig. 2).
TM-FOR11-103
Page 1
Cornell University
FORCAST
Y spacing spread
X spacing spread
60
60
50
50
40
40
30
30
20
20
10
10
0
21
21.05
21.1
21.15
21.2
0
20.9
21.25
21
21.1
21.2
21.3
Fig. 1. Upper Panel: Processed image of the pinhole grid at X µm. Lower Panel: Rectified x
and y pixel spacings for the pinholes in the image.
Y spacing
X spacing
21.25
21.3
21.2
X separation
Y separation
21.2
21.15
21.1
21
21.05
21
21.1
0
2
4
6
Y position
8
10
20.9
12
0
2
4
Y spacing
10
12
8
10
12
21.3
21.2
21.2
X separation
Y separation
8
X spacing
21.25
21.15
21.1
21.1
21
21.05
21
6
X position
0
2
4
6
X position
8
10
12
20.9
0
2
4
6
Y position
Fig. 2. X- and Y- dependencies of the pinhole separation distances given in Fig. 1.
TM-FOR11-103
Page 2
Cornell University
FORCAST
For the LWC, the pinhole grid spacing was rectified to <x> = 21.15  0.15 pixels and <y> =
21.25  0.19 pixels (Fig. 3). There were no x,y dependencies on the spacing (Fig. 4).
Y spacing spread
X spacing spread
70
50
45
60
40
50
35
30
40
25
30
20
15
20
10
10
5
0
20.5
21
21.5
22
0
20.8
21
21.2
21.4
21.6
21.8
Fig. 3. Upper Panel: Processed image of the pinhole grid at X µm. Lower Panel: Rectified x
and y pixel spacings for the pinholes in the image.
TM-FOR11-103
Page 3
Cornell University
FORCAST
Y spacing
X spacing
21.8
21.8
21.6
21.4
X separation
Y separation
21.6
21.2
21
21.2
21
20.8
20.6
21.4
0
2
4
6
Y position
8
10
20.8
12
0
2
4
Y spacing
10
12
8
10
12
21.8
21.6
21.6
21.4
X separation
Y separation
8
X spacing
21.8
21.2
21
21.4
21.2
21
20.8
20.6
6
X position
0
2
4
6
X position
8
10
12
20.8
0
2
4
6
Y position
Fig. 4. X- and Y- dependencies of the pinhole separation distances given in Fig. 3.
2. Astrometry of OMC 2
In-flight images of OMC 2 are given in Fig. 5. These images have been processed and rectified
with the MIRA pipeline. We have computed the RA and DEC given a a tangent plane solution
from 3 stars. The “true” positions of the sources were taken from Spizter/IRAC 3.6 µm images,
except source 3 which has a “true” position taken from the Spitzer 8.0 µm.
Table 1 gives the residual positions between Spitzer-derived positions of the OMC 2 sources and
the FORCAST-derived positions after the tangent plane solution was applied to the FORCAST
images. The residuals are better than 1.
The rectified plate scale is 0.769/pixel as computed using the OMC 2 source pairs 1-6 and 4-5.
TM-FOR11-103
Page 4
Cornell University
FORCAST
1
3
4
5
6
Fig. 5. Processed images of OMC 2 at 19.7 µm (left) and 37.1 µm.
Table 1. Residual positions for FORCAST sources after a tangent plane astrometric solution was
applied from Spitzer 3.6 and 8.0 µm images.
Wavelength Source
(µm)
RA/DEC (2000)
RA/DEC
(Spitzer)
(FORCAST)
05 35 26.863
05 35 26.840
-05 09 24.70
0.436
-05 10 17.38
05 35 26.980
-05 10 17.40
0.088
05 35 24.737
-05 10 30.00
05 35 24.720
-05 10 29.90
0.280
6
05 35 26.875
-05 11 07.53
05 35 26.922
-05 11 07.35
0.731
37
1
05 35 26.863
-05 09 24.43
05 35 26.852
-05 09 24.76
0.365
37
3
05 35 27.078
-05 10 00.32
05 35 27.106
-05 10 01.29
1.049
37
4
05 35 26.974
-05 10 17.38
05 35 27.000
-05 10 17.30
0.398
37
5
05 35 24.737
-05 10 30.00
05 35 24.713
-05 10 29.96
0.372
19
1
19
4
05 35 26.974
19
5
19
-05 09 24.43
 ()
Revision History
Version
Date
1.0
11/7/11
TM-FOR11-103
Comments/Changes
Initial version.
Page 5