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How to fit a linear distortion ?
12-18-2017, 17:21
Post: #1
How to fit a linear distortion ?
Hi

I was testing what happens when using a 1st order polynomial fit in Scamp (DISTORT_DEGREES=1), that is a linear model for the distortions.

The distortion model is then

x' = PV1_0 + PV1_1x + PV1_2y
y' = PV2_0 + PV2_1y + PV2_2x

However, that produced a distortion map which has a circular shape.
Furthermore, the distortion parameters (PV*) are not written in the '.head' file of the image.

I am using SCAMP version 2.0.4 on Centos.

cheers, Kimmo
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01-09-2018, 12:57 (This post was last modified: 01-09-2018 12:58 by Kimmo.)
Post: #2
RE: How to fit a linear distortion ?
(12-18-2017 17:21)Kimmo Wrote:  Hi

I was testing what happens when using a 1st order polynomial fit in Scamp (DISTORT_DEGREES=1), that is a linear model for the distortions.

The distortion model is then

x' = PV1_0 + PV1_1x + PV1_2y
y' = PV2_0 + PV2_1y + PV2_2x

However, that produced a distortion map which has a circular shape.
Furthermore, the distortion parameters (PV*) are not written in the '.head' file of the image.

I am using SCAMP version 2.0.4 on Centos.

cheers, Kimmo

Hi

I have a partial solution.
The appendix B of the SCAMP manual tells the following:

"In the linear distortion case (DISTORT_DEGREES 1 ), the influence of the PV coefficients can be completely described by the standard WCS keywords CDi_j and CRPIXi."

Thus, the distortion parameters are not given in the .head file, as I found.

But I still do not understand why my distortion plot shows a circular shape distortion, not linear.
For example, in the distortion plot at
https://www.astro.uni-bonn.de/theli/gui/.../d_g_2.png
there is a linear distortion map.

What I want to do is to remove the linear distortion from my images.
I cannot do that with SWARP if I do not know the PV parameters...

cheers, Kimmo
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