How to fit a linear distortion ?

12182017, 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 

01092018, 12:57
(This post was last modified: 01092018 12:58 by Kimmo.)
Post: #2




RE: How to fit a linear distortion ?
(12182017 17:21)Kimmo Wrote: Hi 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.unibonn.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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