GeMS (Distortion correction)

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    Daniela Opitz

    Any official/unofficial GeMS code to correct the distortion?

    Thanks a lot in advance


    Erik Kool

    Hi Daniela,

    Over the last 6 months I’ve spend quite some time on correcting the distortion in my GSAOI data. I’ve had most success using Theli, a image reduction package which incorporates Scamp and Swarp, where I’d use a deep K-band image such as VISTA as reference instead of catalog sources. It can be found here:

    For more info and instructions see:
    Schirmer, M. et al, Gemini Frontier Fields: Wide-field Adaptive Optics Ks-band Imaging of the Galaxy Clusters MACS J0416.1-2403 and Abell 2744, 2015, ApJS, 217, 33
    Schirmer, M., THELI: Convenient Reduction of Optical, Near-infrared, and Mid-infrared Imaging Data, 2013, ApJS, 209, 21

    Depending on how many field stars are in your field and common between your reference image and science image, it can be a bit tricky, but I’ve distortion corrected some very sparse fields. Let me know if you run into any issues. Mischa is also quite willing to help out first-time users in my experience, assuming he has the time.



    Software to do this is in an advanced stage of development. It will correct for the static distortion and align images to a common astrometric frame (if an external catalog is provided, it will align images to that frame). Images are stacked, with bad-pixel rejection and optional inverse-variance weighting, and there are options to control the output image’s WCS, including copying the WCS from an existing image so that the output is perfectly aligned.

    If you are interested in using and providing feedback on a pre-release version, please reply here or contact me

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