-- SteveErtel - 21 Apr 2020

LMIRcam distortion correction

The optical distortion of LMIRCam is significant and needs to be corrected for almost all observations. The optical distortion is measured by imaging a pinhole grid near the entrance of NIC. The pinholes are known to be evenly spaced, but the image will be distorted. The distortion caused by the optics in NIC will be the same as for science data. The goal of the distortion correction is to 'undistort' the image so that the pinholes appear regularly spaced. The same correction can then be applied to science images.
  • Type of observation: Closed dome, typically during night time.
  • Duration: 20 min (data acquisition), 2 h (data analysis & publication).
  • Who: Steve Ertel or delegate (data acquisition), Steve Ertel or Emily Mailhot (data analysis & publication).
  • Interval: Data should be acquired once perobserving run. The first data set of a semester and after each significant technical intervention (NIC moved or opened) is to be analyzed and provided to our science PIs.

Acquiring distortion data

This data acquisition procedure assumes that the operator is able to perform basic LMIRCam imaging data acquisition.
  • pinholes.pngSet LMIRCam up for standard, single-sided imaging. Align the pupils as much as possible, use SX or DX aperture. Filter: std-L or N039. Use full frame and fast mode.
  • Insert pinhole grid in NIL NICNAC (not UBC field stops). Manually adjust the wheel position so that the pinhole is reasonably aligned in horizontal/vertical direction and fills as much of the field-of-view as possible (not critical, but saves significant time during data analysis). Good alignment should look similar to the image on the right.
  • Adjust integration time so that peak counts of the pinholes are between 1500 and 2500 counts (not dark/bias subtracted).
  • Set PID to 19.
  • Save 10 frames.
  • Save 10 darks (use blank in LMIR filter wheel 4).
  • Repeat with the other aperture (DX aperture if SX aperture was used aboveor vise-versa, no new darks needed if detector setup is unchanged)

Analyzing distortion data

The data are analyzed using the code Dewarp (link to ADS entry, link to Read the Docs documentation). The results should be documented and published for our users by posting them on this page [link].

Correcting science data

Information on how to apply the distortion correction to science data can be found on the page containing the distortion soultions [link].

Notes

  • The procedure for correcting science frames is currently working on an image-by-image basis and for full frames only. This is not ideal for the large number of frames LMIRCam often produces. Thus, a better procedure should be developed in the future.
  • The distortion correction as currently implemented only corrects optical distortion in NIC (shown to dominate), but not in the UBC and from the telescope. A more complete distortion correction should be explored in the future using the pinhole grids in the UBC field stops. Observations of an astrometric field can in principle be used to implement a complete distortion correction, combined with plate scale and true North corrections.
  • Data should be distortion corrected for almost all scientific applications. After distortion correction, an astrometric solution may optionally be applied if accurate astrometry is required. This can also be done with the Dewarp code. Details on how to obtain an astrometric solution can be found under this link.
Topic attachments
I Attachment Action Size Date Who Comment
pinholes.pngpng pinholes.png manage 240 K 21 Apr 2020 - 21:52 SteveErtel LMIRCam pinhole grid image
Topic revision: r2 - 22 Apr 2020, SteveErtel
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