-- SteveErtel - 21 Apr 2020

LMIRcam astrometric solution

If accurate astrometry is to be obtained from LMIRCam images, the detector plate scale and True North need to be measured. For this, we observe an astrometric field (the Trapezium cluster in Orion) and compare the separations and position angles of pairs of the stars in the images with their astrometry which is known at high precision. If astrometry is precise required, this step needs to be done in addition to (and after) deriving and applying a distortion correction which should be applied to science data in alomst all cases (see this link). The plate scale and True North angle can then be used to convert astrometry of science data derived in detector coordinates to sky coordinates.
  • Type of observation: On-sky.
  • Duration: 2 h (data acquisition, can be done as 2 x 1 h on separate nights), 5 h (data analysis & publication).
  • Who: Steve Ertel or delegate (data acquisition), Steve Ertel & Emily Mailhot to work with E. Spalding to retain expertise on data analysis.
  • Interval: Data should be acquired once per semester (Orion is visible from mid-September to mid-March, so early in each semester is ideal) and -- depending on observability of Trapezium and requirements for planned science observations -- after each significant technical intervention (NIC moved or opened). Each data set is to be analyzed and published.

Acquiring astrometric data

This data acquisition procedure assumes that the operator is able to perform basic LMIRCam imaging data acquisition.
  • Set LMIRCam up for standard, single-sided imaging. Align the pupils, use SX or DX aperture. Filter: std-L. Use full frame and fast mode.
  • Preset to tet1 Ori and find the center of the field (see this image [link] from Close et al. 2012 taken in K band, LMIRCam's field-of-view has a size of approx. 2x2 squares of the blue grid). Try to close the AO on a bright star close to the center.
  • The integration time should be constrained by the background, somewhere in the 1-2 sec range as ususal for std-L imaging. Avoid saturation on any but the brightest few stars.
  • Set PID to 19.
  • Map the center of the field with 5x5 pointings, with 5 arcsec offsets between pointings. Save 10 frames in each position.
  • 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)

Analyzing astrometric data

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

Application to science data

The plate scale and True North angle can then be used to convert astrometry of science data derived in detector coordinates (after distortion correction [link]) to sky coordinates.


  • The procedure and tools for deriving astrometric solutions from the data have been developed by E. Spalding and expertise needs to be transferred to more permanent team members (Steve Ertel, Emily Mailhot). Until astrometric solutions can be provided to our userson a routine basis, the raw Trapezium data are provided to our users and analysis is their responsibility with some support from our team on a best-effort basis.
Topic attachments
I Attachment Action Size Date Who Comment
apj424037f1_hr.jpgjpg apj424037f1_hr.jpg manage 553 K 21 Apr 2020 - 22:44 SteveErtel Trapezium field from Close et al. (2012)
Topic revision: r2 - 22 Apr 2020, SteveErtel
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