Imaging mode tutorial: combination of dithered exposures

Warning

All the commands are assumed to be executed in a terminal running the bash shell (or a compatible one).

Don’t forget to activate the same Python environment employed to install PyEmir. In this document, the prompt (emir) $ will indicate that this is the case.

This tutorial provides an easy introduction to the use of PyEmir (via Numina), focusing on the combination of dithered exposures.

For detailed documentation concerning the installation of PyEmir, see PyEmir Installation.

We strongly recommend to follow the different sections of this tutorial in the provided order, starting with the simplest combination method, before attempting to refine the combination procedure.

Note

• Since PyEmir version 0.17.0 image distortions are corrected by reprojecting each individual pointing into a WCS with a constant pixel scale in X and Y. For this task we are using the package reproject.

  Important: the absolute astrometric calibration of the reduced images is not guaranteed to be better than a few pixels. This means that the same field observed (and reduced) in different times may not overlap perfectly. An absolute astrometric calibration will require the comparison of the derived astrometry with accurate coordinates of objects in the field of view.

• Only integer offsets between (reprojected) images are considered: this is not especially important considering that the PSF is well oversampled. The benefit of using integer offsets is that we avoid reprojecting the images a second time (and we do not increase the reduction time).

Tutorial index:

• Preliminary combination
  • Step 1: basic reduction of individual exposures
  • Step 2: image combination
• Improving the image combination
  • Improving offsets (method #1)
  • Improving offsets (method #2)
  • Improving offsets (method #3)
  • Improving the sky background (problem #1)
  • Improving the sky background (problem #2, old EMIR detector)
  • Improving the sky background (problem #2, new EMIR detector H2RG)
  • Improving the superflatfield