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Dewoff Creation and Modification

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Creating and Modifying Dewoff Files


  • Introduction
  • Useful Tools
  • A Quick Recipe
  • Helpful Hints


Why create or modify a dewoff file?

  • There may not be a dewoff for your instrument configuration
  • You may be using the Long-Camera on IMACS
  • align-mask may be failing to converge
  • For fun

Useful Tools

There are a few programs you need to become familiar with in order to create or modify a dewoff file:

  • adjust-offset  make manual corrections to dewar offset files
  • mosaic  combine all 8 CCD frames into one image
  • apertures  calculate positions of mask apertures using observation definition file
  • spectral-lines  calculate positions of spectral lines using observation definition file
  • spectral-map  calculate positions of spectral lines using a spectral map

A Quick Recipe

  • Display a calibration frame (lamp or direct image). Overlay a list of spectral features or apertures created by spectral-lines or apertures using IRAF or some other program.
  • Use adjust-offset to correct the dewar orientation based on the predicted positions overlaid.
  • Re-run spectral-lines or apertures with the new dewoff and overlay the new positions.
  • Repeat until the predicted positions line up correctly.

Helpful Hints

A few things that can make the process as painless as possible:

  • adjust-offset is setup for 1x1 binned data, you can use it with other binned data, however you will need to modify your offsets to account for the binning.
  • Always make backups of your dewoff files before modifying. A backup is made after using adjust-offset, but the backup will be overwritten after running the program a second time.
  • On occasion apertures does unusual stuff like not outputting any features. We are currently looking into this problem. This has occurred with some binned data and some IMACS Long Camera data. So try using spectral-lines with a lamp if this occurs. Typically for spectroscopic data it makes more sense to just use spectral-lines.
  • The IMACS Long Camera dewoff always seems to be off a little, this is due to the grating angle. It is sometimes sufficiently off enough that align-mask may not find the correct features. In this case you will need to use adjust-offset to line the predicted positions up with the correct features.
  • When correcting the dewar orientation. The positions need to be sufficiently close enough such that they are within the search radius of align-mask. It is usually not wise to run align-mask with a searchbox larger than 50 pixels with a line list with more than 3 features. Running with such a large searchbox, other features might be confused with the correct features.
  • LDSS3 standard long slit masks will need a smaller searchbox than on normal data. You will want to start with a value of 10 and see if it correctly identifies features. If not, you may need to start over with a smaller value.
  • Dewoff files can be converted from Normal orientation to Nod&Shuffle; orientation by subtracting 90 degrees. This is important for instruments such as IMACS in which the default orientation can be changed.
  • D_ALIGNROT in the obsdef may need to be changed if the y-distortion does not seem to be sufficiently mapped by spectral-lines. Typically we see this behavior with LDSS3 data in which the instrument can shift slightly, in which there is a slight rotation between the grism and CCD. A corrected solution is usually less than a degree.

Next: Frequently Asked Questions

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