The ARC echelle was constructed for the 3.5m at the University of Chicago in the 1990s.

ARCES instrument manual on APO website

* the traces and ThAr lines can move around over the course of a night. The reason has never been completely understood.

• This leads to issues with conventional 2D flat-fielding since it is possible that pixels with flux in an object spectra will not have pixels with a lot of flux in a flat-fielded image.
• This also affects the ability to get precision velocities. More frequent ThAr frames may be required, or you may wish to try to use telluric absorption to correct for wavelengths; see note by Patrick Gaulme with his conclusions about the nature of the shift.

* ARCES has a small aperture, giving a very short “slit”. Sky is difficult, if not impossible, to detect/subtract

* as with many echelle spectrographs, scattered light is present in an amount that varies in time, likely to do changing levels of contamination on the detector

* IRAF-based alternatives

• General information on reduction of echelle data with IRAF
  • Chris Churchill's Introduction of Echelle Data Reduction using IRAF is an excellent document describing in detail the process of reducing, extracting and calibrating echelle spectra. Although the document is for the Hamilton spectrograph, many of the steps can be applied to ARCES spectra.
  • Wako Aoki's manual on reducing Subaru echelle data is also provided here. A different method of continuum flattening your spectra is described in this manual.

• The IRAF Data Reduction Guide for ARCES by Julie Thorburn of University of Chicago is the original document for reduction of ARCES data with IRAF, and is the basis for most of the IRAF-related reduction packages below. There are very useful and important descriptions of the peculiarities of the ARCES data, and the user is recommended to read this document before forging ahead with the data processing.
• K. Kinemuchi's ARCES echelle data reduction cookbook provides a step-by-step guide for reduction using IRAF, along with auxiliary files that will aid in the data reduction process.
• J. Mckeever's python script for reduction using PYRAF is available, as well as a gziped tar file that includes all the necessary files to run. See K. Kinemuchi guide for details of the process.

* Python alternatives

• Ceres : a general reduction package for echelle data that has a module for ARCES
• pyvista : a general basic, pedagogical, reduction package that has configuration files for ARCES and a notebook demonstrating basic reduction
• PypeIt : a general package for optimal extraction of spectrograph data, including echelle data. Work is in progress on a configuration for ARCES.

• G. Catanzaro's spectra atlas of atmospheric telluric lines provides a great resource to identify the telluric lines in the ARCES data.
• Paper on a possbility for removing cosmic rays from individual images