Basic Description of the Echelle Spectrograph
for the 4m Blanco Telescope

The echelle spectrograph is a pretty old-fashioned machine... the only parts under remote control from the console room are the width of the slit, the CCD, the shutter and the comparison lamp. But it has reasonably efficient optics, a fairly good selection of gratings, and good detectors.

A Photon's Eye View...

This is a basic walkthrough of the spectrograph optics, highlighting the areas where there are choices to make. Refer to the optical diagram below. The basic elements (in the order they get hit by incoming photons) are:
Acquisition camera & guiding
Slit/Decker
Filter
Folding Flat & Collimator (Choice of Red or Blue Optics)
Echelle Grating (Choice of 31.6 or 79 l/mm)
Cross Dispersing Grating (Many Choices)
Camera & CCD (Choice of Red Long, Blue Long, Air Schmidt or Folded Schmidt camera)


Overall System Efficiency

The overall system efficiency is the fraction of photons striking the telescope's primary mirror which are eventually detected. It has been measured for a small number of setups by observing flux standards on photometric nights, through a 7-10 arcsec slit.

The peak overall system efficiency reaches ~8%. This is reasonable for an echelle spectrograph using a grating cross-disperser.

Available direct measurements:

As of 27 Dec 1995.


Blue Long Camera + Tek 2048
Blue optics, 79 l/mm echelle, KPGL-2 cross disperser.


Red Long Camera + Tek 2048
Red optics, 31.6 l/mm echelle, 226-3 cross disperser.


Air Schmidt Camera + Loral 3K.

csmith@noao.edu
sheathcote@noao.edu