Introduction

LRIS-B receives light by intercepting the collimated beam off the collimator mirror with a deployable dichroic beam splitter (there are 4 different dichroics available, plus an over-coated aluminum mirror that sends all light into the blue spectrograph channel). Wavelengths shortward of the dichroic cutoff (the dichroics are coded by the wavelength of light that corresponds to the 50% point in transmission/reflectance; e.g., the D500 will pass half of the 500 nm photons into the red camera and reflect the other half into the blue camera) are reflected into the blue spectrograph. Typical dichroics will have a transition region that normally spans about 200 centered on the nominal 50% point (see the reflectance and transmission curves for the dichroics. Outside of that range, the dichroics generally have transmission efficiencies of better than 95% and reflective efficiencies of better than 98%. Dichroic beam splitters invariably produce low frequency ``wiggles'' in the efficiency of transmitted and reflected light that can be noticed in high S/N spectral observations; these can be removed through flat fielding. The dichroic substrates are 1-inch thick BK7 glass, and are AR-coated on the back side. The dichroics produce no noticeable shift in the focus for the red and blue side of the spectrograph.

LRIS Dichroics Characteristics

Name Transmission 50% Reflectance Wavelength
[Å]
50% Transmission Wavelength
[Å]
mirror  n/a n/a
D460 Data
Curve
  4874
D500 Data
Curve
4800 5091
D560 Data
Curve
  5696
D680 Data
Curve
6640 6800

Notes on ghosts

At present, for spectroscopy using dichroics, there are ghost spectra that have been determined to be second order light dispersed by the red side grating that is returned to the dichroic (and which passes through because the dichroic is basically a long-pass filter) and then makes it onto the blue side detector as zeroth order light (i.e., it is not further dispersed by the blue grism). The ghosting will thus depend upon red-side grating tilt and by the prism angle of the grism in use. We have ordered a short-pass filter for use in the blue side filter carousel that will block these ghosts for many setups (the ghost line intensity is about 0.5% of the red side line intensity--they are easily seen in arcs but can be quite subtle in taken on the sky)--it is expected to be available by August 2002. In the meantime, if your observations require deep near-UV and blue spectroscopy, I would recommend using the dichroic mirror, which eliminates this ghosting issue. There are no obvious ghosts seen with the 1200 l/mm grism-- because the large prism angle diverts the zero-th order image of the dispersed red light away from the camera. Ghost images should not be a problem for any imaging observations.