The Cambridge OH Suppression Instrument 

Page originally produced by kennico
last update Nov 23 1999 by irp

Above: COHSI Integral Field Unit (left), OH Suppression Unit (middle), J & H band cryogenic spectrograph (right).

COHSI stands for the Cambridge OH Suppression Instrument. COHSI was a special purpose instrument, designed for NIR low resolution (R~500) faint object spectroscopy. Two very important technical successes came out of this project:

1) OH masking technology - The optics required to deliver a high resolution spectrum from 1.0-1.8 microns on to a mask surface worked extremely well. We also developed in-house techniques for making a curved mask surface with very low reflectivity (<0.5%) for the OH lines and very high reflectivity (>98%) for the rest of the spectrum.

2) NIR lens-array, fibre coupled Integral Field Unit (IFU) - We made a 100-element IFU using a macro-lens array. Each fibre was individually positioned with respect to a lens in the array to a precision of ~2 microns giving excellent IFU uniformity and throughput.

COHSI is now being used to construct the next generation spectrograph CIRPASS which exploits both of these technologies to the full.

COHSI was designed when the biggest detectors available were 256x256 arrays. The requirement was for a spectrograph that was not limited by the OH emission but by the much fainter sky emission in between the OH lines. To achieve OH suppression at a suppression mask (at R~3500) and record the whole of the J or H band  (at R~500) on the small detector the light had to be dispersed, masked, fully undispersed and then finally fed into a cryogenic spectrograph where the light was redispersed again. Once larger 1024x1024 and 2048x2048 arrays became available it became possible to capture the spectrum at high resolution without losing wavelength coverage and so the COHSI approach lost its appeal because of its low throughput. We are now therefore using the front end of COHSI (the IFU and the suppression optics) and replacing the backend (the undispersing optics, the second fibre bundle and the cryogenic spectrograph) with a simple camera which uses a large array to image the mask surface. This new instrument configuration is called CIRPASS.

COHSI was designed to suppress the nightsky airglow due to OH emission in the 1.0-1.8 micron region by filtering out the OH using masks. COHSI was a fiber fed instrument, which gave COHSI stability (the bulk of the instrument sat on the observing floor) and telescope independence (a small change of fore-optics in front of the fibers allowed COHSI to move from one telescope to another).

Commissioning on UKIRT March 19, 20 & 25-29, 1998
The first UKIRT COHSI picture list has pictures of COHSI taken during the first commissioning run.

Commissioning on UKIRT Nov, 1998
The second UKIRT COHSI picture list has pictures of COHSI taken during the second commissioning run.

Lab photos of COHSI
We put together a tidy photo gallery with summaries of the main components to COHSI.

Other COHSI links
The COHSI Mask Page.
The COHSI OH Skyline Database Page.

More information about COHSI
Two SPIE papers describing the COHSI

  • optical design SPIE 2871,1332-1341 (1997) (html or postscript)
  • March 1998 commissioning run status SPIE 3354, in press (1998) (html or postscript)

  • have been published.

    The COHSI Team


    We are part of the Institute of Astronomy, which is part of the University of Cambridge, Cambridge, United Kingdom