CFHT, Instruments, Spectroscopy, Gecko: The Coudé f/4 Spectrograph

Web Manual for Gecko - The Coudé Focus

This section presents the characteristics of the Coudé Room where the beam coming from the telescope enters the spectrograph, and CAFE, the CAssegrain Fiber Environment.

NOTE: you may click on some of the images to get a better quality or bigger file.

1. The Coudé Room

The Coudé beam can be sent into either of the two Coudé rooms located inside the telescope pier on the second and third floor of the observatory. Currently, only the third floor Coudé room is used. The beam enters this room horizontally 1.5m above the floor through any one of seven possible crenel positions. One of these is occupied by the slit/slicer assembly for the Coudé f/4 spectrograph. The hardware from the decommissioned f/8 spectrograph is also still located in this room.

The Coudé room is 16m in diameter and is in thermal equilibrium. Instruments with significant heat dissipation are not permitted in this area, and observers are reminded that when (if) they enter this room, they are essentially walking into a precision instrument, so appropriate steps should be taken to ensure that dust is not transported into in. Opening and closing of the outer doors to the Coudé room, and the use of lights (especially the fluorescent lights) inside the room should also be kept to a minimum.

2. CAFE, the CAssegrain Fiber Environment

For the visible and red part of the spectral domain (3700Å to 10,000Å) the mirror train has been replaced by a 27 m fiber optic that runs from the Cassegrain to the Coudé focus. The input fiber end is mounted in the CAFE unit located on the south port of the Cassegrain bonnette, receiving the f/8 beam from the Cassegrain Central mirror (after M2). From the bonnette, the fiber follows the west declination axis and goes down the south pier to the Coudé slit room. The output fiber end goes to a Bowen-Wallraven image slicer.

Input and output ends of the fiber optic, with their microlens.

This slicer is located in a crenel in the Slit Room and injects a f/20 beam into the spectrograph.
The Bowen-Wallraven slicer.

Gecko Interface Optical Characteristic

Schematic view of the fibre optical characteristics

The fiber optic is of the silica/silica type with a very low OH content, H-treated to enhance the transmission shortward of 4500Å. The fiber is also of the "dry" type, so its internal transmission is also free of the red absorption peaks usually seen in OH-contaminated "wet" fibers.

The table below gives a comparative overview of the respective theoretical efficiency of CAFE and the red mirror train. CAFE without the slicer compares very well to the Coudé train. Then, CAFE benefits from the better efficiency of its Bowen-Wallraven slicer (about 90%) when compared to the former Richardson slicer (about 65%). However, this is counterbalanced by the CAFE important central obscuration produced by the camera optics in the spectrograph beam, a characteristic inherent to the loss of the instrument pupil shadow caused by the mode scrambling into the fiber (29% against 6% in the former Coudé configuration).

  4500Å 5700Å 8000Å
5 enhanced Ag mirrors (M3 to M7) 0.780.860.89
2 field lenses 0.960.980.94
Triplet lens 0.970.980.98
Coudé train throughput (from M3 to M7) 73%83%82%
CAFE throughput (from CAFE to fiber output) 79%84%84%
Overall throughputs (slicer and Gecko central obscuration included)    
Coudé train overall throughput 45% 51% 50%
CAFE overall throughput 51% 55% 53%

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