CFHT, Instruments, Detectors, IR, CFHT-IR, Camera Description.

CFHT-IR

Camera Description

Introduction

The new CFHT-IR camera is based on the 1K x 1K Rockwell Science Center HAWAII (HgCdTe Astronomical Wide Area Infrared Imaging) focal plane array and includes a cryostat with two 8-position filter wheels and a pupil mask. The excellent optical and throughput performances of the instrument promise to greatly enhance the near-infrared imaging and spectroscopic capabilities of CFHT. The same pixel scale of 0.211''/pixel is used for both direct imaging and multi-object spectroscopy, leading to a total field-of-view of 3.6' x 3.6' on the sky. The main characteristics of the CFHT-IR camera are summarized in the table below.


   
Detector format  1024 x 1024
Detector material  HgCdTe 
Spectral range  0.85 - 2.40 µm  
Plate scale  0.211 +/- 0.001''/pixel 
Pixel size  18.5 µm / pixel
Total field of view  3.6' x 3.6' 
Operating temperature  77 K 
Readout noise 
~ 15 e-/pixel 
Gain
2.35e-/ADU
Dark current  0.02 e-/sec 
Full well capacity  45 000 ADU/110.000 e- 
Linearity (1%)  35 000 ADU/80.000 e-
Mean quantum  efficiency  (detector only)
76 % (for 99.5% of pixels)
Minimum integration time  0.1 sec (with shutter); 1 sec (in staring mode)  
Maximum integration time  1 hour (not recommanded)  
Readout time (full frame)  1 second 


CFHT-IR main characteristics


 

Cryostat Assembly

The cryostat contains the single fixed reimaging optics, two 8-position filter wheels, corresponding drive motors and position encoders, the FPA, temperature sensors, and electrical connections. The cryostat itself is a custom design from Infrared Laboratories, with a single 8.5 liters cold tank and a surrounding radiation shield. Measured hold times have exceeded 48 hours in a side-looking configuration. Operating temperature is 77 to 80K. The cryostat is internally gold plated to reduce surface emissivity and increase vacuum integrity. All internal surfaces providing optical baffling are black painted using vacuum-qualified paint. The cryostat contains a carbon getter with its own thermal circuit-breaker protected getter heating element and temperature sensing elements. The focal plane mount is on a similar yet completely independent thermal stage. Strict attention was paid to dewar materials and construction to maintain low outgassing and high vacuum.

The following pictures were taken during the preliminary integration of the cryostat at CFHT or at Universitée de Montréeal.

picture 1

picture 2

picture 3
 

Optics

The optics accept a f/8 beam over an unvignetted field of 3.6' x 3.6' for direct imaging at Cassegrain focus and when installed on OSIS. The fixed seven-element optical design reimages the input beam at f/5.03 on the FPA, providing a magnification ratio of 1.523:1.0. This yields a 0.211''/pixel scale. The optical layout is shown here. All optical surfaces are anti-reflection coated. A cold stop is placed at the pupil image. The integrated transmission of the CFHT-IR camera optics (without the filters) is designed to be greater than 85% for all wavelengths in the 0.75 to 2.5 µm spectral range. The transmission of the (now decomissionned) IR camera of the OSIS spectrometer was greater than 95% between 1.1 and 1.8 µm.
 

Detector:

CFHT selected Rockwell HAWAII array number #142 as the science grade array for CFHT-IR in April 2000. This array has a mean QE of 76% for 99.5% of the array. The mean dark current is 0.04 e-/sec.

Here is a dark map and here is a flat field as received from Rockwell.
 

Acquisition system and user interface

CFHT-IR is driven by a second generation San Diego State University (SDSU) CCD controller, which has been selected as the new controller for visible and infrared detectors at CFHT (CFH12K, EEV, etc.), and a 4-channel pre-amplifier mounted on the dewar. The total detector readout and delivery time is 1 second.

The observing sessions provides the observer with a user interface identical to the CFH12K one, called DetCom, incorporated into the CFHT/Pegasus observing environment, through which they configure the camera, control the data acquisition, monitor the data storage and do some pre-processing. Commands can be issued either from a GUI form or at the command line level, the latter feature allowing relatively complex sequences of observations, such as dithering and mosaics, to be prepared in advance using C-shell scripts.
 

Filters:

The CFHT-IR filters have been integrated into the CFHT Interference Filter Database and their detailed characteristics can be retrieved here.

Please note that for the imaging mode, all CFHT-IR filters are identical to the corresponding KIR ones. For the spectroscopic mode, the 3 filters previously available with Redeye on OSIS are still used.
 

Created 24 August 2000.
Updated 14 February 2003.
Page created by Jean-Luc Beuzit and maintained by Thierry Forveille.
Please send comments to: forveill@cfht.hawaii.edu