MEGACAM, the Next Generation Wide-field Camera for CFHT

O. Boulade, X. Charlot

Service d'Astrophysique, DAPNIA, CEA/Saclay,
91191 Gif sur Yvette FRANCE
Electronic-mail: oboulade@cea.fr

Abstract:

MEGACAM is the next generation wide-field camera for the prime focus of the CFHT telescope. This instrument will cover a full 1 square degree and is designed around a mosaic of 36 to 40 2Kx4.5K CCDs. It is scheduled to be available to the CFHT community in the second half of 2001. This paper is a status report on the development of the project.

Introduction

Table 1 shows the main requirements of the MEGACAM camera. This project was presented at the ``Astronomical Telescopes and Instrumentation'' SPIE meeting in Kona in March 1998 (see reference [1]). A postscript version of this presentation which describes the MEGACAM camera in detail, as well as many other informations on the MEGAPRIME project, can be found at the following Web sites: MEGACAM home page; TERAPIX home page.

This paper will present a short report on the status of the project (section 2), the procurement of the CCDs (sect. 3), the test bench being developped in Saclay (sect. 4) and the planning of the project (sect. 5).

 

$$\geq ^{\circ} ^{\circ}$$ field of view

image quality 0.5 arcsec

spectral coverage 3700 - 9000 Å

$$\leq$$ readout time

 

Project Report

 

• A Preliminary Design Review for the camera was held in Waimea in March 1998. The technical solutions (figure 1) presented by the MEGACAM team in Saclay for the different subsystems were all agreed upon, namely:
  • the shutter will be a half-disk spinning always in the same direction and crossing the field of view at a constant speed to ensure a uniform exposure time for all pixels of the mosaic;
  • the filters will be housed in a juke-box system: a filter wheel would be able to hold only 4 filters and there is no room for two superimposed filter wheels in the prime focus cage. Having a juke-box holding up to 8 filters with U, B, V, R and I permanently installed will allow the observer to react efficiently according to the conditions of the night, for example in the case of observations to be done in the U band in a queue scheduling observing mode;
  • the control and acquisition system for the CCDs will be a customized system developped in Saclay, in order to meet the requirement on the short readout time and the constraints on the weight and the power dissipation given by the location of the instrument in the prime focus cage;
  • the CCDs will be cooled down with a pulse tube cryocooler system in order to have the necessary autonomy and reduce the vibrations to a minimum. A possible problem is the dependance of the performances of this kind of system with the inclination of the tube itself, which will follow the orientation of the telescope;
  • all the control/command of the camera will be done through a PLC system using a FIP field bus.

  
  

    

  Figure 1: The MEGACAM camera

  
  
  
• The contract for the procurement of the CCDs has been signed in May (see sect. 3). Two mechanical devices and one engineering grade device have already been received and are being tested in Saclay.
• A prototype of the pulse tube cryocooler, meeting the MEGACAM requirements, has been delivered to Saclay and is being tested.
• A Critical Design Review is scheduled for mid-November, after which the design will be frozen.

CCDs

  There are two main requirements on the CCDs for MEGACAM:

• because of the science programmes that will be performed with the instrument, the detectors must have a very good sensitivity in the blue and ultra-violet (see table 3 for the specifications for the quantum efficiencies);
• because of the size of the field of view, the CCD mosaic must be very flat in order to avoid any defocusing.

 

lambda (nm) required QE (%) expected QE (%)

$$\geq$$ 350 45

$$\geq$$ 400 70

$$\geq$$ 600 85

$$\geq$$ 800 60

 

For these reasons, we have chosen the EEV 42-90 series of CCDs. These devices have 2Kx4.5K pixels, each pixel being 13.5 $\micron$ square, which will give a scale of about 0.185 arcsec/pixel. The contract for the procurement of the CCDs was signed in May: CFHT has ordered a set of 40 CCDs (including 4 spares) which will populate an 18Kx18K mosaic. If additional funding is available, then CFHT will buy 4 more CCDs (see fig. 2).

The CCD packaging that EEV designed will ensure that the whole mosaic will be parallel to the focal plane to better than 60 $\micron$. The gap between the last column of a CCD and the first column of the next CCD will be about 1 mm (ie 13.5 arcsec); there will also be two 5 mm (ie 67.5 arcsec) gaps, one between the first and the second row of CCDs, and one between the third and the fourth row of CCDs, to make room for the CCD connectics.

The delivery of the science grade devices will start in January 1999, and should proceed at a rate of 2 devices per month until November 1999 and 3 devices per month from December 1999 to May 2000. The population of the mosaic will start only after all 40 CCDs have been received, accepted and characterised; this will allow us to put the best CCDs at the center of the field of view.

  

Figure 2: The 40 CCDs mosaic

Test Bench

  A test bench has been built in Saclay for the acceptance and characterisation of the 40 CCDs. It is composed of a cryostat which can simultaneously hold two 2Kx4.5K CCDs, a Fe55 source to do the CTE measurements, a light-tight ``black box'' containing an integrating sphere for flat field measurements and some optics for point source measurements, and a light source made of a tungstene halogen lamp or xenon arc lamp, two monochromators covering the UV and visible ranges, and a fiber feed to the integrating sphere. Photo-diodes and a photo-multiplier tube have been put inside the black box to monitor the output of the integrating sphere (see fig. 3). The operating temperature of the cryostat can be anywhere between 120 and 180K.

Being able to have two CCDs mounted in the cryostat at a time will allow us to speed up the testing: after reception of a CCD from EEV, we will have 40 days to test it and accept it, but the CCDs will be delivered at a rate of 2 to 3 per month, which means that the backlog of untested CCDs would grow very quickly if we were to test them one at a time.

Given the number of CCDs to test, it is very important that all the measurements can be performed in an automated way, using a set of well defined and documented procedures. The purpose of the engineering grade device is twofold: first, it is being used to derive the acceptance test procedure that we will apply to all science grade CCDs as we receive them from EEV; and second, we will also use it to define which additional tests need to be performed - if any -, and to derive the corresponding test procedures. Once defined and tested against the engineering device, these procedures will be frozen in order to ensure an homogeneous acceptance and characterisation of all the science devices.

  

Figure 3: The CCDs test bench

Planning

  The planning for the development of the camera is the following:

• 15 July - 15 September 1998: definition of the acceptance test procedure for the science grade CCDs;
• mid-November 1998: Critical Design Review, after which we will start the fabrication of the different subsystems of the camera;
• January 1999 - May 2000: delivery of the science grade CCDs; a period of 40 days is allocated to test and accept each CCD;
• starting in May 2000, after the acceptance of all the CCDs, we will start populating the mosaic;
• May 2000 - January 2001: integration of the different subsystems of the camera in Saclay;
• January 2001 - July 2001: integration of the camera with the telescope and commissionning at CFHT;
• 2nd semester 2001: MEGACAM available to the community.

References

1

O. Boulade, L. Vigroux, X. Charlot, P. Borgeaud, P. Carton, J. de Kat, J. Roussé, and Y. Mellier, ``Megacam, the next generation wide-field camera for CFHT'', in Optical Astronomical Instrumentation, S. D'Odorico, ed., Proc. SPIE 3355, pp. 614-625, 1998.