The scientific instrument for the JAST/T80 telescope and for J-PLUS is T80Cam, a wide-field camera installed at the Cassegrain focus. It is equipped with a 9.2k-by-9.2k, 10μm pixel, high efficiency CCD that is read from 16 ports simultaneously, allowing read times of 12s with a typical read noise of 3.4 electrons (RMS). This full wafer CCD covers a large fraction of the telescope’s field of view (FoV) with a pixel scale of 0.55"/pixel. J-PLUS will observe more than 8500 square degrees of sky through a set of 12 carefully optimized broad- and narrow-band filters.
T80Cam consists of two main systems (Figure 1): the filter and shutter unit (FSU) and the cryogenic camera. The FSU holds two removable filter wheels and the shutter. The camera system, below, comprises the cryostat, the cooling and vacuum systems, the CCD, an optically powered entrance window and the detector electronics. These two subsystems are described in the following.
The camera is an 1110S cryogenic camera manufactured by Spectral Instruments (Tucson, AZ, USA). It is equipped with a grade-1 e2v CCD290-99 backside illuminated CCD (Figure 2), a 9.2k-by-9.2k, 10μm pixel high efficiency CCD. This state-of-the-art, large format CCD has an image area of 92.2 x 92.4mm, covering a large fraction of the JAST/T80 FoV with a pixel scale of 0.55"/pixel. The CCD is read from 16 ports simultaneously allowing read times of 12s with a read noise of 3.4 e- (RMS). The sensor has a broadband anti-reflective (AR) coating for optimized performance from 380 to 850nm.
The camera control electronics allow 16 different read modes, listed in table 1. Also, regions of interest (RoI) can be selected with consequent shortening in the read time. Image acquisition software provides FITS images with headers containing all the information relevant to the camera and telescope operation.
Together with the CCD and its control electronics, the camera system is completed with a powered cryostat entrance window and cooling and vacuum systems. The CCD is cryocooled to an operating temperature of -100°C with a cryo-tiger, closed cycle refrigeration system. Figure 3 shows the Spectral Instruments 1110S cryogenic camera upon arrival at CEFCA headquarters.
|MODE||Binning||Gain||Read Time (s)||Read Noise (e-, RMS)|
FILTER AND SHUTTER UNIT (FSU)
The FSU has been designed and manufactured by the Instituto Nacional de Pesquisas Espaciais (INPE) and CEFCA. It includes the filter unit, the shutter and the cryogenic camera support flange. The later allows for fine alignment of the camera with respect to the telescope optical axis. The FSU holds two removable filter wheels, each one capable of holding 6 filters. Filter wheels control is performed using stepper motors and absolute rotary encoders. The shutter has been provided by Bonn-Shutter UG. It is a 125mm clear aperture, double curtain shutter that offers a minimum exposure time 0.1s of and an exposure homogeneity better than 1ms over the full FoV. Figure 4 shows the FSU during its integration and testing at CEFCA headquarters.
T80CAM PERFORMANCEST80Cam and JAST/T80 image quality has been carefully analysed and is shown below. The polychromatic encircled energy corresponding to the center (0 deg), mid (0.5 deg) and edge (0.85 deg) of the full FoV is represented in Figure 5. The diffraction limited squared energy (EE) formed by the same elements is shown for comparison. The spot diagrams are represented in Figure 5 for field angles of 0°, 0.25°, 0.5° and 0.85°.
|FoV||1.4º x 1.4º|
|EE50||Ø =6 μm|
|EE80||Ø =13 μm|
|CCD format||9,216 x 9,232 pix, 10 μm/pix|
|Pixel scale||0.55 “/pix|
|Number of read modes||16|
|Read out time (mode05)||12s|
|Read out noise (mode05)||3.4 e- (RMS)|
|Full well||90,000 e-|
|QE||40% (350nm), 86% (400nm), 93% (500nm), 93% (650nm), 61% (900nm)|
|Linearity||1% up to 74ke-|
|Minimum exposure time||0.1s|
|Exposure homogeneity||>1 ms|
|CCD operating temperature||-100ºC|
|Number of filters||12|
T80CAM FILTERS: THE J-PLUS FILTER SYSTEM
The J-PLUS survey has been conceived to perform the calibration tasks for the main J-PAS survey. In this context, the J-PLUS filter system has been optimized to properly recover stellar parameters, Teff, log g, [Fe/H], through the fitting of flux calibrated models of the observed stars. Detailed simulations have proved that this task can be done with a set of 12 carefully selected broad- and narrow-band filters. Table 3 shows the J-PLUS filters main characteristics.
The filters have been manufactured by SCHOTT and have been received, characterized and accepted by CEFCA. Measured filter transmission curves are shown in Figure 6.
The filters have been scanned at CEFCA and a 2D map of each one has been measured and incorporated onto the data reduction pipelines for a proper data analysis. Filters’ transmission curves, and hence their central wavelength, have been measured at CEFCA for a complete characterization of the system. As an example, figure 7 shown the 2D map of J-PLUS filter 9 (H-alpha). Central wavelength shift along the filter’s surface is expressed in % of central wavelength.