The camera unit of the WSO-UV is named ISSIS: the Imaging and Slitless Spectroscopy Instrument. The main requirements for the ISSIS design are:
- High radiometric efficiency in the FUV range, minimizing the number of optical elements involved.
- Low degradation of the spatial resolution.
- Adequate PSF sampling to guarantee accurate astrometric and photometric measurements. The optical design shall provide the necessary magnification for each detector.
- Wide field of view to increase the scientific outcome of the mission.
The original ISSIS design evolved from Phase A of the instrument development, following the constraints in dimensions of the platform, mass budget, environment conditions. The current design was approved in the Preliminary Design Review (PDR) in June 2012, and includes two acquisition channels for imaging and slitless spectroscopy:
- The Far Ultraviolet Channel (FUV), providing the optimum sensitivity in the 1150-1750 Å interval, and equipped with a CsI MCP detector.
- The Near Ultraviolet Channel (NUV) covering wavelegnths in the 1850-3200 Å range; the detector is a CsTe MCP.
ISSIS will be mounted onto the Optical Bench (OB) top base and fixed in three points. Similarly, the three spectrometers UVES, VUVES, and LSS are joined into a single monoblock structural unit (WUVS), and then mounted onto the OB bottom base, by fastening in three points.
The sensor of the FGS is located on the case of each spectrograph near to its input slit. The recording equipment on the OB is mounted in such a manner that the input devices are located in a working zone of the Focal Surface limited by the 30’ field of view of the Telescope.
The layout is arranged as a telephoto system (positive elements are followed by negative elements), to enlarge the overall focal length. A refocusing mechanism (RM), located at the coupling stage between the telescope and ISSIS, changes the distance between the intermediate image from the telescope and the first mirror of ISSIS. The different operation modes for the two channels (imaging, spectroscopy, or calibration) are selected trhough the mode selection mechanism:
- Imaging: the MSM uses flat mirrors aligned to direct the light into the selected channel (FUV or NUV). The optical filters are accomodated on two wheels for each channel: there are long-pass filters and narrow band filters for specific investigations, as well as two neutral filters for each channel, to increase the dynamical range.
- Spectroscopy: the beam is directed to the dispersive elements, which are reflection gratings located at the MSM; in the nominal spectroscopy mode, the filter wheels are positioned into a hole configuration (no filter). The FUV diffraction grating has peak efficiency at 1400 Å and groove density of 450 lines/mm, while the NUV one has the peak at 2300 Å and 250 lines/mm.
- Calibration: a shutter is used to block the light from the telescope, and the beam is directed to the calibration subsystem in order to take flat field images on a pixel-to-pixel basis. The calibration unit includes the lamp, optics and shutter.
- Coronographic mode: is defined to alleviate the detector fatigue when observing faint structures close to bright sources. These dead areas on the detector could be implemented with masks in the photo-cathode.
The next table summarizes the main ISSIS characteristics:
|FUV Channel||NUV Channel|
|Spectral range||1150-1750 Å||1850-3200 Å|
|Peak throughput (imaging)||1400 Å||2300 Å|
|Field of View: imaging||70 arcsec x 75 arcsec||70 arcsec x 75 arcsec|
|Field of View: spectroscopy||36 arcsec x 65 arcsec||36 arcsec x 65 arcsec|
|Detector type||CsI MCP||CsTe MCP|
|Detector diameter||40 mm||40 mm|
|Detector format (equivalent)||> 2048 x 2048 pix||> 2048 x 2048 pix|
|Pixel scale||0.036 arcsec||0.036 arcsec|
|Scale ratio||< 7 %||< 7 %|
|Number of reflections||4||4|
|Temporal resolution||40 ms||40 ms|
|Slitless spectroscopy resolution||R=500||R=500|
At the end of 2012, ISSIS Structural and Thermal Model (STM) was delivered to the Russian company Lavochkin, for the integrated tests of WSO-UV main components. Prior to the delivery, mechanical and thermal tests were performed and successfully passed in the Center for Aerospace Technology (Parque Tecnológico de Miñano, Álava).
At the end of 2014, the Institute of Astronomy of the Russian Academy of Sciences (INASAN) funded the Institute of Space Science (IKI) to develop a simplified version ISSIS's electrical model for the integrated tests. UCM team provided INASAN and IKI with the relevant information in terms of data flow and management and ISSIS operation modes.
ISSIS - Exposure Time Calculator
The Spanish WSO-UV Team at the Universidad Complutense de Madrid has developed an Exposure Time Calculator (ETC) for ISSIS. The ETC allows the astronomers to evaluate the obseving time needed for a scientific program. With the ISSIS ETC the user can choose to calculate the exposure time of an observation to achieve a desired signal-to-noise ratio (S/N), or viceversa. The ISSIS ETC is available here. The user's manual can be downloaded as a PDF.