ISSIS Print

ISSIS design guidelines can be summarized in the next items:

  • Maximize radiometric efficiency in FUV. ISSIS optical design must be developed to reach a high efficiency in the FUV range, minimizing the number of optical elements involved. High FUV QE detectors shall be employed.
  • Minimize spatial resolution degradation. ISSIS design shall minimize any further degradation of the spatial resolution reached with the T-170M telescope optics and the S/C AOCS.
  • Provide adequate PSF sampling. Accurate astrometric and photometric measurement be guaranteed through an appropriated PSF sampling. The optical design shall provide the necessary magnification for each selected detector.
  • Maximize field of view. A high FoV increases the scientific outcome of the mission because larger surveys are obtained in the same observation time. Optics, detectors and processing electronics shall be designed to handle a FoV as high as possible.

  1. ISSIS outline

  2. ISSIS versus HST

  3. More information on ISSIS


ISSIS outline:

The performance of current UV detectors has driven the design. The input radiation is driven to two possible channels:

  • The High Sensitivity Channel (HSC) to provide the optimum sensitivity for UV imaging and slitless spectroscopy in the Far UV range. The detector is a CsI MCP with high sensitivity in the 1150-1750Å
  • The Channel for Surveys (CfS) to provide a large field of view from the far UV (1150Å) to the optical range. The detector is a back thinned CCD. This channel will allow to carry out far UV imaging of bright UV objects such as the Orion nebula. Since the sensor response extends into the optical, red-leakage can be important when observing red sources in the UV. A blocking filter is included in the design at the expense of additional radiometric losses in the FUV.

issis-table

ISSIS will be mounted to the Optical Bench (OB) top base in three points. Three spectrometers – UVES, VUVES and LSS are mounted to the OB bottom base. The three spectrographs are joined into a single structural unit – monoblock– before installation on the OB. The monoblock fastens to OB in three points.

The Guide 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 their input devices are located in a working zone of the Focal Surface limited by the 30’ field of view of the Telescope.

The Optical Bench is a flat triangular frame. The frame consists of tube profiles of rectangular section. The profiles are connected to each other with integral force brackets. The frame mounting points are installed to the brackets. The points are attached to the Optical Bench suspending units. The Spectrograph Assembly and the ISSIS mounting points are provided at the brackets also. The Optical Bench loading structure and its suspending units are made from invar 32NKD.

ISSIS

  1. High Sensitivity Channel The HS Channel shall be optimized for maximum sensitivity at 1216Å (Lyman-alpha). The channel will use a solar-blind MCP detector to provide high FUV efficiency together with high rejection in the optical range. This channel will include at least 10 spectral filters and one dispersive element (or more, to avoid overlapping between spectra from different field positions) positioned with filter wheels. The location of the filters in the optical path shall minimize the filter diameter without degrading the instrument optical performance. The maximum angle of incidence on the spectral filters and dispersive elements shall be minimized. The channel will also include the following elements:
  • Refocusing mechanism.
  • Calibration unit, comprised of a calibration lamp, optics and a mechanism for calibration mirror insertion into the optical path.
  1. CfS will use large format CCD detectors (high dynamic range) optimized for high quantum efficiency it shall include at least 20 spectral filters and one or more dispersive elements (see HSC description) positioned with filter wheels. The location of the filters in the optical path shall minimize the filter diameter without degrading the instrument optical performance. The maximum angle of incidence on the spectral filters and dispersive elements shall be minimized. CfS channel shall provide means to perform coronagraphic-like measurements with a mask deposited directly on the CCD surface. The channel will also include the following elements:

  • Refocusing mechanism. 

  • Calibration unit, comprised of: calibration lamp(s), optics and a mechanism for calibration
    mirror insertion into the optical path.

  • Shutter mechanisms for the CCDs.

ISSIS versus HST

The expected performance of ISSIS/HSC is similar to that of HST/ACS/SBC

 

More information on ISSIS

ISSIS phase A studied was concluded in December 2008. A consortium was built including UCM, INTA, CSIC and the Spanish company SENER that acted as prime contractor. More information on this study is available upon request.