2 Instrument description

  

Figure 1: Optical design of the Echelle spectrometer. Legend: F= telescope focus and entrance diaphragm, CM= collimator mirror, EG= Echelle grating, CD= cross disperser, ED= Echelle detector, L= long wavelength side, S= short wavelength side

Within the ORFEUS telescope a movable mirror is used to switch between the two spectrometers. We will restrict the instrument description to the Echelle spectrometer.

The movable mirror is designed to act at the same time as an off-axis parabolic collimator (CM, Fig. 1). When moved in it feeds the Echelle grating (EG) with a parallel light bundle. When the collimator mirror is moved out, the beam is falling directly into the Berkeley spectrometer.

The Echelle grating has a groove density of 316 lines/mm with a blaze angle of 62$.\!\!^\circ$5. It is operated in the diffraction orders 40 to 61 covering the wavelength range from 90nm to 140nm.

The Echelle diffraction orders are separated by means of a spherical cross disperser grating (CD) with 1200 lines/mm which also serves as the spectrograph camera. This design minimizes the number of reflections, which, in view of the low reflectivities in the FUV (ranging from 10% to 30%), was of critical importance for achieving an acceptable overall efficiency.

The spectrum is recorded by the Echelle detector (ED), which has a plane surface and a sensitive area of 40mm$\times$40mm.

The Echelle spectrometer is designed to achieve a spectral resolution of $\lambda$/$\Delta$$\lambda$=10000 when used with an entrance aperture of 10$^{\prime\prime}$ diameter (Appenzeller et al. 1988).

2.1 The Echelle detector

The Echelle detector is a photon counting microchannel plate detector with a wedge and strip readout system. Three stacked microchannel plates (MCPs), operated in a Z-configuration, provide a gain of 10⁷ to 10⁸ electrons/photon. Each of the three MCPs has its own power supply and an accelerating voltage is applied to the gaps between the MCPs. During flight the gain of the Echelle detector was monitored and could be controlled by variation of the high voltage of the third MCP. The high voltage could be adjusted by commands during flight or by starting an automatic function that adjusted the high voltage so that a predefined gain value was achieved.

A repeller grid in front of the MCPs produces an electric field of about 50V/mm, which is used to force those photo electrons back into the MCP channels, which are released from the areas in between the channels. This improves the quantum efficiency by about 30% but also causes a loss of 10% due to the shading by the repeller grid.

A four electrode wedge-and-strip anode (Martin et al. 1981) was used as readout system for the MCP detector. A special design allowed for a significant reduction of the edge distortions, which are basically unavoidable. Further, by oversizing the active area of the anode to 44mm$\times$44mm, it was possible to avoid any distortions within the sensitive area of the detector. The image format of the detector is 1024 pixels by 512 pixels, which corresponds to the active area of the anode.

The detector sends for each photon event x/y-coordinates to the Echelle onboard processor where these events are integrated onto an image memory. The content of this image memory is stored on magnetic tape at the end of each observation.

In parallel, the x/y-coordinates for each single photon event are stored on tape directly, thus keeping also the photon arrival time to an accuracy of better than 1 s. This method is limited by the ASTRO-SPAS interface to an event rate of less than 3500 counts/s, whereas the onboard integration works with much higher photon rates up to 30000 counts/s. The electronic dead time is about 13$\mu$s per event.

The count rate of the Echelle detector is monitored online and also stored on tape together with many other housekeeping data. This count rate is derived from all events registered by the charge amplifiers falling above the lower threshold of the detector electronics. Not all of these events are finally integrated into the image for the following reasons:

• An upper electronic threshold suppresses pulses too large to be processed by the position decoding electronics.
• At high count rates the dead time of 13$\mu$s for position decoding reduces the rate of registered events accordingly.
• Dead time effects from the interface electronics and from the onboard processor further reduce the electronic efficiency.

These effects are the cause that in general the rate of the integrated counts within an image is between 15% and 25% lower than the count rate of the lower electronic threshold.

In the main dispersion direction somewhat more than 3 pixels correspond to one optical resolution element of $\Delta$$\lambda$=$\lambda$/10000 (161$\mu$m). The electronic resolution was estimated to be about 1.5 detector pixels FWHM. So the electronic detector resolution is sufficient to maintain the optical resolution of the Echelle spectrometer.

Electronic test pulses were fed onto the anode with a frequency of 10Hz at two different detector positions and with 3 different pulse heights. These test pulses were extremely useful during the checkout of the instrument and provide a means of monitoring the electronic performance of the detector throughout the mission.