A filter free dual transmission grating spectrometer for the extreme-ultravioletSeth R. Wieman*a, Leonid V. Didkovskya, Darrell L. Judgea, Andrew R. Jonesb, Matthew Harmonba Univ. of Southern Calif.; Space Sciences Center; SHS-274; UPC; Los Angeles, CA 90089; b University of Colorado, Boulder; LASP; 1234 Innovation Dr.; Boulder, CO 80303 ABSTRACTWe report the design and laboratory testing of a prototype dual-grating filter-free extreme ultraviolet (EUV) spectrometer that has potential as a highly stable instrument for measuring absolute solar irradiance in the X-ray through far ultraviolet spectral range. The instrument is based on the same freestanding transmission gratings and silicon photodiodes used on the successful Solar EUV Monitor (SEM) aboard SOHO and the EUV Spectrophotometer (ESP) part of the EVE instrument suite to be flown on SDO. Its two gratings, placed in series, along with a simple baffle structure provide excellent out of band “white” light rejection. Because the instrument does not use any thin film filters or reflective optics it is not susceptible to the degradation and instability associated with such optical elements. We present photometric efficiency data from laboratory tests with a Helium and Hydrogen discharge light source and measurements of “white” light rejection taken using the Mt Wilson Observatory 60’ solar telescope. Keywords: EUV, transmission grating, spectrophotometer, spectrograph, photometer 1. INTRODUCTION Long-term stable measurements of solar flux in the highly variable X-ray, extreme ultraviolet (EUV) and far ultraviolet (FUV) spectral regions are key to understanding the science and photochemistry underlying many solar system phenomena and require instrumentation that is not susceptible to degradation and time-dependent change1. However, to monitor solar EUV against a background of far more intense visible light, EUV spectrometers conventionally rely on thin metal film filters, which suffer from long-term stability problems. Specifically, transmission characteristics of thin film filters may change over time2,3 due to an interaction with chemical elements, e.g., carbon and may develop pin-holes due to the impact of ambient particles. Some additional problems with thin film filters are that they typically have multiple bandpasses for a given film and do not allow precise selection of the desired wavelength band of interest. We report the design and testing of a prototype EUV spectrometer that does not use thin film filters but instead suppresses background visible light through the use of two freestanding transmission gratings in series (i.e., the incoming EUV radiation is diffracted twice before reaching the detector). This instrument is an enhanced version of our CELIAS Solar EUV Monitor (SEM) instrument aboard SOHO, which uses a single transmission grating and two thin film filters in series to eliminate pinhole overlap and thereby eliminate visible light detection. Having provided EUV irradiance measurements for over 11 years now, SEM has proven to be a highly stable, robust spectrometer, but has, nevertheless, suffered minor degradation related mainly to carbon deposition on its Al filter. Freestanding gratings of the type used in our spectrometer are the subject of considerable previous research4-9. They have been used successfully on space flight missions including SOHO6,7 and multiple sounding rocket flights8, and their diffracting5,9, polarizing10-12 and wave-guide5,13 properties have been investigated. The gratings consist of a set of parallel gold bars, separated by gaps, and supported by larger scale nickel mesh as shown in Figure 1. Gratings with 400 nm and 600 nm periods are used in the prototype reported here. The grating with the 400 nm period has a bar to gap width ratio of 0.8 and thickness of 470 nm. The bar to gap width ratio for the 600 nm period grating is 1.0 and its thickness is 490 nm. These gratings were selected from a limited number that were available to us at the time of these tests, and were not formally optimized based on line density, bar to gap ratio, and thickness, all of which effect transmission. The DGS signal to noise and signal to background ratios reported below could be improved by such an optimization. The gratings *[email protected]; phone 1 213 740 5751; fax 1 213 740 6342;were fabricated at Massachusetts Institute of Technology using a holographic lithography technique described in detail in Schattenburg et al14.Fig. 1. Schematic view of a freestanding transmission gratingHere we assess how effectively the dual-grating spectrometer (DGS) isolates resonance lines of helium at 58.4 nm and hydrogen at 121.6 nm (Lyman-Į) from the rest of the solar spectrum based on two sets of measurements. First, the DGStransmission at these EUV wavelengths is measured using a gas glow discharge lamp and vacuum monochromator as a light source. Second, the degree to which solar “ white” light (as stray light within the optics cavity or as scatter directlyoff of the gratings) reaches the DGS detector is determined with a similar set of measurements made using the 60’ solar telescope at Mt Wilson Observatory - a light source that is comparable in spectral content to the light that it would benecessary to reject in space-based solar measurements. The DGS instrument used in these tests is described in section 2. The procedure for the transmission measurements is described in Section 3. In section 4, the results of thesemeasurements are presented along with a discussion of what they suggest about the instruments’ performance in space-based solar measurements. Concluding remarks are in the summary.2. INSTRUMENT DESCRIPTIONThe laboratory dual grating spectrometer (DGS) prototype (Figure 2) consists of a hermetically sealed optics cavity with2 transmission gratings (labeled G1 and G2, respectively, in Figure 2) and an uncoated EUV sensitive silicon photodiode detector (the “primary” detector, labeled D1). Initial dispersion of incoming radiation is achieved with grating G1,positioned immediately behind a 1 mm u 10 mm entrance slit. The EUV spectral radiation is then diffracted a second time through Grating G2, centered on the first diffracted order from G1, before reaching detector D1. The primarydetector and its aperture (the spectrometer exit slit) are mounted on a linear translation stage that allows them to be scanned (along the x-axis) through the diffraction pattern produced by G2.