FORTIS: A Rocket-Borne Far-UV Spectro -Telescope
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` FORTIS: A Rocket-Borne Far-UV Spectro- Telescope Brian Fleming 1 , S.R. McCandliss 1 , M.E. Kaiser 1 , J. Kruk 1 , P.D. Feldman 1 , A.S. Kutyrev 2 , S.H. Moseley 2 1 Johns Hopkins University, 2 NASA Goddard Space Flight Center Abstract The Far-Ultraviolet Off-Rowland Telescope for Imaging and Spectroscopy (FORTIS) is a rocket- borne spectro-telescope designed to investigate Lyman alpha (Lyα) escape from nearby star-forming galaxies and to quantify its relationship to the local gas-to-dust ratio. By operating in the 900 - 1700 Angstrom bandpass and incorporating a GSFC microshutter array at the prime focus, the FORTIS instrument enables simultaneous observations of both Lyα and the far-UV continuum of the brightest HII regions of low redshift starforming galaxies. Ancillary ground-based optical spectroscopy of the target will provide the necessary Balmer line measurements to calculate the Lyα escape fractions (Fleming et al. 2010). We present the optical design and capabilities of the FORTIS instrument, the first flight of which is currently scheduled for April of 2011 to observe the galaxy cluster Abell 1367. Primary Science Objectives Star-forming galaxies are currently believed to be the dominant sources of the ionizing radiation responsible for re-ionization at z ~ 6, however the fraction of ionizing radiation (Lyman continuum, LyC) that escaped from the first galaxies is currently unknown. Due to increasing HI opacity at redshifts z > 3, direct observation of escaped LyC radiation in the earliest galaxies is considered to be effectively impossible. Lyman alpha (Lyα), which in star-forming galaxies is dominantly produced by the recombination of ionized hydrogen, is a potential proxy for LyC as the Lyα intensity is effectively a measure of the number of LyC photons which do not escape. If a quantitative relationship between escaping LyC and Lyα emission can be established in low-z analogs, then the easily observed Lyα could be used to estimate the contribution of star-forming galaxies to the meta-galactic ionizing background over time. The primary science goal of the FORTIS instrument is to measure to Lyα escape fraction (f Lyα ) and the gas-to-dust ratio as a function of position in nearby resolved star-forming galaxies. We seek to determine whether the observed f Lyα is correlated to the gas-to-dust ratio and ultimately if the same factors which govern f Lyα also determine the amount of LyC escape. The FORTIS instrument is designed to carry out these objectives within the scope of a sounding rocket mission. A full scale program to fully examine the LyC escape fraction and f Lyα could then be completed within the parameters of an orbital mission. FORTIS Overview The FORTIS spectro-telescope design is Gregorian telescope (concave primary and secondary mirrors) with a diffraction grating holographically ruled onto the secondary. The diffractive secondary provides two off-axis spectrally dispersed channels to either side of an on-axis imaging channel all in the same plane. This two-bounce design expands the efficiency, angular coverage, and spectral multiplexing capability of FORTIS in comparison to that of a traditional longslit three-bounce far-UV Rowland circle spectrograph and telescope. Astigmatism in the spectral orders is controlled by slightly altering the radius of curvature of the secondary mirror/grating in the direction perpendicular to the dispersion, making the secondary mirror into the form of a triaxial ellipse. Correcting for astigmatism in the spectral channels introduced astigmatism into the imaging channel. A cylindrical doublet is used to remove this aberration. Multi-object capability is provided by an array of microshutters at the focus of the primary. The FORTIS microshutter arrays (MSA) are small prototype devices of the larger arrays developed at Goddard Space Flight Center (GSFC) for use in Optical Elements The FORTIS optical system consists of a parabolic primary mirror, a diffractive secondary triaxial ellipse, a zero-order CaF 2 and MgF 2 cylindrical doublet to correct for astigmatism in the imaging focal plane, and an optical bench. We recently accepted delivery of two astigmatic secondary ellipses and have shipped them to the grating vendor. The optical bench is built up from an Al mirror baseplate, which provides the attachment points for the central hub of the primary mirror, the secondary metering tower (carbon fiber facesheet over Al honeycomb (fig.1)) and the detector focusing bellows. The MSA sits at the prime focus. The entire optical path is evacuated to < 10 -5 torr prior to launch to avoid undue gas load of the detectors. Fig. 2 – (left) Raytrace of the tangential focus of the FORTIS secondary aspherical ellipse illuminated by a 5μm pinhole lightsource located at the prime focus. (middle) Measured image of the tangential focus of one of the FORTIS secondary mirrors illuminated by a 5μm pinhole at prime focus. (right) Projection of the two tangential focii onto the y-axis. Fig. 3 – (left) Far-UV zero-order cylindrical achormat. (middle) Zero-order raytrace PSFs at the focal plane without achromat and (right) with achromat. Microshutter Array The MSA consists of a 2D array of closely packed clear aperture slits, each with an independently selectable shutter (Kutyrev et al. 2004). An array is composed of a thin shutter membrane and light shield mounted to a 100 μm × 200 μm pitch of rectangular holes. The matching shutter array is machined into a silicon nitride membrane 0.5 μm thick. An individual shutter blade is suspended from the shutter frame by a torsion flexure. The shutters are coated with a layer of high permeability magnetic material to allow for rotation of the shutters by a passing magnet. Electrodes, deposited on one of the interior walls of the shutter support grid and the shutters themselves, allow for electrostatic latching and release. Fig. 4 – (left) Rendering of the MSA mount with scanning magnet supports and electronics. (middle) 64 × 128 microshutter array with an area of 12.8 mm 2 . (right) Schematic cross section of a microshutter unit cell showing key elements. Detector System The FORTIS detector array consists of three CsI coated microchannel plate (MCP) stacks readout by a crossed delay-line anode. The detector is housed in a vacuum vessel with a vacuum door that will be opened during flight. One of the features of the FORTIS design is that a judicious choice of platescale (41.25” mm -1 ) and dispersion of 20 Å mm -1 results in a flat focal plane in the spectral channels, eliminating the need for curved microchannel plate detector. This allows for a large spatial FOV (0.5°) 2 and 900 – 1800 Å in two outrigger spectral channels. The imaging detector area is ~ (45 mm) 2 and the two adjacent spectral areas are 45 mm × 62.5 mm. Fig. 5 – Rendering of detector inside the mounting flange. FORTIS Optical Parameters Launch Targets Targets for observation were selected based on redshift and the estimated Lyα surface brightness. A redshift of z ≥ 1000 km/s is required to move the Lyα emitted from the target out of the geocoronal Lyα window. We use the GALEX flux calibrated FUV and NUV images to simulate Lyα emission from potential targets. We assume that each pixel emits a powerlaw with a slope of β o and the Lyα EW is ~ 20 Å. We have selected five targets that FORTIS can observe in a 400s flight to achieve the science goals. Fig. 7 – Simulation of Lyα emission from the cosmic web (Furlanetto et al. 2003). The bright points in (b) are galaxies with surface brightness > 10 6 photons cm -2 s -1 sr -2 . (right) GALEX composite of the Antennae galaxy, which would have similar Lyα brightness if f Lyα ~ 20% Observation Strategy FORTIS will initially launch with all of the shutters in the MSA closed. There will be an unknown number of failed shutters in the “open” position which will provided a minimum backup in case of addressing failure. When the telescope has slewed onto target, the payload shutter door will open and the magnet will scan the face of the MSA, transitioning into the “all open” configuration. The imaging (zero-order) MCP will observe the target for ~ 30 seconds while an onboard National Instruments cRIO FPGA unit will analyze the accumulated image on the fly to determine the brightest “pixel” (shutter) in each row of shutters. Only one shutter per row may be open during spectral acquisition to reduce confusion. A command sent from the ground will order the magnet to rescan the array and the latching voltages will be applied in such a way so that only the brightest shutter in each row remains open. It will remain in this configuration for the duration of the flight. Fig. 6 – Simulated data for NGC 1365. The zero-order image of the target with the open microshutter apertures overlaid is in the center. The wavelength scale at the top is for the topmost spectra only. Future Work Calculating f Lyα : Ancillary measurements of the Balmer series lines Hα and Hβ are necessary for each target so that the intrinsic Lyα flux can be calculated from recombination theory. This data will be obtained through either public databases or observing time at the Apache Point Observatory in New Mexico. The ratio of the observed Lyα flux to the calculated intrinsic Lyα flux determined from extinction corrected measurements of Balmer lines yields f Lyα . Science Enabled by FORTIS: In addition to providing an unparalleled classroom for training experimental astronomers, sounding rocket programs enable the development of new observing strategies due to the high risk/low cost nature of the program. The technologies employed in FORTIS; large format UV detectors, clear aperture MSA and on-the-fly targeting algorithms, are relevant to those required to enable science studies detailed in many Astro2010 whitepapers (Bertone et al. 2009; McCandliss 2009; Martin et al. 2009; Postman 2009; Scowen et al. 2009a,b; Strickland et al. 2009). Some examples of science that could be done with a FORTIS-like orbital instrument are: • Search for the epoch of HeII reionization • Baryon acoustic oscillations with the Lyα forest • Visualizing the Lyα web • Directly measuring LyC and Lyα escape from nearby galaxies FORTIS is ideally suited to explore a number of science goals in addition to its primary science mission if it were launched as part of the proposed long duration sounding rocket program discussed in the Astro2010 decadal survey. Acknowledgements This work is funded by NASA grant number NNX08AM68G to JHU (McCandliss, PI). References •Bertone, S. et al. 2009. The Cosmic Web. Astro2010: The Astronomy and Astrophysics Decadal Survey , Vol. 2010, pp. 270 •Kutyrev, A. S. et al. 2004. Programmable Microshutter Arrays for the JWST NIRSpec: Optical Performance. IEEE J. Select. Topics Quantum Electron. 10, pp. 652-718 •McCandliss S.R. et al. 2004. FORTIS: Pathfinder to the Lyman Continuum. Proceedings of the SPIE, Vol. 5488, pp. 709- 718 •McCandliss, S.R. 2008. Project Lyman. SPIE Conference, Volume 7011, pp. 20:1-12 •McCandliss, S.R. 2009. Project Lyman: Resolving the Physics Behind Reionization. Astro2010: The Astronomy and Astrophysics Decadal Survey, vol. 2010, pp. 196 •Martin, C. et al. 2009. The Science Enabled by UV Emission Line Mapping of the Intergalactic Medium and Circum- Galactic Medium. Astro2010, Vol. 2010, pp. 195 •Postman, M. 2009. Advanced Technology Large-Aperture Space Telescope (ATLAST): A Technology Roadmap for the Next Decade. ArXiv e-prints. •Strickland, D.K. et al. 2009. Starburst Galaxies: Outflows of Metals and Energy into the IGM. Astro2010: The Astronomy and Astrophysics Decadal Survey . Pp. 289 • Fig. 1 – Schematic drawing of the FORTIS array and photo of the optical bench.
description
FORTIS: A Rocket-Borne Far-UV Spectro -Telescope. Brian Fleming 1 , S.R. McCandliss 1 , M.E. Kaiser 1 , J. Kruk 1 , P.D. Feldman 1 , A.S. Kutyrev 2 , S.H. Moseley 2. 1 Johns Hopkins University, 2 NASA Goddard Space Flight Center. Optical Elements - PowerPoint PPT Presentation
Transcript of FORTIS: A Rocket-Borne Far-UV Spectro -Telescope
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FORTIS: A Rocket-Borne Far-UV Spectro-TelescopeBrian Fleming1, S.R. McCandliss1, M.E. Kaiser1, J. Kruk1, P.D. Feldman1, A.S. Kutyrev2, S.H. Moseley2
1Johns Hopkins University, 2NASA Goddard Space Flight Center
AbstractThe Far-Ultraviolet Off-Rowland Telescope for Imaging and Spectroscopy (FORTIS) is a rocket-borne spectro-telescope designed to investigate Lyman alpha (Lyα) escape from nearby star-forming galaxies and to quantify its relationship to the local gas-to-dust ratio. By operating in the 900 - 1700 Angstrom bandpass and incorporating a GSFC microshutter array at the prime focus, the FORTIS instrument enables simultaneous observations of both Lyα and the far-UV continuum of the brightest HII regions of low redshift starforming galaxies. Ancillary ground-based optical spectroscopy of the target will provide the necessary Balmer line measurements to calculate the Lyα escape fractions (Fleming et al. 2010). We present the optical design and capabilities of the FORTIS instrument, the first flight of which is currently scheduled for April of 2011 to observe the galaxy cluster Abell 1367.
Primary Science ObjectivesStar-forming galaxies are currently believed to be the dominant sources of the ionizing radiation responsible for re-ionization at z ~ 6, however the fraction of ionizing radiation (Lyman continuum, LyC) that escaped from the first galaxies is currently unknown. Due to increasing HI opacity at redshifts z > 3, direct observation of escaped LyC radiation in the earliest galaxies is considered to be effectively impossible. Lyman alpha (Lyα), which in star-forming galaxies is dominantly produced by the recombination of ionized hydrogen, is a potential proxy for LyC as the Lyα intensity is effectively a measure of the number of LyC photons which do not escape. If a quantitative relationship between escaping LyC and Lyα emission can be established in low-z analogs, then the easily observed Lyα could be used to estimate the contribution of star-forming galaxies to the meta-galactic ionizing background over time.
The primary science goal of the FORTIS instrument is to measure to Lyα escape fraction (fLyα) and the gas-to-dust ratio as a function of position in nearby resolved star-forming galaxies. We seek to determine whether the observed fLyα is correlated to the gas-to-dust ratio and ultimately if the same factors which govern fLyα also determine the amount of LyC escape. The FORTIS instrument is designed to carry out these objectives within the scope of a sounding rocket mission. A full scale program to fully examine the LyC escape fraction and fLyα could then be completed within the parameters of an orbital mission.
FORTIS OverviewThe FORTIS spectro-telescope design is Gregorian telescope (concave primary and secondary mirrors) with a diffraction grating holographically ruled onto the secondary. The diffractive secondary provides two off-axis spectrally dispersed channels to either side of an on-axis imaging channel all in the same plane. This two-bounce design expands the efficiency, angular coverage, and spectral multiplexing capability of FORTIS in comparison to that of a traditional longslit three-bounce far-UV Rowland circle spectrograph and telescope. Astigmatism in the spectral orders is controlled by slightly altering the radius of curvature of the secondary mirror/grating in the direction perpendicular to the dispersion, making the secondary mirror into the form of a triaxial ellipse. Correcting for astigmatism in the spectral channels introduced astigmatism into the imaging channel. A cylindrical doublet is used to remove this aberration.
Multi-object capability is provided by an array of microshutters at the focus of the primary. The FORTIS microshutter arrays (MSA) are small prototype devices of the larger arrays developed at Goddard Space Flight Center (GSFC) for use in the Near Infrared Spectrograph (NIRSpec) on JWST. FORTIS will provide the first flight opportunity for this device, which is ideally suited to far-UV applications because the shutters are clear apertures devoid of material. In spectral acquisition mode only one shutter per row is open to reduce spectral confusion. Targets are selected on the fly using the zero-order imaging channel to determine the brightest regions of the targets in the far-UV.
Optical ElementsThe FORTIS optical system consists of a parabolic primary mirror, a diffractive secondary triaxial ellipse, a zero-order CaF2 and MgF2 cylindrical doublet to correct for astigmatism in the imaging focal plane, and an optical bench. We recently accepted delivery of two astigmatic secondary ellipses and have shipped them to the grating vendor.
The optical bench is built up from an Al mirror baseplate, which provides the attachment points for the central hub of the primary mirror, the secondary metering tower (carbon fiber facesheet over Al honeycomb (fig.1)) and the detector focusing bellows. The MSA sits at the prime focus. The entire optical path is evacuated to < 10-5 torr prior to launch to avoid undue gas load of the detectors.
Fig. 2 – (left) Raytrace of the tangential focus of the FORTIS secondary aspherical ellipse illuminated by a 5μm pinhole lightsource located at the prime focus. (middle) Measured image of the tangential focus of one of the FORTIS secondary mirrors illuminated by a 5μm pinhole at prime focus. (right) Projection of the two tangential focii onto the y-axis.
Fig. 3 – (left) Far-UV zero-order cylindrical achormat. (middle) Zero-order raytrace PSFs at the focal plane without achromat and (right) with achromat.
Microshutter ArrayThe MSA consists of a 2D array of closely packed clear aperture slits, each with an independently selectable shutter (Kutyrev et al. 2004). An array is composed of a thin shutter membrane and light shield mounted to a 100 μm × 200 μm pitch of rectangular holes. The matching shutter array is machined into a silicon nitride membrane 0.5 μm thick. An individual shutter blade is suspended from the shutter frame by a torsion flexure. The shutters are coated with a layer of high permeability magnetic material to allow for rotation of the shutters by a passing magnet. Electrodes, deposited on one of the interior walls of the shutter support grid and the shutters themselves, allow for electrostatic latching and release.
Fig. 4 – (left) Rendering of the MSA mount with scanning magnet supports and electronics. (middle) 64 × 128 microshutter array with an area of 12.8 mm2. (right) Schematic cross section of a microshutter unit cell showing key elements.
Detector SystemThe FORTIS detector array consists of three CsI coated microchannel plate (MCP) stacks readout by a crossed delay-line anode. The detector is housed in a vacuum vessel with a vacuum door that will be opened during flight.
One of the features of the FORTIS design is that a judicious choice of platescale (41.25” mm-1) and dispersion of 20 Å mm-1 results in a flat focal plane in the spectral channels, eliminating the need for curved microchannel plate detector. This allows for a large spatial FOV (0.5°)2 and 900 – 1800 Å in two outrigger spectral channels. The imaging detector area is ~ (45 mm)2 and the two adjacent spectral areas are 45 mm × 62.5 mm.
Fig. 5 – Rendering of detector inside the mounting flange.
FORTIS Optical Parameters
Launch TargetsTargets for observation were selected based on redshift and the estimated Lyα surface brightness. A redshift of z ≥ 1000 km/s is required to move the Lyα emitted from the target out of the geocoronal Lyα window. We use the GALEX flux calibrated FUV and NUV images to simulate Lyα emission from potential targets. We assume that each pixel emits a powerlaw with a slope of βo and the Lyα EW is ~ 20 Å.
We have selected five targets that FORTIS can observe in a 400s flight to achieve the science goals.
Fig. 7 – Simulation of Lyα emission from the cosmic web (Furlanetto et al. 2003). The bright points in (b) are galaxies with surface brightness > 106 photons cm-2 s-1 sr-2. (right) GALEX composite of the Antennae galaxy, which would have similar Lyα brightness if fLyα ~ 20%
Observation StrategyFORTIS will initially launch with all of the shutters in the MSA closed. There will be an unknown number of failed shutters in the “open” position which will provided a minimum backup in case of addressing failure. When the telescope has slewed onto target, the payload shutter door will open and the magnet will scan the face of the MSA, transitioning into the “all open” configuration. The imaging (zero-order) MCP will observe the target for ~ 30 seconds while an onboard National Instruments cRIO FPGA unit will analyze the accumulated image on the fly to determine the brightest “pixel” (shutter) in each row of shutters. Only one shutter per row may be open during spectral acquisition to reduce confusion. A command sent from the ground will order the magnet to rescan the array and the latching voltages will be applied in such a way so that only the brightest shutter in each row remains open. It will remain in this configuration for the duration of the flight.
Fig. 6 – Simulated data for NGC 1365. The zero-order image of the target with the open microshutter apertures overlaid is in the center. The wavelength scale at the top is for the topmost spectra only.
Future WorkCalculating fLyα: Ancillary measurements of the Balmer series lines Hα and Hβ are necessary for each target so that the intrinsic Lyα flux can be calculated from recombination theory. This data will be obtained through either public databases or observing time at the Apache Point Observatory in New Mexico. The ratio of the observed Lyα flux to the calculated intrinsic Lyα flux determined from extinction corrected measurements of Balmer lines yields fLyα.
Science Enabled by FORTIS: In addition to providing an unparalleled classroom for training experimental astronomers, sounding rocket programs enable the development of new observing strategies due to the high risk/low cost nature of the program. The technologies employed in FORTIS; large format UV detectors, clear aperture MSA and on-the-fly targeting algorithms, are relevant to those required to enable science studies detailed in many Astro2010 whitepapers (Bertone et al. 2009; McCandliss 2009; Martin et al. 2009; Postman 2009; Scowen et al. 2009a,b; Strickland et al. 2009). Some examples of science that could be done with a FORTIS-like orbital instrument are:
• Search for the epoch of HeII reionization• Baryon acoustic oscillations with the Lyα forest• Visualizing the Lyα web• Directly measuring LyC and Lyα escape from nearby galaxies
FORTIS is ideally suited to explore a number of science goals in addition to its primary science mission if it were launched as part of the proposed long duration sounding rocket program discussed in the Astro2010 decadal survey.
AcknowledgementsThis work is funded by NASA grant number NNX08AM68G to JHU (McCandliss, PI).
References•Bertone, S. et al. 2009. The Cosmic Web. Astro2010: The Astronomy and Astrophysics Decadal Survey, Vol. 2010, pp. 270•Kutyrev, A. S. et al. 2004. Programmable Microshutter Arrays for the JWST NIRSpec: Optical Performance. IEEE J. Select. Topics Quantum Electron. 10, pp. 652-718•McCandliss S.R. et al. 2004. FORTIS: Pathfinder to the Lyman Continuum. Proceedings of the SPIE, Vol. 5488, pp. 709-718•McCandliss, S.R. 2008. Project Lyman. SPIE Conference, Volume 7011, pp. 20:1-12•McCandliss, S.R. 2009. Project Lyman: Resolving the Physics Behind Reionization. Astro2010: The Astronomy and Astrophysics Decadal Survey, vol. 2010, pp. 196•Martin, C. et al. 2009. The Science Enabled by UV Emission Line Mapping of the Intergalactic Medium and Circum-Galactic Medium. Astro2010, Vol. 2010, pp. 195•Postman, M. 2009. Advanced Technology Large-Aperture Space Telescope (ATLAST): A Technology Roadmap for the Next Decade. ArXiv e-prints.•Strickland, D.K. et al. 2009. Starburst Galaxies: Outflows of Metals and Energy into the IGM. Astro2010: The Astronomy and Astrophysics Decadal Survey. Pp. 289•
Fig. 1 – Schematic drawing of the FORTIS array and photo of the optical bench.
