Far-ultraviolet astronomical narrowband imaging

Far-ultraviolet astronomical narrowband imaging

Timothy A. Cook,* Brian A. Hicks, Paul G. Jung, and Supriya ChakrabartiCenter for Space Physics, Boston University, Boston, Massachusetts 02215, USA

*Corresponding author: [email protected]

Received 7 October 2008; revised 16 February 2009; accepted 22 February 2009;posted 4 March 2009 (Doc. ID 102371); published 26 March 2009

We describe an all-reflective system for narrowband imaging suitable for imaging emission lines in thefar ultraviolet. The system, which we call a monochromatic imager, combines a pupil plane gratingmono-chromator with a telescope and camera to image a scene in one or more very narrow bands. The mono-chromator uses physical stops at its input and output apertures, and, as a result, the system has excellentrejection of out-of-band and off-axis light. © 2009 Optical Society of America

OCIS codes: 110.4234, 120.4820.

1. Introduction

Narrowband filters are used ubiquitously for astro-nomical imaging. They allow observers to routinelysuppress background and continuum emission to im-age individual emission lines. This simple proceduremakes it possible to access the spatial distribution ofthe ionic, atomic, and molecular emission lines,which are diagnostics of temperature, pressure, den-sity, etc. Unfortunately, this technique cannot be ap-plied in the 912–1216Å far-ultraviolet (FUV) band,where transmissive materials are unavailable. LiFhas the shortest cutoff wavelength of any commonlyavailable optical material (1050Å), while the morestandard, and less hygroscopic, MgF2 cuts off at1150Å. At wavelengths between these and thesoft-x-ray regime, where thin metal filters becomepractical (e.g., thin tin filters provide a 100Å widebandpass around 600Å), it is currently not possibleto make a transmissive optical element. Reflectivemultilayer interference filters are commonly usedin the soft-x-ray regime [1] and have been proposedin the FUV [2], but are not yet widely available.The narrowband filters used at visible and near

ultraviolet (UV) wavelengths are generally interfer-ence filters with fixed bandpasses and central wave-lengths. Switching from one line to another requiresswitching filters. As a result only one wavelength canbe observed at a time, and each new wavelength

requires a new filter. While tunable filters do exist,their application is limited by a variety of technologyfactors [3].

Instead, to produce narrowband images, onetypically uses a long slit spectrometer and “pushbrooms” it across a scene of interest (e.g., [4]). Whilethis produces excellent spectroscopy, it is very ineffi-cient in observing time, particularly if only a singlespectral feature is required. Since much of the spec-trometer’s detector is used to record spectra awayfrom the wavelength(s) of interest, its multiplex ad-vantage is used to record information at unwantedwavelengths.

Astronomical FUV observations are necessarilycarried out from space, where the UV background en-vironment brings its own set of obstacles. The Earth’supper atmosphere and interplanetary medium arerich in foreground lines. The brightest such line isLyman α at 1216Å. The Lyman α intensity rangesfrom about 3 × 109 PU [40kR, where R is rayleighs;1PU (photon unit) is 1photon=cm2=s=sr] on theday side in low Earth orbit (LEO) [5] to an interpla-netary value of 4 × 107 PU (500R) at solar minimum[6] at about 6R⊕ [7]. ] This can pose a considerabledifficulty because the astronomical emissions of in-terest are often as faint as a few hundred [8] or afew thousand [9] photon units.

Imaging low-surface-brightness objects shortwardof Lyman α is particularly hindered by bright geocor-onal and interplanetary Lyman α emission, sinceshort-pass filters are not possible in this band.Geocoronal Lyman α scatters off optical elements,

0003-6935/09/101936-07$15.00/0© 2009 Optical Society of America

1936 APPLIED OPTICS / Vol. 48, No. 10 / 1 April 2009

in particular a system’s grating, and contaminatesthe resulting data. High-quality gratings scatter ata level of 1∶105 Å−1 to 1∶106 Å−1 in the plane of dif-fraction [10], and gratings have been reported withscattering figures as low as 1∶107 Å−1 [11]. In a typi-cal grating spectrometer, the light thus introducedinto an incorrect spectral element then contributesan effective surface brightness of 30,000–250PU=Åfor observations from LEO or 400–4PU=Å from highEarth orbit. For all but the highest-spectral-resolu-tion observations this precludes LEO observationsof lines fainter than a several hundred photon unitsand is a significant source of background even in highEarth orbit or interplanetary orbits.This paper presents a new instrument design,

called the monochromatic imager (MI), which over-comes all of these difficulties. It provides an all-reflective configuration, which is suitable for useat all wavelengths (especially including the UVand FUV bands) with a scanning monochromator,which can be tuned to adjust both the bandpassand the central wavelength of the instrumentresponse. The system can be multiplexed to imageseveral different wavelength bands at the same time.

2. Pupil Plane Spectroscopy

The MI is an instantiation of pupil plane spectro-scopy. In pupil plane spectroscopy, an image of thepupil is presented to the dispersive system, andthe pupil is dispersed to form a spectrum (Fig. 1).By contrast, in standard image plane spectroscopya field is imaged to a field stop, often in the formof a slit, and that image is dispersed to form a spec-trum. Other examples of pupil plane spectroscopyinclude the Cotton mount spectrometer [12,13],the TIGER spectrometer [14], and the IMAGEFUV-SI [15].One advantage of dispersing the pupil rather than

the image is that the system’s field of view is inde-pendent of the monochromator’s slit width (whichsets the spectral resolution). As a result, pupil planespectroscopy can be used to view a wide field of viewat high spectral resolution. The system’s etendue–resolution product (AΩR) is still limited by the f -number, the dispersion of the spectrometer, andthe size of the detector (see Table 1). In pupil planesystems, this limit simply applies to the collectingarea rather than the field of view.It also follows that, since the pupil is dispersed, the

shape of the pupil stop (and thus frequently the pri-mary) can be a long thin slit. Like its image planeanalog, the use of a long slit allows a long collectingarea while keeping the spectral resolution elementsmall. In pupil plane spectroscopy this long slit isa pupil stop; in conventional spectroscopy it is a fieldstop.To show that the geometric etendue is the same re-

gardless of the system configuration we can comparesimilar image plane and pupil plane systems—aconventional push broom spectrometer and the MIdescribed here (see Table 1). Each system uses a

51 cm telescope, an f =6 spectrometer, and a 20mmdetector. Because the pupil plane system uses a slitin the pupil plane, it uses its primary aperture inef-ficiently—only a 51 × 1:1 cm section is used—whilethe push broom spectrometer uses the full aperture.The push broom spectrometer puts a 25″ × 200 slit inthe image plane and thus samples the 200 field ofview more inefficiently than the full 200 diameterfield of view of the MI. In each case, narrowing theslit will both improve the spectral resolution of the

Fig. 1. (Color online) Schematic representation illustrating theoperation of the MI. The same optical elements are used to simul-taneously limit the spectral throughput to a narrow spectral bandby imaging the pupil to the monochromator entrance slit (toppanel) and to image the sky onto the detector (bottom panel).The MI images the scene in a single emission line by using a tun-able monochromatic imager. The narrow spectral response is setby the monochromator, which is formed by an entrance slit atthe image of the pupil, a concave grating, and one or more exit slitsalong the Rowland circle. The exit slits limit the light enteringeach image to a narrow bandpass. Multiple exit slits each contri-bute light to a different image in a different band (schematicallyshown in the top panel). The imaging properties (schematically de-picted in the bottom panel) of the system are largely unaffected bythe monochromator. The Gregorian telescope’s image plane is nearthe monochromator grating; thus any grating aberrations intro-duced by that optic do not significantly affect the image quality.The final imaging optics (one per exit slit) relay the image formedon the grating onto the detector. The monochromator entrance andexit slits are both images of the long thin pupil formed by the longthin off-axis primary and thus have little effect on the imagingperformance.

Table 1. Comparison of Pupil and Image Plane Systemsa

Characteristic

Image Plane,Push BroomSpectrometer Pupil Plane, MI

Primary size 51 cm diameter 51 × 1:1 cmTelescope geometric area 2042:8 cm2 56:1 cm2

Spectrometer f -number f =6 f =6Spectral resolution 0:5Å 0:5ÅSlit width 381 μm 381 μmTelescope focal length 3060mm 14477mmSpatial resolution 25″ (slit width) 1–30″

System field of view 25″ × 200 200 diameterSystem field of view 7:1 × 10−7 sr 2:6 × 10−5 srGeometric etendue 1:4 × 10−3 cm2 sr 1:5 × 10−3 cm2 sraOne can see that systems with similar f -numbers, dispersions,

and detector sizes have similar geometric etendues regardless ofthe system’s configuration.

1 April 2009 / Vol. 48, No. 10 / APPLIED OPTICS 1937

system and increase the system’s inefficiency. TheMIwill use less of the primary collecting area, and thepush broom will see a smaller portion of the field ofview. The systems use identical Rowland spectro-meters [16] and thus have identical spectral resolu-tion. Each system has the spectral response of thisspectrometer; it defines the spectral line spread func-tion of the push broom system and the bandpass ofthe MI system. The push broom telescope focallength is chosen to match the telescope f -numberto the spectrometer; The MI telescope focal lengthis chosen to match the system field of view to thespectrometer f -number.As can be seen in Table 1, the resulting systems

have identical geometric etendues. The push broomspectrometer samples a single spatial column and∼50 spectral elements (the 50 spectral elements re-sult from dispersing the 381 μm slit across the 20mmdetector) in a single pointing, while the MI samples afull image at a single wavelength. If the instrumentswere to be used to collect a full hyperspectral datacube, the push broom would require ∼50 pointingsto scan its 25″ slit across the 200 field of view, whilethe MI would require ∼50 pointings, each scanned toa different wavelength. Thus each system would re-quire the same number of pointings with the samegeometric etendue to fill the data cube. Each wouldcomplete the observation set in the same amountof time.The details of the systems are, of course, different.

The assumed circular detector results in a circularfield of view for the MI. The scanning slit of the pushbroom system results in a square field of view, but thesame circular detector would clip the field of viewaway from the central wavelength. The MI requiresfour optical elements, while the push broom can beimplemented with three. Reflectivities vary from40% to 90% between 900 and 1400Å; so the reflectiv-ity loss may be significant. The push broom spectro-meter is behind the telescope, resulting in a longerpackage, while the MI spectrometer is beside thetelescope, making packaging easier.Given the same f -number, spectrometer spectral

resolution, and detector area, the push broom andMI configurations yield identical geometric eten-dues. The push broom is the preferred system if aparticular observation requires full spectral informa-tion but only sparse sampling of the slit across thefield. The MI’s strength is in observations requiringlarge images at a few wavelengths.

3. Instrument Description

TheMI consists of a telescope, a monochromator, anda camera. The telescope is an off-axis aplanatic Gre-gorian (Fig. 2). Because the system uses pupil planedispersion, its usable primary mirror area is limitedto a long, thin segment by a pupil stop at the en-trance slit of the monochromator. As a result it ismore convenient to use an off-axis system and a con-tinuous segment of the primary mirror rather thanan on-axis system with the secondary obscuration

andmonochromator dividing the primary mirror intotwo segments. The Gregorian configuration allows usto place a pupil stop, which will act as the monochro-mator’s entrance slit, at the real image of the pupilformed by the secondary. The virtual image of thepupil formed in a Cassegrain system would not bea practical entrance aperture. An aplanatic correc-tion is applied to the Gregorian optics to widen thefield of view. This is equivalent to the Ritchie–Cretian correction applied to a Cassegrain systemand has a similar effect.

The monochromator is a simple aberration-corrected Rowland system [16,17]. The Rowlandconfiguration consists of an entrance slit, a toroidalconcave grating [18], and an exit slit. The entranceand exit slits are positioned on a construction circlewhose diameter is along the grating normal and isequal in magnitude to the central radius of curvatureof the grating. The grating is used in a near-Littrowconfiguration (i.e., with equal angles of incidence anddiffraction). We have elected to use a Rowland config-uration for its simplicity and high spectral resolu-tion. Similar systems are possible with any othermonochromator layout. Being near Littrow makesthe packaging compact, while being slightly off ofthe Littrow alignment allows us to slightly separatethe entrance and exit slits.

We position the monochromator so that the focus ofthe telescope is near the surface of the monochroma-tor grating. As a result, any aberrations introducedby that grating are introduced as angular deflectionsin the image plane, not the pupil plane. This meansthat the aberrations will affect the illumination pat-tern on the exit slit of the monochromator but thatmonochromator grating aberrations have no effecton the image quality of the system. We positionthe image near the monochromator grating to avoid

Fig. 2. (Color online) The MI consists of a telescope, a monochro-mator, and a camera. Because the system uses pupil plane disper-sion, its usable primary mirror area is limited to a long thinsegment by a pupil stop at the entrance slit of the monochromator.The Gregorian configuration allows us to place a pupil stop, whichwill act as the monochromator’s entrance slit, at the real image ofthe pupil formed by the secondary. The monochromator is a Row-land spectrometer consisting of an entrance slit, a toroidal concavegrating, and an exit slit.


a peculiar physical effect; if a point source were to bebrought to a perfect focus on the surface of the grat-ing, then it would illuminate very few rulings. Theresulting diffracted beam would be highly diffractionlimited. To avoid this, we move the grating slightlyoff focus to blur the image sufficiently to avoid de-grading the system’s spectral resolution. However,since the grating is slightly off the image plane,the monochromator grating aberrations will contri-bute slightly to the system’s imaging performance.Since the grating is close to the image plane, the con-tribution can be kept small.The MI configuration allows one to adjust both the

system’s bandwidth and central wavelength. Thebandpass adjustment is accomplished by varyingthe width of the entrance and exit slits. Narrowingthese slits increases the spectral resolution at the ex-pense of the system’s effective area, but has no effecton the field of view or spatial resolution. The centralwavelength adjustment can be effected by either ro-tating the grating in a manner similar to a Seya–Namioka monochromator [19] or by moving the exitslit along the Rowland circle (Fig. 3). In addition, one

can use multiple exit slits to image multiple bandswith a single telescope and grating [15]. If the multi-ple slits each feed a separate camera, then the sys-tem will produce images of the same scene inmultiple wavelengths. If the multiple slits all feedinto the same camera system, then a single imagewith a complex wavelength response function canbe recorded [15].

We have investigated several different camera con-figurations. These include all-reflective systems, lenssystems, and systems with an additional dispersiveelement in the camera. In the all-reflective system apowered mirror is used to reimage the image, whichthe telescope projects near the grating, onto the de-tector. Both the grating and the detector are at finitedistances, and, as a result, the mirror figure is ellip-soidal. The lens system is, obviously, only useful forwavelengths where transmissive optical elementsare available, but otherwise acts in the same manneras the reflective system.

A dispersive camera can be used to create a double-dispersion system to further suppress out-of-bandlight. Since gratings can be obtained with scatterat a level of less that 1∶1 × 105, a double-dispersionsystem can suppress out-of-band light at a levelgreater than 1∶1 × 1010. Such a system is createdby substituting a concave grating for the reflectiveimaging element.

There are two important constraints when select-ing this dispersive element. The dispersion must behigh enough to fully separate the background (i.e.,Lyman α) image from the desired image. This isbecause the dispersive camera optic is effectivelyan objective spectrometer. Unless the dispersion issufficiently high, it will leave the two images con-volved in an “overlapogram.” The background imagedoes not need to be well resolved, or even on the

Fig. 3. (Color online) The MI consists of a telescope, a monochro-mator, and a camera. The telescope is an off-axis Gregorian. TheGregorian configuration allows us to place a pupil stop, which willact as the monochromator’s entrance slit, at the real image of thepupil formed by the secondary. The monochromator is an aberra-tion-corrected Rowland system consisting of an entrance slit, a tor-oidal concave grating, and one or more exit slits (four are shown).The imaging optic can be either a reflective ellipsoid or a dispersiveelement (as modeled below) with no change in configuration. Thesystem’s central wavelength is tuned by scanning the exit slit car-riage along the track that corresponds to the Rowland circle. Thecomponents pictured are the (1) primary mirror, (2) secondary mir-ror, (3) grating, (4) aperture cover, (5) monochromator entranceslit, (6) monochromator exit slits, (7) detector housing, (8) detectorcover, (9) instrument housing, (10) electronics package, (11) cameraoptic.

Table 2. Parameters of the Modeled Double-DispersionMI (Figs. 4 and 5)

Parameter Value

Primary focal length 510mmPrimary radius of curvature 1020mmPrimary eccentricity 0.99995628Primary aperture 11 × 510mmSecondary radius of curvature 35:976629mmSecondary eccentricity 0.93309988Telescope focal length 14477mmTelescope plate scale 14″=mmMonochromator grating radius of curvature 611 × 400mmMonochromator grating ruling density 3700 g=mmMonochromator grating order ThirdMonochromator slit width 381 μmMonochromator incidence angle (α) 33:4°Monochromator diffraction angle (β) 36:4°–40:4°Camera grating radius of curvature 191 × 145mmCamera grating ruling density 3800 g=mmCamera incidence angle (α) 56:6°Camera diffraction angle (β) 19:8°–23:3°System focal length 3438mmSystem plate scale 10=mm


detector; it does need to be fully separated from thedesired image. In addition, the dispersion must below enough that the blurring due to the camera dis-persion is acceptably low. In practice, this limits thedouble-dispersion system to observations modestlyremoved from the background line, but acceptablecompromises between blur and proximity to Lymanα can be found.

4. Modeling Results

To illustrate the performance of the MI we presentray-trace results for a single specific configuration.We have elected to model a double-dispersion systemwith a modest field of view (200 diameter), spectralresolution of 2000 λ=Δλ, and a double-dispersionimaging optic. We have selected this configuration

because the 200 field of view is comparable with manyother astronomical systems (e.g., IRAC [20] at 5:20 ×5:20 or GALEX [21] at 1:2° diameter), while the reso-lution is typical of medium-resolution spectroscopicsystems (e.g., medium-resolution HST STIS [22] orHST COS [23] modes). The double-dispersion mode,which is suitable for FUV observations from LEO, isthe most challenging optical configuration.

We have modeled the MI optical system using bothan author modified version of IRT [24] and ZEMAX[25]. The parameters for the system modeled are gi-ven in Table 2. The model demonstrates a resolutionof better than 30″ (50% enclosed energy) over the 200field of view at three wavelengths from 1030 to1080Å (Fig. 4). The point spread function (PSF) froma monochromatic point source is shown in Fig. 5.

Fig. 4. We have modeled the resolution of the double-dispersion system at a variety of wavelengths. The resolution varies from betterthan 1″ to about 30″ across the field of view. The systemwill be diffraction limited in width at the 2″ level. We report here the 50% encircledenergy width in the vertical and horizontal directions. The contours are labeled in arc seconds with the dashed curves indicating the widthof the PSF and the dotted curves indicating the height. Since the pupil, and thus the f -number, is asymmetric, the resolution is verydifferent in the vertical and horizontal directions. (a) 1030Å, (b) 1055Å, (c) 1080Å.


TheMI’s PSF is highly asymmetric and has the ap-pearance of an astigmatic system. However, theasymmetry is not due to astigmatism. It is a resultof the highly asymmetric pupil shape and the defocusdue to the curvature of the focal plane. This can bestbe seen by examining the image produced at the focalplane of the telescope (Fig. 6). The spots in this plane

have not yet interacted with any grating or cameraoptic yet show a highly asymmetric PSF. The succes-sive optics will further curve the focal plane, exacer-bating this effect, and the aberrations from thecamera optic, especially coma, will act to twist thePSF (Fig. 5). This defocus is the dominant limit tothe system’s imaging performance; the system hasbeen configured so that it just barely acceptable.The defocus can be mitigated either by using acurved detector or by reducing the field curvature.The field curvature is, in turn, due to the relativelyhigh magnification of the telescope (m ¼ 28). Sincethe magnification is needed to match the 200 fieldof view to the f =6 spectrometer, lowering the tele-scope magnification would, in turn, necessitateeither a larger field of view or a slower f -number.

5. Summary

We have described an all-reflective instrument for re-cording very narrowband images. The system can beused at any wavelength from IR to UV, but is parti-cularly useful in the far UV (FUV) where interfer-ence filters and transmissive optical elements arenot routinely available.

The instrument bandpass is defined by a gratingmonochromator operating in the pupil plane of a Gre-gorian telescope. Themonochromator can be scannedin wavelength and bandpass to fit both the centralwavelength and bandpass to any particular observa-tion’s requirements. As a result, the system is robustand adaptable. By equipping the monochromatorwithmultiple exit slits, multiple cameras can be usedto simultaneously image a field in multiple emissionlines or pass bands.

The authors thank Stephen Mende, Jerry Edel-stein, and Paul Glenn for their useful discussionsof the design of the instrument described here. Thiswork was supported by internal Boston Universityfunding, the Boston University Photonics CenterSenior Photonics Fellowship, and NASA grantNNG05WC17G.

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Far-ultraviolet astronomical narrowband imaging