KEYWORDS: Stray light, Phase modulation, Observatories, James Webb Space Telescope, Near infrared, Space telescopes, Telescopes, Adaptive optics, Mirrors, Aerospace engineering
The James Webb Space Telescope (JWST) is a large cryogenic telescope observing over a spectral range from 0.6 μm to 29 μm. A large sun shield blocks sunlight and provides thermal isolation for the optics. Analyses characterizing the stray light reaching the instrument focal planes from the galactic sky, zodiacal background, bright objects near the line-of-sight, and earth and moon shine are presented along with the self-generated thermal infrared background from Observatory structures. The latter requires thermal analysis to characterize the Observatory temperatures. Dependencies on the surface properties of BRDF and emittance are discussed for the underlying materials and the effects of contamination
The James Webb Space Telescope (JWST) is a large space based astronomical telescope that will operate at
cryogenic temperatures. The architecture has the telescope exposed to space, with a large sun shield providing
thermal isolation and protection from direct illumination from the sun. The instruments will have the capability to
observe over a spectral range from 0.6 µm to 28 µm wavelengths. The following paper will present updated stray
light analysis results characterizing the stray light getting to the instrument focal planes from the full galactic sky,
zodiacal background, bright objects near the line of sight, and scattered earth and moon shine. Included is a
discussion of internal alignments of pupils at relevant interface planes to stray light. The amount of self-generated
infrared background from the Observatory that reaches the instrument focal planes will be presented including the
tolerance to the alignment of the edges of the sun shield membranes relative to each other and the telescope.
The open telescope design of the James Webb Space Telescope (JWST) allows light from off-axis sources to scatter into the instrument field of view. The significant sources of stray light in the near IR and the mid-infrared waveband are galactic light and reflected sunlight and thermal emission from the zodiacal dust. The stray light from these sources was calculated with the ASAP software. Backward ray tracing was efficiently used in the prediction of the stray light from the sky. Since the galactic and zodiacal light is distributed over the whole sky, the sky was divided into 7200 patches of size 3 degrees by 3 degrees, and the contribution from each patch was calculated. The instrument geometric susceptibility for each sky patch was calculated with backward ray tracing. Multiplying the geometric susceptibility and the sky radiance, we are able to calculate the stray light from each sky patch. Total stray light from the full sky is then calculated by summing the individual patch contributions. The stray light from the galactic sky and zodiacal light has been calculated for different orientations of the observatory relative to the sky.
KEYWORDS: Stray light, Observatories, James Webb Space Telescope, Sun, Phase modulation, Space telescopes, Scattering, Near infrared, Telescopes, Light scattering
The James Webb Space Telescope (JWST) is a large space based astronomical telescope that will operate at
cryogenic temperatures. The architecture has the telescope exposed to space, with a large sun shield providing
thermal isolation and protection from direct illumination from the sun. The instruments will have the capability to
observe over a spectral range from 0.6μm to 29 μm wavelengths. The following paper will present the stray light
analysis results characterizing the stray light getting to the instrument focal planes from the full galactic sky,
zodiacal background, bright objects near the line of sight, and scattered earth and moon shine. The amount of self-generated
infrared background from the Observatory that reaches the instrument focal planes will also be presented.
Ball Aerospace uses several techniques in radiance calibrations of the SBUV/2 instruments. The instrument Primary Test Fixture (PTF) and Normal Incidence Test Fixture (NITF) both use Spectralon diffusers as radiance targets. Diffuser BRDF (Bidirectional Reflectance Distribution Function) is measured for a central spot at several scatter angles and at several wavelengths. Weighted BRDF is then calculated across the instrument FOV, based on diffuser BRDF measurements, spatial uniformity test data, instrument vignetting, and test geometry. This weighted BRDF curve is then fitted spectrally to determine BRDF at each wavelength of the SBUV/2 instrument. The PTF and NITF have their own BRDF curves, since each fixture has a unique diffuser plate and test geometry. A third test fixture is used for the last SBUV/2 instrument radiance calibration, using a Labsphere Uniform Source System (USS) and an external source for reference. The large aperture of the sphere provides a uniform radiance target with no need for BRDF knowledge. Comparison of instrument calibrations from all three radiance targets shows a small discrepancy of about ±1% among these calibration methods, which indicates that BRDF calculations for both PTF and NITF test diffusers are acceptable.
A formation flying interferometer suffers a unique stray light problem: it is the only space astronomical instrument which looks at starlight in the presence of a bright object (the sunlit collector spacecraft) which is intentionally placed in the field of view. The combiner instrument is a collection of astronomical telescopes looking at stellar targets relayed by the collectors; but other parts of the collector spacecraft can scatter or emit substantial fluxes. Screening the glare while collecting the starlight is the challenge. The TPF formation-flying interferometer is especially vulnerable because the planets it seeks are so faint. We present a preliminary estimate of stray light from various sources appearing in the interferometer science detector. We assumed a pinhole field stop would be employed, but that two mirrors preceding that pinhole in the combiner optical system would scatter off-axis light into the pinhole. Sources include scattering of direct sunlight and thermal emission, both from the multi-layer thermal shades which permit passive cooling of the instruments to about 40K. We find that the estimated stray light fluxes are of order 104 times the planet flux. We conclude that the optics at the combiner entrance must be blocked from any direct view of the thermal shades.
New developments in interference filter technology utilizing low thermal expansion coefficient materials which are deposited free of voids using variants of ion-assisted- deposition techniques have made possible the development of a new class of low-cost, lightweight remote sensing instruments. These instruments can easily have a throughput two orders of magnitude larger than similar dispersive monochromators in addition to having a stray light rejection somewhere between that of a single and a double monochromator for a bandpass of the order of one nanometer. Results from environmental testing, measurements of stability in space, and unique problems associated with spectral radiance calibrations with these interference filter instruments are described.
Beginning in the mid 1960's large aperture scanning radiometers have been used in space to determine spectroradiometric properties of earth scenes in the red and near infrared regions. Panel diffusers as calibration sources for these radiometers were abandoned in favor of internally illuminated integrating spheres because of problems of illuminating the panel diffuser uniformly.1 Since 1970 spectroradiometric instruments used in space for remote sensing of the atmosphere in the ultraviolet for the determination of stratospheric ozone and total column amounts have used spectral radiance calibrations derived from calibrated panel diffusers illuminated by NIST standards of spectral irradiance. An advantage of the panel diffuser technique is simplicity of the experimental set up. Stratospheric ozone profiles and total column amounts are derived from ratios of atmospheric radiances to corresponding solar irradiances incident at the top of the atmosphere in the wavelength region of 250 - 340 nm. An inherent problem associated with measurements for the remote sensing of stratospheric ozone which is not shared with remote sensing measurements of earth scenes at longer wavelengths of the solar scattering and reflective region is the extremely large dynamic range of atmospheric radiances and the steep gradients of radiance with wavelength. For a typical wavelength scan the spectral radiance changes by about lO and the average signal level of a spectral scan can shift by another factor of 25 or more due to solar zenith angle changes between the subsolar point and the solar zenith angle limit of useful scan information which is within a couple of degrees of the terminator. The derivation of spectral radiance calibrations using either the sphere or panel diffuser techniques for ultraviolet remote sensing instruments are single point calibrations at each wavelength. A subsequent linearity calibration of the detector and electronics is made in non dispersed or white light over the entire dynamic range of the instrument of more than six decades. Consequently the derived radiometric calibration constants consist of a radiometric sensitivity term and a signal dependent linearity correction. An initial comparison of spectral radiance calibrations of SBUV-2 instruments using spherical integrator and panel diffuser techniques has been given by Heath et al.2. Subsequent work by Heath et al. describes the results from comparisons of four spectral radiance calibrations derived using panel diffuser techniques with five spectral radiance calibrations derived using spherical integrator techniques for three different SBUV-2 instruments. The comparability of the sphere and panel diffuser spectral radiance calibration techniques is assesed by comparing derived average BRDF values of panel diffusers based upon the sphere technique with laboratory measurements of BRDF of the panel diffusers. The sphere radiances determined relative to NIST standards of spectral irradiance are compared with measurements of sphere radiance relative to a NIST high temperature blackbody source. This work describes the evaluation of the consistency of spectral radiance calibration scales established using panel diffuser and internally illuminated spherical integrator techniques for the SSBUV and SBUV-2 Flight Model 5 instruments using zenith sky radiance measurements as a function of solar zenith angle (UMKEHR) which coincident in space and time. Also described are the spectral radiance calibrations of the Global Ozone Monitoring Experiment (GOME) with the NASA sphere which has been used to intercalibrate SBUV-2 and SSBUV instruments. These spectral radiance calibration constants are compared with those derived using a Spectralon panel diffuser whose BRDF was measured at NASA Goddard Space Flight Center by J. Butler.
Spectral radiance calibrations have been made for several SBUV/2 instruments using techniques based upon an internally illuminated spherical integrator and diffuse reflectance panels with BRDF measurements from NIST. Both techniques are referenced to NIST standards of spectral irradiance which are used to derive the spectral irradiance calibrations of the instruments. The spectral radiance of the aperture of the internally illuminated spherical integrator also has been calibrated by NIST against a high temperature blackbody. The consistency of the spectral radiance calibrations and the problems specific to each of the techniques are described.
A comparison has been made of spectral radiance calibrations of SBUV-2 instruments using two techniques. In one, a source of spectral radiance is obtained by illuminating a spectralon panel diffuser whose BRDF has been measured by NIST with a 1000 W FEL quartz-tungsten- halogen standard of spectral irradiance from NIST. In the other, the spectral irradiance of the aperture of an internally illuminated spherical integrator is measured relative the same NIST standard of spectral irradiance used with the panel diffuser. The spectral radiance of the sphere is calculated for a source of irradiance which is uniform and radiates as a Lambertian source over the area of the aperture. The agreement between the average of four spectral radiance calibrations of SBUV-2 instruments obtained with the spectralon panel diffuser technique with those obtained from five spectral radiance calibrations with the internally illuminated spherical integrator technique is better than one percent in the wavelength range of 250 - 340 nm. Repeated use of two spectralon panel diffusers over a two year period indicates a stability of diffuser BRDF of better than one percent.
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