The Ritchey-Chrétien design minimizes spherical aberration and coma over large fields of view. Field curvature varies quadratically with field angle. Similarly, the rms wavefront error at best focus varies quadratically with field angle and equals 0.52 waves ( = 0.6328 microns) at the edge of the field. Essentially all of this error is due to astigmatism.
The surface figure for the primary mirror was measured at cryogenic temperatures to be 0.067 microns rms over the entire clear aperture, meeting the specification of 0.075 microns rms (Figure 3.12).
Figure 3.12: The deviations in the flight primary mirror surface. The RMS error was measured at cryogenic temperatures to be 0.067 microns rms over the entire clear aperture, meeting the specification of 0.075 microns rms.
3.9.2 Wave-front Errors
The telescope is required to provide a beam to the telescope focal surface that is diffraction limited (transmitted wave front error </14 rms) at 5.5 microns over the entire field at operating temperature. At a wavelength of 3.5 microns, the telescope produces a wave front error of less than 0.13 rms over the IRAC field of view, and the image of a point source contains 45% of the encircled energy within a diameter of 2 arcseonds. The actual performance of the telescope is slightly better than the requirements; see the IRAC Instrument Handbook more information.
The requirements for the telescope assembly are that it shall provide a minimum end-of-life throughput no less than that given in Table 3.4. Telescope throughput is defined as the ratio of energy from a point source reaching the telescope focal surface to the energy collected by an 85 cm diameter mirror. Factors that degrade telescope throughput are the central obscuration (including spiders), mirror reflectivity as a function of wavelength and losses due to contamination.
Table 3.4. Telescope Throughput
End of Life Throughput
3.9.4 Stray Light Rejection
The cryostat, telescope, multiple instrument chamber and science instruments are designed and baffled such that, at all wavelengths from 3.6 to 160 microns (center of passbands), celestial stray radiation and internal stray radiation:
· Do not, except for lines of sight near bright sources, increase by more than 10% the photon noise of the natural background in the direction of the line of sight of the telescope. This requirement implies that the combination of celestial stray radiation and internal stray radiation must be < 21% of the natural background at the instrument detector arrays.
· Display no gradients or glints in the celestial stray light that will increase confusion noise over natural levels or produce false sources.
· Do not significantly decrease the contrast of the first dark ring of the diffraction-limited point spread function.
The conformance of the Spitzer design to its stray light requirements was verified by analysis using the APART stray light analysis program and an analytical test source designed to approximate the brightest celestial source expected in each of Spitzer’s wavelength bands. The actual scattered light performance of the Spitzer telescope was characterized on-orbit during the early parts of the mission and will continue to be characterized during nominal operation of the telescope. In some cases, there are modifications or caveats to the requirements above. Discussion of stray light as it pertains to each instrument is covered in the instrument chapters later in this document.
A useful output from the stray light analysis is a set of predicted point source transmission curves. The point-source transmission function (PST) is the inverse of the ratio of the flux density (W/m2/Hz) of an off-axis source to the flux density at the telescope focal plane due to light scattered from that source. The separate PST curves in Figure 3.13 refer to different azimuthal locations of the celestial point source. An azimuth of 0º refers to the anti-Sun direction. The differences among the azimuths are mainly due to the changing illumination of struts supporting the secondary mirror. Figure 3.14 shows the variation as a function of wavelength.
Figure 3.13: Theoretical PST for 24 microns off-axis as a function of azimuth.
Figure 3.14: Theoretical PST as function of wavelength for fixed azimuth on-axis.
These plots can be used to estimate the stray light contribution from a given source. For example, at 8 microns, Vega has a flux density of ~62 Jy. From Figure 3.14, the 8 micron PST at 1º off axis is ~2x10-3, so the predicted flux density in the Spitzer focal plane due to Vega at 1º off axis is 120 mJy. [The natural background at 8 microns near an ecliptic pole is 5.3 MJy/sr, which is imaged to a flux density of 5.3 MJy/sr x /(4x122) = 2.9x104 Jy in the focal plane of the f/12 Spitzer telescope. The scattered light from Vega at 1º off-axis is far below the natural background. The first stray light trouble from Vega at 8 microns should come from the outer parts of its diffraction-limited PSF.]
3.9.5 Telescope Temperature/Thermal Background
The telescope is cooled by helium vapor vented from the cryostat. The helium vaporization is driven by power dissipated by the Science Instrument cold assemblies. A supplemental heater is available to provide additional vaporization, if needed. The required telescope temperature and heat load drive the required helium flow rate. As discussed in section 3.4.1, the telescope was launched warm and gradually cooled by a combination of radiative cooling and helium boil-off to ~6 K during the first ~45 days in orbit. The telescope temperature varies between ~6 K and 12 K depending upon how much power is dissipated in the cryostat. The power thus dissipated depends primarily upon the science instrument in use; the “make-up” heater may be used to ensure that the telescope is maintained at ~6 K when needed for 160 microns observations.
3.9.6 Telescope Focus
Spitzer is equipped with a secondary mirror focus mechanism, which was operated both on the ground and on orbit. Prior to launch, the end-to-end image quality was measured on the ground and the mechanism was set to the position that was predicted to give optimal focus following in-orbit cool-down. During In-Orbit Checkout (IOC), it was found that the telescope focus point was about 1.85 mm above (toward the back of the primary mirror) the optimum focus position for the science instruments. Thirty-eight days after launch, when the telescope had become thermally stable, the secondary mirror was moved toward the primary mirror to bring the telescope into focus. The instruments are confocal, so that separate adjustment of the focus for each instrument is not be necessary.