Near Field Maps
4.2 Frequency Band Definition
Spider’s frequency bands are set by the design of the antennas and the on-chip LC fil- ters (§3.2). Band edges must be defined sharply and appropriately to minimize response to temporally variable atmospheric line emission that can increase loading and contaminate sky maps.
Measured Bands and Band Edges
We have measured the spectra of devices in a test focal plane using a Fourier Transform Spectrometer (FTS) clamped over the window of the Spider test cryostat. The FTS is a Michelson interferometer built by the Princeton group, with some modifications by the Caltech group. The field of view is filled by an unmodulated liquid nitrogen source and the off-source beam is directed on to an Eccosorb absorber at ambient lab temperature. In our testing, the moving mirror was scanned across 300 mm at 2 mm/s. The results of forward and backward scans were indistinguishable above the noise. Scans were taken with the FTS centered on the window and oriented at three different axial clockings, each separated by 45◦. At each clocking, scans were made at five different FTS pointing angles (adjusted by a single goniometer). More than ten devices achieved significant S/N in each scan. In this
way, adequate measurements of nearly all active devices on the focal plane were made in just a few hours. Most devices were illuminated by multiple different scans. The shape of the measured spectra showed no appreciable dependence on the orientation or pointing angle of the FTS.
A fourth degree polynomial was subtracted from each device interferogram and then a Hanning window applied. A S/N-weighted average interferogram was calculated for each tile. The FFT was taken without folding the interferogram over onto itself. The magnitude of the result was divided byν2to deconvolve the device spectrum and the blackbody source spectrum (Bν ∝ν2 becausehν kT). This factor ofν2 is responsible for the slope in the noise floor of the semilog plots.
Four notably different tiles were installed when we took spectral measurements in Run 2.1 of the Spider test cryostat. One was a test tile designed to measure the variation of the ILD index (see §4.1). Another was designed for a 99 GHz band center. The basic functionality of this tile was confirmed, but the spectra were affected by severe fringing because the optics of the telescope were optimized for the 148 GHz band. A third tile (designation JAB090323.1) was an older wafer, designed for a band center of 145 GHz.
The target band center has been shifted up because a band centered at 145 GHz shows significant response at the 115 GHz galactic carbon monoxide line and at the 118 GHz atmospheric oxygen line. The final tile (designation JAB100104.1) was fabricated with the current design, targeting a band center of 148 GHz. The weighted average spectra of these two tiles are plotted in Figure 4.7.
The bandwidth of the 148 GHz tile Δν= (
F(ν)dν)2
F2(ν)dν (4.1)
is 41 GHz, so Δν/ν0= 28%.
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Figure 4.7: Top: The average passband for devices from tile JAB090323.1. This passband was judged to be too low, and tiles are now made to target a higher passband. Bottom:
The average passband for devices from tile JAB100104.1. This newer tile was fabricated using the same filter design and ILD thickness which we use in the fabrication of 148 GHz science flight tiles. Both passband averages have been normalized to one at peak response.
Plotted in grey is the atmospheric emission in pW/GHz (solid line) and the location of the J=1-0 CO transition (dashed).
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Figure 4.8: Measured band centers for devices on tile JAB100401.1. Left: Histogram of band centers. Due to noise in the spectral measurements this represents an upper limit on the actual dispersion in band centers. Right: Band centers mapped by the physical row and column of each device on the tile. There is no obvious spatial pattern.
Uniformity
Uniformity in the location of band edges is important, especially to control differential response to atmospheric and galactic CO emission lines. To date we have performed spec- troscopy on just one full tile ofSpiderdevices made with flightlike filters, so we do not have a good measurement of the tile-to-tile variation in band center. However, I performed spec- troscopy in the White Dewar (single pixel test bed described in§4.1) on individual devices from several different 99 and 145 GHz tiles. The band centers from those measurements are tabulated in Appendix B.11. The variation in measured band centers is at the level of a couple gigahz both within a tile and between tiles. In Figure 4.8 I histogram band centers for devices on a single 148 GHz tile measured in the Spider test cryostat. The standard deviation is about 1 GHz. The spectra and spectral differences for A/B polarization pairs from three pixels are shown in Figure 4.9. In-band spectral differences are small and show no apparent bias, so they should subtract effectively for common mode broadband sources.
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Figure 4.9: Spectra from both devices of both polarizations (A and B) for three represen- tative pixels on test tile JAB100104.1. The relative normalization of the spectra are chosen such that the integral of the difference spectrum vanishes. The absolute normalization is arbitrarily chosen such that the mean value in the center of the band is close to unity.
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Figure 4.10: Convolution of the atmospheric model shown in Figure 2.1 with the measured passband of JAB100104.1 using the measured optical efficiency (η0= 33%).
Atmospheric Lines and Interstellar CO Lines
Atmospheric emission includes both broadband water emission and narrow water and oxy- gen line emission (Figure 2.1). In Figure 4.10 I convolve our atmospheric model with the average measured spectrum from the 148 GHz tile (normalized by the measured optical responsedP/dTRJ = 0.16). The total predicted atmospheric loading is 0.10 pW per single- polarization device. This is comparable to the CMB loading, but a factor of a few smaller than the internal loading. The photon noise contributed by atmospheric loading is quite small compared to the measured dark noise level.
Atmospheric turbulence generates fluctuations in atmospheric emission, especially in water. A large fraction (∼85%) of the atmospheric emission absorbed by the 148 GHz band is broadband water emission, so the response to these fluctuations will be mostly common mode across the focal plane. Variations in spectral response do not average out as well for narrow emission lines. In band spectral differences between devices at the level of 10%
(Figure 4.9) could result in differential coupling to atmospheric lines at a level of ∼1 fW (∼1% in gain). Variations in band center at the level of 1 GHz (Figure 4.8) would result in differential coupling of∼2 fW to the 118 GHz oxygen line and∼0.5 fW to the 181 GHz water line. There is little to be gained by moving the band or shrinking it. Further, the effect of in-band variation is comparable to the effect of band center variation, so improvements
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Figure 4.11: Early tiles were made with a filter that produced band centers around 99 GHz.
The cyan spectrum here is a model based on measurements of individual 99 GHz devices.
The blue model is a 6% lower band (the target for future tiles). Plotted in grey is the atmospheric emission in pW/GHz (solid line) and the location of the J=1→0 CO transition (dashed).
in band center uniformity would not have a dramatic impact on differential atmospheric response.
We have not yet performed spectroscopy on any devices with a band center of 93 GHz (the intended science design). Early devices exhibited a band center of 99 GHz. A few such device spectra were measured in the White Dewar. Spectroscopy was performed on an entire 99 GHz tile in the Spider test cryostat, but the measured spectra exhibited severe fringing because the telescope optics were optimized for the 148 GHz band. I have combined existing data on 99 GHz devices to form a passband model (Figure 4.11). The 99 GHz model exhibits unacceptably high exposure to the CO line at 115 GHz. Future tiles will be made with an LC filter designed for a band center of 93 GHz. In Figure 4.12 I convolve our atmospheric model with the 93 GHz band model normalized to η0 = 33%.
The total predicted atmospheric loading is <0.03 pW per single-polarization device. Like the 148 GHz band, most (∼85%) of the atmospheric loading is due to broadband water emission. In band spectral differences between devices at the level of 10% would result in differential coupling to atmospheric lines at a level of0.4 fW (∼1% in gain). Variations in
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Figure 4.12: Convolution of the atmospheric model shown in Figure 2.1 with the 93 GHz band model (Figure 4.11) assuming an optical efficiencyη0= 33%.
band center at the level of 1 GHz would introduce differences in atmospheric coupling at a level of0.8 fW, primarily in the 118 GHz oxygen line and the broadband water emission.
Above Band Reponse
Before the Spider test cryostat was commissioned, I optically characterized individual 145 GHz test pixels in a windowed dewar with no lenses. To measure above band response, I coupled the main beams of the detectors to a chopped blackbody source and then inserted a high-pass thick grill filter (TGF) with a cutoff around 185 GHz in to the optical path.
The measured leakage was small (0.5%).
When we mated detectors to our full telescope optics we discovered much higher leakage (∼2%). This pickup was the result of radiation coupling directly to the TES island. Direct island coupling is not as strongly directional as the antenna beam, so it increased in a relative sense with the addition of the lenses. To reduce this response, we decreased the size of the cutout in the niobium ground plane around the TES island and added niobium to the silicon nitride support legs. We also added a low-pass metal mesh filter (7 cm−1 cutoff for 148 GHz focal planes) on the detector side of the eyepiece lens with a light-tight seal to the shielding enclosure. The above-band response of 148 GHz detectors coupled through
the optics is now no longer visible above our measurement noise (∼0.1%). We have not yet performed a comparable measurement for 93 GHz detectors, but the TES island geometry is the same for detectors at all frequencies.