4.3 Optics Reconfiguration
4.3.6 Responsivity
Table 4.12: Median optical loading after optics reconfiguration, calculated using all detectors of the specified band.Texcis determined from the hot/cold data.Tspillis determined from the skydip data.Tskyis determined by convolving FTS measurements of the bandpass with the atmospheric transmission spectrum corresponding toτ225.
Band Texc[K] Tspill[K] Tsky[K] Tload[K] Popt[pW]
0 15 40 10 70 3.5
1 20 35 25 80 7.8
2 20 35 35 90 5.6
3 20 55 95 165 5.4
Figure 4.25: Response comparison after the optics reconfiguration. Light blue diamonds denote the ratio of measured to predicted response to an unresolved astronomical source (in this case Uranus). Orange circles denote the ratio of measured to predicted response to small changes in airmass. Blue and red lines are linear fits to the data.
Figure 4.26:Left:The intermediate size (100%) EccosorbR collar.Right:Steve Baca showing the EccosorbR collar attached to the secondary mirror of the CSO. Photos taken by Simon Radford.
secondary mirror is 23 inches in diameter, so the area of the intermediate collar is approximately equal to the area of the secondary. The other two collars are 28 inches and 40 inches in diameter, which correspond to 50% and 200% the area of the secondary, respectively.
Multiple skydips are collected:
1. Two redundant skydips without a collar attached to the secondary.
2. Skydips with the 50%, 100%, and 200% collar attached to the secondary.
3. Skydips with each of the four quadrants of the 200% collar attached to the secondary.
The entire dataset was collected on the night of January 23, 2015. Each skydip measurement consists of IQ sweeps taken with the dome open and the telescope pointed at 6 elevation angles between 20◦and 70◦. From these sweeps we extract the resonant frequency as a function of elevation angle fres(e)and calculate the shift relative to the resonant frequency under room-temperature loading. The room-temperature loading reference measurement was obtained by placing a large piece of EccosorbR over the opening in the hex plate and was collected on the same night as the skydips. The same room-temperature reference measurement was used for all of the skydips.
The top panels of Figure 4.27 compare the skydips collected with the full collars of varying sizes. On the left, we have a detector located towards the center of the focal plane. On the right, we have a detector located towards the edge of the focal plane. Focusing first on the right, we find that the frequency shift relative to the
[!Az, !El] = [-0.6, 1.1] arcmin, R = 1.2 arcmin [!Az, !El] = [-5.2, 6.1] arcmin, R = 8.0 arcmin
Figure 4.27: Skydips collected with various secondary collar configurations. 01 and 02 refer to the two redundant measurements without a collar. 50, 100, and 200 refer to the measurements with full collars of increasing radial size. LL, LR, UL, and UL are the lower-left, lower-right, upper-left, and upper-right quadrant measurements with the largest (200) collar. The left and right columns show two Band 1 detectors on the lower half-band of the B2 array that are located near the center and edge of the focal plane, respectively.
The azimuthal, elevation, and radial offset of the detectors from the center of the focal plane are quoted in the title.
warm load is larger without a collar than with a collar of any size. The frequency shift either gets slightly smaller or stays the same as you increase the collar size from 50% to 100% to 200%, suggesting that most of the spillover occurs within a radius of 14 inches. The two redundant skydips without a collar were collected four hours apart. They show similar results, suggesting that the systematics due to changing conditions throughout the night are small. Now, focusing on the left, we find that the skydip is almost identical whether or not we have a collar (of any size). This suggests that the detectors located in the center of the focal plane do not see the secondary collar. We find that the majority of the resonators have behavior similar to that shown on the left; it is only the detectors located at the edge of the focal plane that the collar has a noticeable effect on the skydips. Since the large discrepancy in the measured and predicted response to an unresolved astronomical source is observed in all detectors, secondary spillover cannot be the explanation.
The bottom panels of Figure 4.27 compare the skydips obtained with each of the four quadrants of the 200% collar attached to the secondary. The results corroborate the story presented in the previous paragraph.
The skydips are identical for detectors located in the center of the focal plane. For detectors located at the edge, the spillover past the secondary is clearly localized to a specific quadrant.
Even for the detectors located at the edge, the frequency response between sky and room-temperature col- lar is.15% of the frequency response between sky and room-temperature beam-filling load. This suggests that the secondary spillover is a small effect.
4.3.6.2 Large-Angle Beammaps
Knowledge of the effective area of the CSO is necessary to predict the response to an unresolved astronomical source. We calculate Aeff using the throughput theorem: Aeff =λ2/Ωbeam. The beam solid-angle Ωbeam is determined by fitting a two-dimensional Gaussian to a beammap of Uranus, taking the average value of the
FWHMin the two directions, and employing the equation
Ωbeam=πFWHM2
4 ln(2) . (4.13)
Note that theFWHM in the two directions are approximately equal and a Gaussian shape provides a good fit to the main beam. If there is wide-angle response outside of the main beam, then this will result in an underestimation ofΩbeamand overestimation of the response to an unresolved astronomical source. Hence, wide-angle response is a possible explanation for the observed discrepancy between the measured and pre- dicted response to Uranus. In order to test this hypothesis we have made precise measurements of the beams of our detectors. This analysis is primarily the work of Jordan Wheeler; however, the results bring some resolution to the aforementioned discrepancy, so we briefly summarize them in this section.
The flux density of Jupiter is two orders of magnitude larger than Uranus, and enables a study of the beams at very large angles. It also drives the detectors into a nonlinear regime at small angles and distorts the beammap within the central arcmin. We use Uranus to measure the beam at small angles and Jupiter
to measure the beam and large angles. By splicing the Uranus and Jupiter beammaps together in this way we are able to measure the beams out to approximately 10 arcmin. Multiple wide-angle beammaps of both Uranus and Jupiter were collected. For each source we compute the inverse-variance weighted average of the beammaps of all detectors of a given observing band. We find that indeed there are significant sidelobes and diffuse wide angle response outside of the main beam. The fraction of the total power outside of the main beam is [0.77, 0.75, 0.73, 0.52] for the four observing band. This explains more than half of the observed discrepancy between our measured and predicted response to an unresolved astronomical source.
We have compiled a list of the wide-angle response of millimeter and submillimeter instruments that are similar to MUSIC in order to determine if our measured values are reasonable. The Atacama Cosmology Telescope (ACT) measures approximately 20% of the beam solid angle outside of a Gaussian fit to the main beam at 148 and 218 GHz [169]. These measurements extend out to 15−20 arcmin. The APEX-SZ experi- ment measures 30% of the beam solid angle outside of the main beam at 150 GHz [170]. This measurement extends out to 4 arcmin. The SCUBA-2 instrument measures 24% of the beam solid angle outside of the main beam at 350 GHz and 39% at 660 GHz [171]. These measurements extend out to 3.33 arcmin and 2.1 arcmin, respectively, but note that the SCUBA-2 beams are much smaller than the other experiments and MUSIC. Finally, the South Pole Telescope (SPT) measures that 15% of the beam solid angle isoutside of 15 arcminat 150 and 220 GHz [172]. Our results are consistent with all of these measurements, and suggest that millimeter and submillimeter instruments on∼10 meter telescopes can expect to find 20−30% of their beam solid angle outside of the main beam. We are unable to probe our beams beyond 10 arcmin due to atmospheric noise combined with the relatively slow scan speed of the CSO. But if the measurements made by SPT also hold for MUSIC, which is not unreasonable given the similarities between the two instruments, then we can expect to find an additional 15% of the beam at these very large angles. This would bring the measured and predicted response into agreement at the 10% level.