Chapter 1 Cosmology
Z- Spec has been operating as a PI instrument at the Cal- tech Sub-millimeter Observatory (CSO) on Mauna Kea, Hawaii
3.3 Data Collection and Reduction
3.3.3 Synchronization and Pointing Correction
Excessive pointing and timing uncertainty will effectively increase the beam FWHM, when large amounts of data are coadded. Both effects are modeled and removed from the data.
TheBolocam data acquisition clock has a long time-scale drift and is actively synchronized to the GPS-synchronized telescope on a nightly basis. Figure 3.4 depicts how the timing offset can change throughout the course of the night. The y-axis is in units of the 50Hz sample rate. Therefore, a change in 50 units over the course of the night would cause an overall shift of 1 second in synchronization. As the telescope scans approximately 4 arcminutes per second, this offset could significantly smear if the beam if not accounted for. The data is corrected using a simple linear time-drift model, which can be constructed from the timing uncertainty indicated by the red line in Figure 3.4.
There are two stages to the pointing correction: one at the bolometer level and one at
Figure 3.3 A sample diagnostic plot used to remove one of the RXJ 0451 (aka MACSJ 0451.9+0006) observations from the final cluster coadds. From top to bottom: DC signal in Volts, AC signal in Volts, and AC noise power spectral density in Volts/Hz1/2 for the October 2009 observing run. Red lines indicate the signal averaged over all observations (including the 5-minute pointing observations), and the black lines represent a particular observation which was cut from the final coadds. One can see that the sky noise for this particular observation is rather severe.
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Figure 3.4 Sample diagnostic plot used to model the clock drift for each night of observation.
The y-axis is in units of the data sample rate: 1 unit represents 10 ms. The clock has drifted a bit more than a second over the course of the night, and this is well-modeled by a linear fit. Uncorrected, this would result in a systematic error on the pointing correction that is time-dependent over the night. Given the scan speed, this error would be large compared to the beam FWHM and effectively smear the FWHM for coadded observations.
the telescope level. Pointing models are constructed by performing two consecutive 5-minute observations of stationary millimeter-bright point sources (generally quasars) approximately once every fourth observation. The centroids of the pointing maps determine the position of the center of the focal plane on the sky with respect to the telescope coordinates. The relative positions of individual bolometers with respect to the center of the focal plane are determined using beam maps of the nightly 20-minute observations of a calibration source:
either Uranus, Neptune, or a secondary calibrator given in Sandell [251]. Flux calibration is discussed separately in Section 3.3.4.
As Bolocam is mounted at the Cassegrain focus of the telescope, it must be dismounted every time another instrument is scheduled to observe at that position. The mounting of the warm optics has fairly loose tolerances, which results in slightly different optical path for each mount/dismount cycle. This is accounted for by constructing pointing models for each observation run relative to an ensemble average over many runs. In Figure 3.5, one can observe that the overall trend in pointing has been that the beams of the bolometers in the
Figure 3.5 Individual bolometer pointing corrections superimposed with the relative bolome- ter positions on the Bolocam focal plane in units of the mean bolometer spacing for the November 2006 observing run. Left: Bolometer position offsets magnified by a factor of 4. Black: the raw offsets. Red: the local trend in offsets, obtained by averaging over all bolometers within 1.5 bolometer spacings of the given position. Right: the raw offsets over all observing runs from November 2006 to October 2010. The bolometer positions are gen- erally consistent from run-to-run, although the upper-right hextant appears to experience more positional variation than the others.
upper right corner of the focal plane are shifted towards the center. Note that the trend is magnified by a factor of four in the plot for visibility. The right-hand plot depicts the overall trend of the pointing offsets through several different observing runs.
The second stage of pointing corrections occurs at the telescope stage. In general, these are more significant than the focal plane pointing corrections. A sample nightly-pointing diagnostic plot is given in Figure 3.6 and Figure 3.7 shows the positions of the pointing sources for MACSJ 0744.8 over an entire observing run in 2009. Pointing generally does not vary from night-to-night, and these diagnostics are solely performed as an additional quality check. By far the largest systematic pointing offset is caused by offsets in the telescope and source coordinates which vary telescope position. The left-hand plot of figure 3.8 depicts a relatively rare situation with a gross offset of approximately 30 arcseconds in the azimuthal direction (which can also be identified in Figure 3.6). This is most likely due to an incorrect telescope setting and, if left uncorrected, this offset would source a significant amount of effective beam smearing. Fortunately, as can be seen in the right-hand plot of the same
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Figure 3.6 Diagnostic plot for the nightly pointing of MACSJ 0744.8 on the night of November 4th, 2009. (Left) elevation pointing offset, (right) azimuthal pointing offset, (black diamonds) raw data, and (red diamonds) corrected data. Note the overall 30 arcsecond pointing offset in azimuth. When corrected with the pointing model, the residuals drop to about 5 arcseconds.
figure, these pointing erors can be well-modeled and removed. These models are accurate to
∼5 arcseconds, and this pointing uncertainty produces an effective broadening of the point- spread function (PSF). Specifically, an effective PSF is determined by convolving Bolocam’s nominal PSF, which has a full-width at half-maximum (FWHM) of 58 arcseconds, with a two-dimensional Gaussian profile with σ = 5 arcsec. Fortunately, this broadening of the PSF due to pointing uncertainty is small, and it does not have a significant impact on the derived results (especially for resolved objects like galaxy clusters).