COSMOCAL PROJECT

monday - september 30 2024

fifth and last day of measurements:

• the analysis of the map taken yesterday showed a circular pattern in the polarization intensity as well as in the polarization angle; by comparing this map with the optical simulation, it is quite clear that the circular pattern is the product of diffraction at the edge of the secondary mirror's shadow on the primary. However, this circular structure was not in the centre of the map, so we concluded that we were not in an optimal alignement condition. So in order to improve the alignement of the box, we performed two 5'x 5' maps of the source, with different offsets in azimuth, calculated from the map derived yesterday and we compared the results; unfortunately these maps were not conclusive, as no particular pattern was detected. For these maps, the angle of the polarizer was fixed at 90º, the power of the source at 0.2 mW and the chopper frequency at 0.22 Hz.
• we opted for the az-offset which was offering the best alignement (by eye) and we performed another set of angle measurements, starting at 90º and turning by 20º until 70º, with a source power of 0.5 mW.
• finally, in the remaining 20 minutes, we performed another map with the same parameter as before, but this time not chopping the source and not applying any new offset, so keeping the [-2,-15] from yesterday.

sunday - september 29 2024

fourth day of measurements:

• the first part of the slot was dedicated to the track scans of different orientations of the source's polarizer : the first round, we turned from 270º to 240º, with steps of 30º, then we did another round from 255º to 225º, again by steps of 30º, so in the end we sampled two periods every 15º
• in the second part of the slot (last 20 mins), we acquired a map of 5'x 5', centered in the new coordinates estimated by the alignement yesterday, with steps of 15 arcsecs and a scanning speed of 5 arcsec/s.

saturday - september 28 2024

third day of measurements:

• all observational time was dedicated to the alignement
• we run multiple ON scans, changing the azimuth and the elevation of the antenna, keeping the source fixed
• we estimated (through on-the-fly intensity polarization maps) that the best alignment is found when the antenna is set with offsets of -2' in Az and -15' in El

  • SL: Note that 2.31 km * tan (2') = 1.3 m and 2.31 km * tan (15') = 10 m ==> If what you call the best alignment is the maximum intensity measured, and supposing that the (0', 0') offset corresponds to the the telescope optical axis being aligned with the source, that is to say with the shadow of M2 at the center of the detectors array, then what you measure is consistent with the optical simulations. In fact I would expect that you find maximums of signal not only at (-2', -15') but also at (+2', +15'), (+15, -2') and (-15', +2')... Or maybe not: it depends how strong are the constructive interferences caused by the diffraction on the rim of M2...

friday - september 27 2024

second day of measurements:

• acquired ON-OFF scans of the mm source
• we changed the polarizer's orientation by the following angles: 90º, 105º, 120º, 135º, 150º, 165º, 180º, 195º, 210º, 225º, 240º, 255º, 270º, 285º, 300º, 315º, 330º
• we acquired files to check the noise level of the mountain, the pws of the source with and without the chopper and finally with the hwp
• we had issues with the tuning: within every scan a tuning was done, so we observed a flat interval in the signal

thursday - september 26 2024

in the first day of measurements, we performed the following activities:

• with the source off, the hwp placed on the optical path but not spinning, we pointed the sky (close to the north pole) and then a source: on it we performed focus and pointing and applied corrections
• we moved to the mm source by running track command on pako, using the coordinates known from holography
• on the acquisition software, we looked for resonances and tuned them (with hwp still off); the saved parameter was already well adapted to the mountain load
• we started spinning the hwp at nominal 3Hz frequency
• we switched on the mm source along with the chopper, modulating at 1Hz
• at first, we saw no signal, but then we realised that the source's polarizer was set to 0º, hence minimising the signal, so we turned it to ~45º the signal appeared, with the correct hwp + chopper modulations

• at this point, we started procedure to maximise the signal, by tilting the mm source in Az and El : here came some problems --> lost resonances, irregular signal (appearing and disappearing) and we could not go back to initial situation

• so we closed the vertex to see if the problem was in kids or in real signal, and we saw a constant signal of ~2kHz
• we opened the vertex again, tuned the resonances and the situation was quite stable again, so we took 3 ON-OFF scans