SC-MST telescope performance

The Schwarzschild-Couder medium-sized telescopes (SC-MST) is the first ever designed IACT with a double mirror optics setup, allowing an improved angular resolution and a decrease of the focal plane scale, making possible the use of more compact cameras. This telescope is designed to hold a 11328 SiPM camera covering up to 9◦ of sky. Such densely populated camera allows unprecedented detail in the extracted information of the development of the cascade, although current analysis methods may still not be tuned for such capabilities. In fact, the only CTA MC analysis properly tuned to correctly assess SC-MST telescopes performance is the SLAC analysis. As described in Sec. 3.3.2, this analysis uses a parallel simplified simulation of telescopes response and cannot be cross-checked by parallel studies. For this reason, an accurately tuned analysis of the available Prod-2 SCMST configuration was required.

some parameters had to be tested in order to properly assess their performance. Both the trace integration time window and the image cleaning thresholds had to be tuned. Taking into account the form of a single photo-electron response of the SiPMs (broader than classical PMTs), a 10 ns window was chosen. Note the 2-pass trace extraction method applied (described in Sec. 3.3.3.1) shows much better performance for classical pixel sizes compared with the SC-MST camera. As shown in Fig. 3.24, in the case of the MSTs NSB contribution (bump at low pixel amplitudes) is reduced up to a factor 4, while in the SC-MSTs NSB contribution is barely reduced, up to a ∼ 20%. The reason may be related with the wider width of the window of integration, extracting more NSB regardless of the position (in time) of the extraction window, or with the lower ratio of signal pixels with respect to background pixels of the camera. The SC optics should not be the cause, taking into account the 2-pass trace extraction is also efficient for SC-SST telescopes, reducing NSB contribution by a factor 3.

(a) DC-MST (b) SC-MST

Figure 3.24: Histograms of the logarithm of the pixel amplitude (extracted charge from individual pixels) of two different MST types. Black lines show the extracted signal in the first pass, and red lines show it after the second pass, using information from the time of arrival. Note SC-MSTs highly pixelated digital camera is not tuned for the 2-pass trace integration method.

Regarding cleaning thresholds, as accidental runs were not available, several options were tested (shown in Fig. 3.25) resulting in the selection of the 2-level cleaning thresholds of 2.5/1.25 phe. Note this cleaning algorithm is not ideal for such densely pixelated cameras either, as the probability of creating higher

number of islands significantly increases with cameras with 5 times more pixels and geometries with higher number of neighbours. An alternative cleaning method, the “sum cleaning” introduced in [144], was implemented in the analysis but further tuning needs to be applied for competitive results.

Figure 3.25: Histogram of the logarithm of the pixel amplitude (extracted charge from individual pixels) of the SC-MST for different image cleaning thresholds. Note the low amplitude peak saturates with a cleaning above 2 phe.

To test SC-MST performance and their inclusion within the CTA MARS anal- ysis, 2 layouts were studied and compared: Both arrays have equal telescope dis- tribution, but different MST telescope type. The first layout is the 50 DC-MSTs contained within layout “2KA” and the second one is the 50 SC-MSTs present in the candidate array “2KB”. Comparing pure DC/SC MST layouts will allow us to compare each telescope performance, and to find out their strong and lack- ing characteristics. Figures 3.26a and 3.26b show the differential sensitivity and angular resolution of these layouts, composed by 50 DC-MSTs and 50 SC-MSTs respectively, simulated at the Leoncito site (in 50 hours of observation, averaged between ± 20◦ of zenith angle).

Although SC-MSTs do not help much below ≈ 100 GeV due to their smaller reflectors area, they show good performance above that point, improving the sen- sitivity of the core energies of CTA up to a 50% with respect to the DC-MSTs.

(a) Differential sensitivity (b) Angular resolution

Figure 3.26: Comparison of the differential sensitivity in 50 hours (Left ) and the an- gular resolution (Right ) between 2 layouts of equal telescope distribution, one composed by 50 DC-MSTs and the other one by 50 SC-MSTs. Both layouts were simulated at the Leoncito site, with ±20◦ of zenith angle. Comparison shows SC-MSTs outperform DC-MSTs in the core energies of CTA, mainly due to their improved angular resolution.

Comparing θ2 cut efficiencies and background rates shows that this improvement comes mainly from the enhanced angular resolution of these telescopes (due to the higher pixelization of the shower image), allowing a greater background rejection.

The next reasonable step is to compare the MST types performance under a more reasonable scenario: with full layouts composed by the mixture of LSTs, MSTs and SSTs. For this reason layouts “2KA” and “2KB” were analyzed. Re- sults, in Fig. 3.27, show the loss in the low energy performance caused by the smaller reflecting area of SC-MSTs is compensated with their improved recon- struction in the core energies. Note these results rely on an analysis that is far from ideal, and has been mainly tuned for classical IACTs, so they should be considered as very conservative regarding SC-MST performance.

Figure 3.27: Comparison of the differential sensitivity of the “2KA” and “2KB” layouts (see Fig. 3.16) of N+S pointing average performance in 50 hours, simulated in the Leoncito site. These layouts correspond to the same telescope distributions of DC- MSTs and SC-MSTs shown in Fig. 3.26, with the addition of 4 central LSTs and 72 SC-SSTs. DC-MSTs show better low energy performance while the SC-MSTs boost the sensitivity within the CTA core energies.