Narrative:

Aircraft X checked on descending via the new STAR. I descended him to 10000 feet. On my initial scan of his route I didn't see the unidentified VFR target (probably because it was too far east of aircraft X's planned route of flight. Prior to optimization of airspace and procedures in the metroplex (oapm) these arrivals came in on a route about 5-10 miles east of their new oapm route and they also used to be at a higher altitude which kept them above most of the unidentified VFR aircraft landing and departing L35 airport. These VFR aircraft generally transition at altitudes between 8500-10550 feet due to terrain avoidance.oapm has forced the sectors to look at about 10-15 miles more airspace and this changes the way controllers scan. It forces your attention to be placed away from potential conflicts. Prior to aircraft X reporting the TCAS-RA I received a call from march approach with unnecessary coordination. They wanted to know if I wanted to talk to a previously coordinated aircraft since he would be moving east. Hemet radar (hemr) had already coordinated a point out with this aircraft prior to me taking position so the second call was really unnecessary; especially since the planned route of this aircraft was all in their class 'C' airspace. With my attention diverted south of aircraft X (by approximately 35 miles) I missed the potential conflict. When I got off the line with march approach aircraft X reported the RA; they asked me if I was talking to the aircraft; I said no. When aircraft X reported the RA I did not issue traffic since it was about 1 mile east of aircraft X and I determined them to be no factor since the targets had passed and were diverging.the conflict alert alarm did not go off until targets were no longer a factor. Conflict alert alarm went off after aircraft X told me they were responding to an RA. If the conflict alert had gone off with adequate time for me to react I could have taken measures to ensure separation even while on the line with march approach. My attention would have been re-engaged to the conflict in the northern part of my airspace. Hemr sector has too much airspace. That combined with the oapm changes leads to a very complex sector. March approach needs to take more airspace to relieve the hemr sector of the low level VFR aircraft. We also seem to be working more picture and survey flights on the weekends. Can we mitigate this by not allowing so many at one time? This and the radar feed are probably the most pressing issues in this TRACON area.descend via's could also play a role in the event; I understand that I descended aircraft X to 10000 feet; however the old procedures these arrivals came over stopped at 12000 feet and not descending to 9000 feet allowing the controller to make adjustments after multiple scans for the hundreds of VFR aircraft that transition this area at or above 9000 feet for terrain. We can all sit back and play monday morning quarterback and I'll be the first to admit I missed the traffic and should have taken action but; the amount of airspace hemr and norr have to look at combined with the oapm procedures are going to lead to more significant events and possibly worse.

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Original NASA ASRS Text

Title: SCT TRACON Controller reported not noticing a VFR aircraft conflicting with an IFR RNAV arrival due to focus required on a different portion of the airspace.

Narrative: Aircraft X checked on descending via the new STAR. I descended him to 10000 feet. On my initial scan of his route I didn't see the unidentified VFR target (probably because it was too far east of Aircraft X's planned route of flight. Prior to Optimization of Airspace and Procedures in the Metroplex (OAPM) these arrivals came in on a route about 5-10 miles east of their new OAPM route and they also used to be at a higher altitude which kept them above most of the unidentified VFR aircraft landing and departing L35 airport. These VFR aircraft generally transition at altitudes between 8500-10550 feet due to terrain avoidance.OAPM has forced the sectors to look at about 10-15 miles more airspace and this changes the way controllers scan. It forces your attention to be placed away from potential conflicts. Prior to Aircraft X reporting the TCAS-RA I received a call from March Approach with unnecessary coordination. They wanted to know if I wanted to talk to a previously coordinated aircraft since he would be moving East. Hemet Radar (HEMR) had already coordinated a Point Out with this aircraft prior to me taking position so the second call was really unnecessary; especially since the planned route of this aircraft was all in their Class 'C' airspace. With my attention diverted South of Aircraft X (by approximately 35 miles) I missed the potential conflict. When I got off the line with March Approach Aircraft X reported the RA; they asked me if I was talking to the aircraft; I said no. When Aircraft X reported the RA I did not issue traffic since it was about 1 mile East of Aircraft X and I determined them to be no factor since the targets had passed and were diverging.The Conflict Alert alarm did not go off until targets were no longer a factor. Conflict Alert alarm went off after Aircraft X told me they were responding to an RA. If the Conflict Alert had gone off with adequate time for me to react I could have taken measures to ensure separation even while on the line with March Approach. My attention would have been re-engaged to the conflict in the northern part of my airspace. HEMR sector has too much airspace. That combined with the OAPM changes leads to a very complex sector. March Approach needs to take more airspace to relieve the HEMR sector of the low level VFR aircraft. We also seem to be working more picture and survey flights on the weekends. Can we mitigate this by not allowing so many at one time? This and the radar feed are probably the most pressing issues in this TRACON area.Descend via's could also play a role in the event; I understand that I descended Aircraft X to 10000 feet; however the old procedures these arrivals came over stopped at 12000 feet and not descending to 9000 feet allowing the controller to make adjustments after multiple scans for the hundreds of VFR aircraft that transition this area at or above 9000 feet for terrain. We can all sit back and play Monday morning quarterback and I'll be the first to admit I missed the traffic and should have taken action but; the amount of airspace HEMR and NORR have to look at combined with the OAPM procedures are going to lead to more significant events and possibly worse.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.