B-737 first officer reported descending below glideslope intercept altitude during vector to final.

Attributes
ACN	1618695
Time
Date	201902
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Experience	Flight Crew Last 90 Days 465
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance Flight Deck / Cabin / Aircraft Event Other / Unknown Inflight Event / Encounter CFTT / CFIT

Narrative:

On downwind to xxr; cleared for visual; I set 2800 feet. Captain verified it. On base leg; we were told of a king air on the opposite side. We saw it on TCAS. I'd planned and briefed joining final outside FAF; as we were still descending to gsia (glide slope intercept altitude). As I turned to base; about 3 miles outside FAF; I looked left to see the runway and adjusted the heading bug slightly to left for better intercept angle; or so I thought. All of a sudden; the sight picture looked lower; I looked up and the aircraft was at 2100-2200 feet MSL. I looked at the alerter and it was set to 1500 feet. Clicked autopilot off; climbed back up to 2800 feet MSL. At this time; ATC gave us a low altitude alert. We acknowledged; and at 2800 MSL; intercepted glideslope and continued on approach to landing. I think I may have inadvertently twisted the altitude alerter instead of heading bug to the left as I was setting a better intercept angle. But because the visual picture looked low; I was able to recognize and correct the situation.

Original NASA ASRS Text

Title: B-737 First Officer reported descending below glideslope intercept altitude during vector to final.

Narrative: On downwind to XXR; cleared for visual; I set 2800 feet. Captain verified it. On base leg; we were told of a King Air on the opposite side. We saw it on TCAS. I'd planned and briefed joining final outside FAF; as we were still descending to GSIA (Glide Slope Intercept Altitude). As I turned to base; about 3 miles outside FAF; I looked left to see the runway and adjusted the heading bug slightly to left for better intercept angle; or so I thought. All of a sudden; the sight picture looked lower; I looked up and the aircraft was at 2100-2200 feet MSL. I looked at the alerter and it was set to 1500 feet. Clicked autopilot off; climbed back up to 2800 feet MSL. At this time; ATC gave us a low altitude alert. We acknowledged; and at 2800 MSL; intercepted glideslope and continued on approach to landing. I think I may have inadvertently twisted the altitude alerter instead of heading bug to the left as I was setting a better intercept angle. But because the visual picture looked low; I was able to recognize and correct the situation.

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.