Revenue (Disaggregation) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 28, 2020	Mar. 30, 2019	Mar. 28, 2020	Mar. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 211,463	$ 233,046	$ 417,306	$ 436,227
Americas
Disaggregation of Revenue [Line Items]
Revenue	83,991	90,013	167,962	166,902
Europe
Disaggregation of Revenue [Line Items]
Revenue	67,302	57,189	116,359	111,164
Asia
Disaggregation of Revenue [Line Items]
Revenue	60,170	85,844	132,985	158,161
Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	134,920	180,374	271,765	337,614
Over time
Disaggregation of Revenue [Line Items]
Revenue	76,543	52,672	145,541	98,613
Product
Disaggregation of Revenue [Line Items]
Revenue	183,223	206,690	362,081	381,769
Service
Disaggregation of Revenue [Line Items]
Revenue	28,240	26,356	55,225	54,458
Operating Segments | Test & Simulation
Disaggregation of Revenue [Line Items]
Revenue	125,497	151,032	246,227	276,592
Operating Segments | Test & Simulation | Americas
Disaggregation of Revenue [Line Items]
Revenue	39,375	51,399	77,223	90,888
Operating Segments | Test & Simulation | Europe
Disaggregation of Revenue [Line Items]
Revenue	41,135	29,536	67,650	58,163
Operating Segments | Test & Simulation | Asia
Disaggregation of Revenue [Line Items]
Revenue	44,987	70,097	101,354	127,541
Operating Segments | Test & Simulation | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	57,616	98,360	117,720	177,979
Operating Segments | Test & Simulation | Over time
Disaggregation of Revenue [Line Items]
Revenue	67,881	52,672	128,507	98,613
Operating Segments | Test & Simulation | Product
Disaggregation of Revenue [Line Items]
Revenue	102,288	126,511	197,784	225,419
Operating Segments | Test & Simulation | Service
Disaggregation of Revenue [Line Items]
Revenue	23,209	24,521	48,443	51,173
Operating Segments | Sensors
Disaggregation of Revenue [Line Items]
Revenue	86,198	82,375	171,733	160,325
Operating Segments | Sensors | Americas
Disaggregation of Revenue [Line Items]
Revenue	44,848	38,975	91,393	76,704
Operating Segments | Sensors | Europe
Disaggregation of Revenue [Line Items]
Revenue	26,167	27,653	48,709	53,001
Operating Segments | Sensors | Asia
Disaggregation of Revenue [Line Items]
Revenue	15,183	15,747	31,631	30,620
Operating Segments | Sensors | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	77,536	82,375	154,699	160,325
Operating Segments | Sensors | Over time
Disaggregation of Revenue [Line Items]
Revenue	8,662	0	17,034	0
Operating Segments | Sensors | Product
Disaggregation of Revenue [Line Items]
Revenue	81,161	80,540	164,935	157,040
Operating Segments | Sensors | Service
Disaggregation of Revenue [Line Items]
Revenue	5,037	1,835	6,798	3,285
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(232)	(361)	(654)	(690)
Intersegment Eliminations | Americas
Disaggregation of Revenue [Line Items]
Revenue	(232)	(361)	(654)	(690)
Intersegment Eliminations | Europe
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Intersegment Eliminations | Asia
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Intersegment Eliminations | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	(232)	(361)	(654)	(690)
Intersegment Eliminations | Over time
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Intersegment Eliminations | Product
Disaggregation of Revenue [Line Items]
Revenue	(226)	(361)	(638)	(690)
Intersegment Eliminations | Service
Disaggregation of Revenue [Line Items]
Revenue	$ (6)	$ 0	$ (16)	$ 0