Financial Statement Components - Selected Financial Statement Components (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016
Inventories:
Raw materials and sub-assemblies	$ 29,880		$ 32,187
Work in process	17,655		13,498
Finished goods	6,703		7,175
Inventories	54,238		52,860
Inventories-delivered systems	1,855		1,534
Total inventories	56,093		54,394
Property, plant and equipment, net:
Total property, plant and equipment, gross	90,376		89,229
Accumulated depreciation and amortization	(47,326)		(44,419)
Total property, plant and equipment, net	43,050		44,810
Other Current Liabilities:
Accrued warranty	4,906	$ 4,643	4,863	$ 4,602	$ 4,244	$ 3,838
Accrued taxes	1,926		813
Accrued professional services	496		534
Other	1,180		1,039
Total other current liabilities	8,508		7,249
Land [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	15,571		15,573
Building and improvements [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	21,195		20,880
Machinery and equipment [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	40,235		36,380
Furniture and fixtures [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	2,461		2,420
Software [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	9,806		9,558
Capital in progress [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	$ 1,108		$ 4,418