Financial Statement Components - Selected Financial Statement Components (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Jun. 30, 2018	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Dec. 31, 2016
Inventories:
Raw materials and sub-assemblies	$ 31,619		$ 32,187
Work in process	20,590		13,498
Finished goods	6,675		7,175
Inventories	58,884		52,860
Inventories-delivered systems	1,858		1,534
Total inventories	60,742		54,394
Property, plant and equipment, net:
Total property, plant and equipment, gross	91,777		89,229
Accumulated depreciation and amortization	(48,642)		(44,419)
Total property, plant and equipment, net	43,135		44,810
Other Current Liabilities:
Accrued warranty	4,258	$ 4,906	4,863	$ 4,542	$ 4,602	$ 3,838
Accrued taxes	1,201		813
Third party commissions	837		76
Accrued professional services	537		534
Other	1,637		963
Total other current liabilities	8,470		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	21,226		20,880
Machinery and equipment [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	39,652		36,380
Furniture and fixtures [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	2,494		2,420
Software [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	9,915		9,558
Capital in progress [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	$ 2,919		$ 4,418