Financial Statement Components - Selected Financial Statement Components (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Mar. 26, 2016	Dec. 31, 2016	Dec. 26, 2015
Inventories:
Raw materials and sub-assemblies	$ 23,185		$ 23,506
Work in process	14,383		10,347
Finished goods	3,976		4,984
Inventories	41,544		38,837
Inventories-delivered systems	3,280		2,457
Total inventories	44,824		41,294
Property, plant and equipment, net:
Total property, plant and equipment, gross	87,490		86,545
Accumulated depreciation and amortization	(44,314)		(42,319)
Total property, plant and equipment, net	43,176		44,226
Depreciation and amortization expense	1,800	$ 1,700
Accrued warranty	4,244	$ 4,377	3,838	$ 4,504
Customer deposits			581
Retrofit liability	984		432
Accrued professional services	570		424
Accrued royalties	60		1,233
Other	1,435		1,539
Total other current liabilities	7,293		8,047
Land [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	15,571		15,568
Building and improvements [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	20,566		20,532
Machinery and equipment [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	37,122		35,659
Furniture and fixtures [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	2,419		2,282
Software [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	9,776		9,756
Capital in progress [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	$ 2,036		$ 2,748