Financial Statement Components - Selected Financial Statement Components (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2017	Jun. 25, 2016	Jul. 01, 2017	Jun. 25, 2016	Apr. 01, 2017	Dec. 31, 2016	Mar. 26, 2016	Dec. 26, 2015
Inventories:
Raw materials and sub-assemblies	$ 28,556		$ 28,556			$ 23,506
Work in process	14,692		14,692			10,347
Finished goods	5,614		5,614			4,984
Inventories	48,862		48,862			38,837
Inventories-delivered systems	1,674		1,674			2,457
Total inventories	50,536		50,536			41,294
Property, plant and equipment, net:
Total property, plant and equipment, gross	87,557		87,557			86,545
Accumulated depreciation and amortization	(45,539)		(45,539)			(42,319)
Total property, plant and equipment, net	42,018		42,018			44,226
Depreciation and amortization expense	1,600	$ 1,700	3,400	$ 3,400
Accrued warranty	4,602	$ 4,556	4,602	$ 4,556	$ 4,244	3,838	$ 4,377	$ 4,504
Customer deposits						581
Retrofit liability	613		613			432
Accrued professional services	720		720			424
Accrued royalties	60		60			1,233
Other	1,373		1,373			1,539
Total other current liabilities	7,368		7,368			8,047
Land [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	15,571		15,571			15,568
Building and improvements [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	20,628		20,628			20,532
Machinery and equipment [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	37,247		37,247			35,659
Furniture and fixtures [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	2,440		2,440			2,282
Software [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	10,532		10,532			9,756
Capital in progress [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	$ 1,139		$ 1,139			$ 2,748