Financial Statement Components - Selected Financial Statement Components (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 30, 2017	Apr. 01, 2017	Dec. 31, 2016
Inventories:
Raw materials and sub-assemblies	$ 29,781	$ 32,187
Work in process	15,781	13,498
Finished goods	7,548	7,175
Inventories	53,110	52,860
Inventories-delivered systems	1,493	1,534
Total inventories	54,603	54,394
Property, plant and equipment, net:
Total property, plant and equipment, gross	89,970	89,229
Accumulated depreciation and amortization	(46,175)	(44,419)
Total property, plant and equipment, net	43,795	44,810
Other Current Liabilities:
Accrued warranty	4,643	4,863	$ 4,244	$ 3,838
Accrued taxes	721	813
Accrued professional services	470	534
Other	1,286	1,039
Total other current liabilities	7,120	7,249
Land [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	15,573	15,573
Building and improvements [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	21,212	20,880
Machinery and equipment [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	39,629	36,380
Furniture and fixtures [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	2,470	2,420
Software [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	9,584	9,558
Capital in progress [Member]
Property, plant and equipment, net:
Total property, plant and equipment, gross	$ 1,502	$ 4,418