Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net income	$ 2,973	$ 16,381
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	2,406	1,724
Stock-based compensation	3,114	2,338
Disposal of fixed assets	2	45
Inventory write-down	1,747	95
Deferred income taxes	4	2,062
Changes in assets and liabilities:
Accounts receivable	6,604	8,035
Inventories	(2,864)	315
Inventories-delivered systems	(169)	41
Prepaid expenses and other	(1,189)	(454)
Operating lease - right of use assets	148
Accounts payable, accrued and other liabilities	(13,656)	(1,474)
Deferred revenue	1,403	2,172
Operating lease liabilities	(90)
Income taxes payable	(1,289)	1,434
Net cash provided by (used in) operating activities	(856)	32,714
Cash flows from investing activities:
Payments to acquire certain assets		(1,000)
Sales of marketable securities	739	17,435
Maturities of marketable securities	11,450	6,500
Purchases of marketable securities	(24,021)
Purchases of property, plant and equipment	(4,132)	(1,319)
Net cash provided by (used in) investing activities	(15,964)	21,616
Cash flows from financing activities:
Proceeds from sale of shares under employee stock option plans and purchase plan	1,548	545
Taxes paid on net issuance of stock awards	(803)	(476)
Repurchases of common stock under share repurchase plans		(22,987)
Net cash provided by (used in) financing activities	745	(22,918)
Effect of exchange rate changes on cash and cash equivalents	336	(399)
Net increase (decrease) in cash and cash equivalents	(15,739)	31,013
Cash and cash equivalents, beginning of period	110,951	34,899
Cash and cash equivalents, end of period	95,212	65,912
Supplemental disclosure of non-cash investing activities:
Transfers between inventory and property, plant and equipment, net	406	91
Unpaid property, plant and equipment	$ 516	$ 265