Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 24, 2016
Cash flows from operating activities:
Net income	$ 19,404	$ 17,381
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	5,219	6,440
Stock-based compensation	6,775	5,432
Disposal of fixed assets	98	162
Inventory write-down	1,412	1,451
Deferred income taxes	2,153	(14)
Changes in fair value of contingent payments to Zygo Corporation		158
Changes in assets and liabilities:
Accounts receivable	1,418	(986)
Inventories	(16,828)	6,652
Inventories-delivered systems	(864)	(1,560)
Prepaid expenses and other	(3,850)	889
Accounts payable, accrued and other liabilities	709	(4,310)
Deferred revenue	(1,204)	3,825
Income taxes payable	942	(1,419)
Net cash provided by operating activities	15,384	34,101
Cash flows from investing activities:
Payments to acquire certain assets	(2,000)
Sales of marketable securities	28,624	2,093
Maturities of marketable securities	62,923	25,461
Purchases of marketable securities	(104,984)	(63,840)
Purchases of property, plant and equipment	(2,342)	(3,349)
Net cash used in investing activities	(17,779)	(39,635)
Cash flows from financing activities:
Payments to Zygo Corporation related to acquisition		(315)
Proceeds from sale of shares under employee stock option plans and purchase plan	3,678	5,984
Taxes paid on net issuance of stock awards	(3,838)	(1,578)
Net cash provided by (used in) financing activities	(160)	4,091
Effect of exchange rate changes on cash and cash equivalents	(466)	710
Net decrease in cash and cash equivalents	(3,021)	(733)
Cash and cash equivalents, beginning of period	47,062	38,154
Cash and cash equivalents, end of period	44,041	37,421
Supplemental disclosure of non-cash investing activities:
Transfer of inventory to property, plant and equipment, net		$ 1,208
Transfer of property, plant and equipment to inventory, net	$ 205