Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 24, 2016	Sep. 26, 2015
Cash flows from operating activities:
Net income	$ 17,381	$ 4,718
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	6,440	6,826
Stock-based compensation	5,432	4,664
Disposal of fixed assets	162	578
Inventory write-down	1,451	1,971
Deferred income taxes	(14)	173
Changes in fair value of contingent payments to Zygo Corporation	158	137
Changes in assets and liabilities:
Accounts receivable	(986)	(14,873)
Inventories	6,652	(15,893)
Inventories-delivered systems	(1,560)	370
Prepaid expenses and other	889	4,436
Accounts payable, accrued and other liabilities	(4,310)	9,634
Deferred revenue	3,825	(688)
Income taxes payable	(1,419)	295
Net cash provided by operating activities	34,101	2,348
Cash flows from investing activities:
Sales of marketable securities	2,093	2,884
Maturities of marketable securities	25,461	30,279
Purchases of marketable securities	(63,840)	(27,298)
Purchases of property, plant and equipment	(3,349)	(1,365)
Net cash provided by (used in) investing activities	(39,635)	4,500
Cash flows from financing activities:
Payments to Zygo Corporation related to acquisition	(315)	(614)
Proceeds from sale of shares under employee stock option plans and purchase plan	5,984	3,642
Taxes paid on net issuance of stock awards	(1,578)	(1,104)
Repurchases of common stock		(1,721)
Net cash provided by financing activities	4,091	203
Effect of exchange rate changes on cash and cash equivalents	710	(166)
Net increase (decrease) in cash and cash equivalents	(733)	6,885
Cash and cash equivalents, beginning of period	38,154	34,676
Cash and cash equivalents, end of period	37,421	41,561
Supplemental disclosure of non-cash investing activities:
Transfer of inventory to property, plant and equipment, net	1,699	$ 1,068
Transfer of property, plant and equipment to inventory, net	$ 491