Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jul. 01, 2017	Jun. 25, 2016
Cash flows from operating activities:
Net income	$ 13,640	$ 9,498
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	3,552	4,305
Stock-based compensation	4,327	3,432
Disposal of fixed assets	98	128
Inventory write-down	1,015	1,046
Deferred income taxes	798	(30)
Changes in fair value of contingent payments to Zygo Corporation		95
Changes in assets and liabilities:
Accounts receivable	(4,720)	(14,277)
Inventories	(10,090)	6,457
Inventories-delivered systems	783	(6,737)
Prepaid expenses and other	(947)	(151)
Accounts payable, accrued and other liabilities	(521)	(1,511)
Deferred revenue	1,837	12,080
Income taxes payable	486	(1,115)
Net cash provided by operating activities	10,258	13,220
Cash flows from investing activities:
Payments to acquire certain assets	(2,000)
Sales of marketable securities	27,391
Maturities of marketable securities	40,923	21,816
Purchases of marketable securities	(78,787)	(12,880)
Purchases of property, plant and equipment	(1,536)	(2,528)
Net cash provided by (used in) investing activities	(14,009)	6,408
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	2,865	3,466
Taxes paid on net issuance of stock awards	(3,392)	(1,288)
Net cash provided by (used in) financing activities	(527)	1,863
Effect of exchange rate changes on cash and cash equivalents	(430)	387
Net increase (decrease) in cash and cash equivalents	(4,708)	21,878
Cash and cash equivalents, beginning of period	47,062	38,154
Cash and cash equivalents, end of period	42,354	60,032
Supplemental disclosure of non-cash investing activities:
Transfer of property, plant and equipment to inventory, net	$ 550	$ 278