Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Mar. 26, 2016
Cash flows from operating activities:
Net income	$ 5,352	$ 3,467
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	1,854	2,176
Stock-based compensation	2,164	1,689
Disposal of fixed assets	63	99
Inventory write-down	406	651
Deferred income taxes	(479)	(30)
Changes in fair value of contingent payments to Zygo Corporation		55
Changes in assets and liabilities:
Accounts receivable	(6,874)	(5,197)
Inventories	(2,695)	(1,871)
Inventories-delivered systems	(823)	(1,283)
Prepaid expenses and other	(88)	605
Accounts payable, accrued and other liabilities	970	(1,468)
Deferred revenue	3,039	2,643
Income taxes payable	186	(1,037)
Net cash provided by operating activities	3,075	499
Cash flows from investing activities:
Sales of marketable securities	10,181
Maturities of marketable securities	24,531	13,153
Purchases of marketable securities	(36,514)	(12,953)
Purchases of property, plant and equipment	(47)	(610)
Net cash used in investing activities	(1,849)	(410)
Cash flows from financing activities:
Payments to Zygo Corporation related to acquisition		(84)
Proceeds from sale of shares under employee stock option plans and purchase plan	1,217	934
Taxes paid on net issuance of stock awards	(1,755)	(658)
Net cash provided by (used in) financing activities	(538)	192
Effect of exchange rate changes on cash and cash equivalents	(333)	238
Net increase (decrease) in cash and cash equivalents	355	519
Cash and cash equivalents, beginning of period	47,062	38,154
Cash and cash equivalents, end of period	47,417	38,673
Supplemental disclosure of non-cash investing activities:
Transfer of property, plant and equipment to inventory, net	$ 22	$ 491