Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 26, 2016	Mar. 28, 2015
Cash flows from operating activities:
Net income	$ 3,467	$ 2,556
Reconciliation of net income to net cash provided by (used in) operating activities:
Depreciation and amortization	2,176	2,324
Stock-based compensation	1,689	1,571
Disposal of fixed assets	99	485
Inventory write-down	651	403
Deferred income taxes	(30)	55
Changes in fair value of contingent payments to Zygo Corporation	55	35
Changes in assets and liabilities:
Accounts receivable	(5,197)	(13,340)
Inventories	(1,871)	(2,442)
Inventories-delivered systems	(1,283)	1,164
Prepaid expenses and other	605	(94)
Accounts payable, accrued and other liabilities	(1,468)	3,947
Deferred revenue	2,643	(2,468)
Income taxes payable	(1,037)	(70)
Net cash provided by (used in) operating activities	499	(5,874)
Cash flows from investing activities:
Sales of marketable securities		1,082
Maturities of marketable securities	13,153	9,592
Purchases of marketable securities	(12,953)	(10,613)
Purchases of property, plant and equipment	(610)	(502)
Net cash used in investing activities	(410)	(441)
Cash flows from financing activities:
Payments to Zygo Corporation related to acquisition	(84)	(224)
Proceeds from sale of shares under employee stock option plans and purchase plan	934	2,357
Taxes paid on net issuance of stock awards	(658)	(764)
Repurchases of common stock		(1,721)
Net cash provided by (used in) financing activities	192	(352)
Effect of exchange rate changes on cash and cash equivalents	238	(74)
Net increase (decrease) in cash and cash equivalents	519	(6,741)
Cash and cash equivalents, beginning of period	38,154	34,676
Cash and cash equivalents, end of period	38,673	27,935
Supplemental disclosure of non-cash investing activities:
Transfer of inventory to property, plant and equipment, net		$ 435
Transfer of property, plant and equipment to inventory, net	$ 491