O’Reilly AlphaTech Ventures, a venture fund affiliated with O’Reilly Media, said it has officially finished raising its first $51 million fund for early stage investing.

It is a small venture firm focused on the Internet, akin in some ways to First Round Capital, a fund run by Josh Kopelman. These firms have more formality than wealthy individual angel investors, but they invest much earlier than the large venture capital firms that like to put millions of dollars into a company straight off the bat. OATV will typically invest $1 million or less in a seed round, and then support a company with follow-on financing of up to $4 million. The idea is that Internet companies are cheap to build, and so the firm wants to invest just enough so that the company can get off the ground — but gives it the freedom to entertain acquisitions offers without having to worry about returning a profit on a large investment.

OATV will invest in areas such as Web 2.0, mobility and telephony applications, location-aware services, open hardware devices, sensor networks and renewable power.

The fund’s managing directors include Mark Jacobsen and Bryce Roberts. Earlier, Jacobsen managed O’Reilly Media’s own investments for nine years. Roberts began his VC career at Wasatch Venture Fund, a Draper Fisher Jurvetson affiliate. In 2004, he co-founded the Open Source Business Conference (since sold to IDG).

Roberts told VentureBeat that the fund will try to build upon O’Reilly’s experience in supporting the work, through its publishing and conference businesses, of engineer inventors who like to hack and modify new technologies. These hackers are likely to create interesting new businesses, he said.

OATV has made four investments to date. The three announced investments are Instructables, a service for collaborating on do-it-yourself services, Chumby Industries, which is producing an open, hackable, WiFi-enabled device, and Wesabe, a Web-based tool that lets people track their spending habits and communicate with others. Wesabe said today it has raised $700,000 from OATV.