Your device takes a CR-123 / 3 Volt lithium battery.
Here is a link to the product sheet for your device:
http://www.nightowloptics.com/documents/NOnightScopeSELLSHEET.pdf
To view the product sheet just use your mouse to click on the link, which is the underlined and highlighted text just above. This will open a new web browser page automatically for you and allow you to view the information at the website indicated.
You may need ADOBE READER installed on your computer to read PDF documents.
You can download this free at
WWW.ADOBE.COM if necessary.
Click on the DOWNLOAD ADOBE READER button on right side of the ADOBE home page when you get there, then follow the onscreen instructions to install the software on your computer.
I hope that was helpful to you, good luck and take care.