I bought a telescope with the following specs.:
Objective lens: 2.99"
Focal length: 900mm
Eyepieces: 20, 12.5, 4mm
Finder scope: 6 X 25mm
Barlow: 3X max
Magnification: 575X

I know it is a beginner model which will suit me fine, my question is that I don't know what actually these terms mean!

Can anybody advise and also suggest how can I use it (and what I can observe with it)?

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The focal length of an optical system is a measure of how strongly it converges (focuses) or diverges light. A system with a shorter focal length has greater optical power than one with a long focal length.An objective in optics is the lens or mirror in a microscope, telescope, camera or other optical instrument that receives the first light rays from the object being observed. The objective is also called the object lens, object glass, and objective glass.An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device.A finder scope is a little telescope that mounts on your main telescope. It is used to help find objects in the sky. It is usually 2 inches in diameter and has a power of 8. This will allow you to gather some light and magnify a little bit. It will let you see fainter guide stars and some brighter deep sky objects.The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, effectively increases the focal ratio of an optical system as perceived by all components after it in the system.Magnification is the process of enlarging something only in appearance, not in physical size. Magnification is also a number describing by which factor an object was magnified.I hope this explains what you need.

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