Optical Instruments

• Plano-Convex lens: Produces real images for objects outside focal length; produces virtual images only when object is inside focal length. Thus, not always virtual.
• Convex mirror: Always forms virtual, erect, and diminished images, irrespective of object distance. Therefore, always produces virtual images.
• Biconvex lens: Produces real images for objects beyond focal length; virtual images only when object is within focal length. Thus, not always virtual.
• Concave mirror: Can produce both real and virtual images depending on the object position. Thus, not always virtual.

Final Conclusion:
Among the given options, only a Convex mirror always produces virtual images.

An astronomical telescope works by having a large objective lens and an eyepiece. It produces a virtual, inverted, and magnified image of distant objects. This is because the objective lens gathers light from distant objects and forms a real, inverted image, which is then magnified by the eyepiece. The final image appears upside down (inverted) and larger (magnified) than the original object.