50mm diameter lens Telescope?

No, because you would still have the same size lens.

Can you say "duh"? I knew that you could.

Depending on the geometry, it might or might not cause vignetting.

The best it would do is act as a baffle to block out stray light (especially if it were black).

It's not going to increase the light gathering power. If it vingnets, it will reduce it.

This is known as aperture which is the diameter of the primary objective lens or mirror.

The structure supporting this lens/mirror has nothing to do with gathering light.

Lens hoods are a common accessory in photography for blocking light glare at the lens.

Now if your PVC did not infringe on the field of view and jet black as to not reflect light, it would make for a suitable hood.

The opening on my 300MM telescope is just over 345MM Its effective aperture is still 300MM, the OD of the primary mirror.

50 MM refractor compares to a rile scope, monocular or binocular,,equates to about 840 times the light gathering over the unaided eye.this allows for magnification to the tune of 25X without loss of light

You'd probably mess up the view.

The objective lens (or opening) of a telescope needs the incident light to come in as parallel rays. What you propose would bring additional light in from random directions.