Hubble Telescope: What's the Big Flap/Door?

[clel miller]

I did a cursory search on the forum, and I browsed Wiki...but maybe I missed it.
What is that big Flap/Door on the end of the scope.?
Is it part of the optics, or is it a protection device, or a...?
Thank You

 

[Bandersnatch]

It's the aperture door. It closes to protect the optics from sunlight.

Read more here:
asd.gsfc.nasa.gov/archive/sm3a/downloads/sm3a_media_guide/HST-systems.pdf

 

[clel miller]

10-4
Thank You...