That is for taking "dark frames". "Flat frames" need light going through the optics so that things like dust and vengetting can be detected.

This link explains them in more detail:

https://www.highpointscientific.com/astronomy-hub/post/astro...

You're right! I read too quick.

Instead, you want one of these!

https://support.phaseone.com/knowledgebase/article/KA-01239/...