I would like to use my Nikon D3100 for astrophotography. I have attached the camera to my telescope with a T-ring, after which the camera recognized the absence of the lens, which is perfectly normal. However, without the lens on, I am unable to adjust the aperture, which is extremely important for astrophotography. I can set the ISO, the shutter speed, and some other, minor things but not the Aperture.

I’m using the camera in manual mode.

Is setting the aperture without the lens even possible?