Is it possible to adjust the aperture without a lens?
Whereas the opening is a part of the lens, not the body of the camera, no it is not possible.
As a rule, telescopes do not have variable openings: it's not necessary to stop yourself to limit the incoming light (which is absolutely the opposite of what you're looking for the star photography) and the control of the depth of field does not make sense when not all objects are closer than the moon.
The opening of the telescope is the diameter of the front element. In number ƒ, it is the ratio of the focal length of the telescope divided by the diameter of the front element. Be sure to use the same units (that is, inches or mm / cm / m).