and
daniejb. I suspect that because the 4mm huygens has such a short eyerelief you are not able to get the lens of you camera close enough to get an image of the moon. If you put in a larger eyepiece and stand back from the eyepiece you will see the secondary and spider with your naked eye.
I would suggest using a barlow to increase the focal length of the telescope and a 20mm lens to shoot afocally through. Get that focus by eye, then use your camera to focus on the image in the eyepiece. If you can give a bit more detail of you scope and camera I can probable work out the distances and zooms that would be optimal for your scope and camera combination.