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10" Dobs are generally Rich Field Telescopes. Rich Field Telescope are generally an F/4 Focal Ratio or less. They have a very wide field of View, and provide bright images. If the magnification of the eyepiece is too low, part of the Secondary Mirror will be seen as a dark spot through the eyepiece.
Low magnification of the Planets, especially bright Jupiter, will show no detail. You need to have a minimum of around 150x to spread out the light a bit, and show details. Otherwise the planet is just too bright, using your size of telescope.
Although the hole in your Dust Cap is quite small, observing Jupiter using just this small hole will cut down the light considerably. It will also change the focal ratio of the telescope to a longer one (better for planets). It's important to align the hole where the Spider (cross hairs) isn't. Using this method, the Spider and Secondary Mirror are no longer an obstruction to the Main Mirror, since the Main Mirror only sees the light coming through the small Dust Cover hole. The small hole means the Resolution of Telescope is reduced. This reduces the quality of the clarity of the image. So it's a case of getting the right size hole to suit your type of Telescope. Use a piece of cardboard, instead of the Dust Cap, with a larger circle cut out of it. The circle can be oval shaped, to be larger, and better fit the gap between the Spider Vanes, as long as the shape is curved, it won't bother the optics. You can cut more than one circle, to fit between the Spider Vanes. If you cut 2 circles, opposite each other - hey, you've made a Hartmann Mask! Used for getting perfect focus. Just adjust the Focuser until the 2 images of Jupiter become one.
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