Is collimation the answer?
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Collimation will improve image clarity by correcting optical aberrations from improper mirror alignment, but it will have no affect on image brightness. That is a function of the magnification/eyepiece you are using and your telescope's aperture.
What size scope do you have? A basic law of optics is that image brightness drops as magnification goes up, which it is when you use the 6 mm and, especially the 4 mm. In other words, it is perfectly normal for things to get darker as eyepiece focal length gets shorter. The only way to compensate for this loss of light is to go to a scope with a larger mirror or objective lens. In a small telescope, though, things get dark in a hurry, since small mirrors or objectives don't transmit enough light to support higher magnifications, In other words, a small telescope can't support as much magnification as a large telescope and, if you have a small scope, that 4 mm and 6 mm eyepiece may be too much. Don't overdo on magnification - the number one beginner's mistake in astronomy. SeeTen Top Telescope Eyepiece Questions.
Then, too, you field of view is also pinching down, drastically, when you go from your 32 mm (low power) to your 4 mm (high power eyepiece). If you did not center the object you wish to see, perfectly, in the center of the 32 mm eyepiece field of view, it will not be visible when you switch to the 6 mm or 4 mm. That is quite a drastic jump in magnification. Your finder scope will really have to be exactly adjusted, too, at such magnifications, but always start out at lower magnifications and work your way up when trying to locate objects. You may also want to see my article, Telescope FAQsas how to set up your telescope
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