All telescopes have design problems. The reason is that there are always tradeoffs, but the following tips are for the more simple and common problems.
Diffraction: Diffraction is caused by light bouncing off an object in and out of the visual light cone that feed a telescope. The way to cut down on diffraction is to paint all objects in your telescope flat black so they will reflect less light, except for your mirrors obviously. But you might want to blacken the sides of your secondary mirror if it is not yet black (be careful). Then try to eliminate or shorten anything that protrudes to the inside of your telescope such as screws (many times turning the screw around will help alot, this way the extra thread is on the outside of the tube). If you use mirror clips to hold your mirror in place, try finding another method to attach the mirror such as clear silicone rubber. If you have all of that under control you might consider baffling your tube. Your tube is a large source of circular diffraction, which is not very visible in the eyepiece unless your are looking for it. It will show up as a not quite black background when viewing objects in the telescope. By placing baffles in the tube the diffraction is more limited to the edges of the baffle, not the whole length of the tube.
Optimizing obstruction: If your are building a telescope from scratch or doing a major remodel, keep in mind that you can reduce your secondary obstruction by using a low profile focuser. Remember the light going to the eyepiece is cone shaped, so the closer in toward the tube your eyepiece is the smaller your secondary mirror can be. A good low profile focuser is a little pricey but look at the bright side, you will save a little money by using a smaller secondary mirror. If your are modifying an existing scope to a low profile focuser you will need to move your primary mirror down the tube (away from the secondary) the same distance that you have shortened your focuser length in order to have the benefit of using a smaller secondary mirror.

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