Nick Smith @ Altair Astro if inside you can generally unscrew the front objective after removing the dew-shield and clean the back, but sometimes it looks as if the dust is on the back or inside when it's really on the front. You can blow the dust off first with a rocket blower. Never use compressed air as it will damage the lens by blasting dust grains against it. You can use an optical cleaning fluid or even specsavers glasses spray (it should say "for coated optics"). Spray it on lens tissue (not Kim wipes) or a cotton wool ball or swab, and gently roll the dirt off from centre to edge, never in circles because if you scratch it you want the scratches radiating outwards. Don't drag or you'll scratch the surface with the dirt which is embedded in the swab (that's why you roll it to pick up the dirt so you are always using a fresh part of the swab or cleaning tissue or whatever). Don't use microfibre cloths which are included with optical fluids because they trap dust and scratch the surface. Don't use cotton wool balls which are saturated with perfume or moisturisers as it just sticks to the lens. Don't ever shine an LED torch on a lens because it will show up the tiniest dust grains which won't affect the image and you'll keep trying to clean them off forever :) If you have pollen sticking to the coatings or tree sap then use isopropyl 99% to dissolve it. Never put whatever cleaning fluid on the lens surface always on the swab first. Can't think of more tips but will mention if any more occur to me...