: : : I want to see the Red Spot and cloud of Jupitar in detail through eyepiece. : : : In order to do that, I have made 6" dobsonian telescope using meade f5 primary mirror. It is good mirror but I can only realise the existance of red spot and cloud with it. I also tried to see the Jupiter through celestron's 11"schmidt - cassegrain at star party, but in fail. Now I am trying to build 16" f4.5 dobsonian using Meade mirror hoping that 16" is enough to gather the light.

: : : Are there anybody who knows what size of aperture is enough to see the red spot and cloud of Jupiter in detail through eyepiece? (I believe there exists some threshold of aperture size to see the Jupitar clearly. )

You have asked a very simple question, You should be able to clearly see the great red spot as well as the cloud bands on Jupiter with a 4in or large scope. Im assuming that your 6in f/5 scope is properly colluminated you should have no problem seeing either of the above listed structures on jupiter. Also keep in mind that weather and seeing conditions have a lot to do with what you can see. Take heart, the GRS (Great red Spot) is not necessarily red for the most part is is a pale pinkish color, try using color filters to enchance the contrast of finding the spot. Also remember that the GRS can only be seen when it transits infront of the disk of jupiter. Take care happy hunting