On Monday, 22 May 2017 03:07:39 UTC-4, wrote:
Hi,I've always used celestron stardiagnol,, recently I purchased a celestron 90° erect image correcting prism,I've not yet used it.can anyone tell me what differences to expect and the advantage and disadvantage, if there is any,thanks.

Some thing prisms provide the best images, but the light has to traverse 2 glass surfaces and reflect off another, all while traveling through 2-5 inches of glass. A mirror only requires one reflection. Also, some believe prisms add chromatic aberration to the image. I had what was considered the best prism (Celestron 2" Baader from the 1990's) and IMO, a good mirror is better.