It looks that there are many opinion about minimum acceptable size of 100% Illuminated Image. Many books says 100% Illuminated Image Size should be larger than 12mm or so (or 20mm for larger telescopes) if we don't want vignetting (light loss). But some other book says that even under 12mm of 100% Illuminated Image Size, nothing happens in the human eyes except the background sky gets blacker and the objects we look at (especially low contrast objects like galaxies and dim nebulae) become much clearer and easier to observe provided 100% Illuminated Image is at least 2.5mm. Does anyone know what will happen in actual when I choose 100% Illuminated Image Size between 2.5mm and 12mm (say 3mm) to reduce a diagonal size?

• Re: 100% Illuminated Image Size - bobmay - June 12, 1998 - 13:02 UTC