I want to upgrade from my ES ED102. Budget is no longer an issue. I have already thought about weight, mounts, yada-yada-yada. I'd like to develop a list of great refractors that I can be on the lookout for on AM, CN, etc. I'm also ready to purchase new. APO is required. Target specs are simple. Aperture: 125mm - 140mm Focal ratio: f/6 - f/10 Please make a convincing case for your suggestions.
There are many factors that influence the level of detail you can experience. Let me concentrate on just the numbers. Think of your scope / eyepiece combination in terms of exit pupil (which directly translates to magnification per mm of aperture). It's all about aperture, and you can only ask your scope to perform within its aperture constraint. You already noticed that your 9mm ep with a 2x barlow was great, and Ricochet mentioned that this is the optimum for your scope. I wholeheartedly agree. This combination gave you 144x at an exit pupil of 0.90mm. A great exit pupil for planetary detail is 1.0mm . You can push this to .75-.8mm, but I do not recommend anything beyond that. An exit pupil of .5mm is often used for double stars, but I don't recommend it for planets. So how does this relate to your eyepiece/barlow selection? Exit pupil is easily calculated by taking the focal length of your eyepiece (divide by 2 if using a 2x barlow), and dividing that by the focal ratio of your telescope. Yours happens to be f/5. If you want more magnification than a .8mm exit pupil can provide on your scope, then you need a bigger scope. Here are some figures you can use: