As said in a previous post, I've recently become concerned over image sampling because of a recent experiment in response to poor seeing. My setup has contained a 2.5x or 3x barlow for most of my 4.5 years of planetary imaging. I've always used a barlow of those magnifications because my 2x and 5x barlows gave inferior images. However, I've realized for a while that the 2.5x and 3x barlows, by the numbers, are over-sampling, especially with the tiny pixels of the ASI290MM, but they've given me excellent results when conditions permit. Also, the 2x barlow I have on hand is a cheap model that usually come in beginner kits, so I've been starting the think the inferior images from that is due to the quality of the barlow instead of under-sampling like I once thought.
The way I see it, less magnification than what I have now should give a brighter image, which would mean shorter exposures, less gain, less noise, and more frames per second, all without losing details and perhaps recovering details in poor seeing, so it would be smart for me to get a lesser magnification barlow if it's a big deal.
I currently get about f/27.5 with my 2.5x Powermate. Based on the numbers I get from the Useful Formulae web page from Wilmslow Astro, the most optimal focal ratio for my setup is F/14, so it looks like I would be best served by a 1.25x barlow. The only one I know of is the 1.25x Magic Dakon Barlow from Brandon.
I think it's important for me to hear about personal experiences from other planetary imagers before I make further decisions, so I would be glad to get some opinions on this!
BTW, here's my imaging train as of this post: C14 EdgeHD -> MoonLite Focuser -> USB Filter Wheel -> 2.5x Powermate -> ASI290MM.