Achro to APO upgrade advice.
Life and local sky conditions have resulted in my grab-and-go refractors being what I use 90% of the time. (The C8 hardly makes it out of the door these days). I do visual observing, wide field, almost entirely DSOs. My grab-and-gos are 2 Meade Infinity scopes, 80mm f5 and 102mm f6, both upgraded with 2” dual speed crawfords. I recently acquired a older 90mm f5.5 StarSky (re-branded Long Perng) “semi APO” and noticed the 90mm has somewhat sharper views than either of the Meades. As a result I am looking at upgrading at least one of my grab-and-gos to an APO.
My primary question is, in these aperture and focal lengths, what image quality differences have others experienced when moving from an Achro to an APO?
Your 102mm F6 achromat will have very similar, if not nearly equal performance on deep space objects (referring to nebula, galaxies, etc, things that are not bright point light sources or solar system objects). The aperture gives you the light collection. Getting a same aperture ED/APO scope will not really change how nebula and galaxies look in that sense. What will change however, in an ED/APO is the CA on bright subject matter such as stars, clusters, solar system objects, where the CA masks detail and reduces contrast. It's not a magic bullet. A same size aperture ED/APO will not make DSO brighter. And if you're into DSO, aperture is what matters for light collection (a smaller aperture with best glass ever will not show a nebula better, it will lose light). So while I would normally say a 102mm F7 Astrotech (or equivalent) would be an excellent 4" ED frac grab & go, it will really not do much for DSO viewing compared to your current 106mm. If you are interested in seeing brighter nebula and stuff like that, not bright objects, more aperture would be more beneficial. Also, most globs/clusters are better in larger aperture, not just having better glass, and these fast achromats don't show severe CA at low power on these objects. So really, I would likely look at something like a 120mm F5 or even 150mm F5 achromat (or just get a 6 inch reflector really.... F5~F6) if DSO is your thing. ED/APO correction is a lot more important on brighter things.
Unfortunately the moment you get into 120mm+ aperture ranges on ED/APO's, they require significant mounts to handle them, they're big heavy scopes, even the fast ones. So grab & go sort of goes away there.
Biggest grab & go ED frac is likely going to be the 102mm F7 flavors. But again, aperture for aperture, you're not gaining anything there. You would have better views of bright things though. But nebula aren't going to be better. Also the 102ED F7 will have a little CA still unfortunately, it's not an APO.
For cost, the alternative really is a 150mm F5~F6 reflector. No CA. More aperture over a 4" scope so everything will be brighter (and will be equivalent in many ways to a 5" frac).
If you still want a ED/APO, a doublet with FPL53 glass will be virtually CA-free for visual use and ideal there.
Here's my 120mm F5 achromat and 80mm F7.5 ED (FPL53) side by side. The ED80 is better on planets, solar, lunar, some brighter stars, etc, no visible CA. But the 120/5 achromat eats the ED80's lunch on DSO like M42, M13, galaxies, etc. Aperture just brings in more light to see things that are faint and fuzzy. ED glass is great on already really bright things where the CA from the achromat masks detail and/or robs contrast. I like both. But for DSO I choose the 120mm aperture each time. Similar sized scopes.
Edited by MalVeauX, 23 January 2020 - 02:10 PM.