You are better off with a Triplet and Field Flattener or a Quad.
Visible light is made up of 3 colors: Red, Green and Blue light.
A Triplet or Quad gets all three colors of light in focus at the same point.
A doublet only gets 2 of those colors in focus at the same point. So with a mono camera when you focus the third filter the scale of your image on the sensor will be slightly different. This makes the object slightly larger or smaller in the one color. It also causes the distances between stars to be slightly different when using that third filter.
Using a doublet with a
OSC (One Shot Color) camera you get
CA (Chromatic Aberration), which usually shows up as a blue or purple halo around bright stars.
A well corrected Doublet will do a decent job and you can produce nice
AP images. Just not quite as good as a well corrected Triplet. It really all boils down to what amount of imperfection are you willing to accept and how much money are you willing to spend.
The type of glass used makes a difference too. The better the glass, the more crisp and pinpoint the stars will be. Abbe is a measure of the material's transparency or dispersion, higher is better.
Here are the Abbe numbers (Vd) of some commonly used ED glasses:
Crown & Flint (Schott): 60
FPL-51 (Ohara): 81.54
H-FK61 (CDGM): 81.61
FCD1 (Hoya): 81.61
OK-4 (LZOS): 92.1
FCD100 (Hoya): 94.66
FPL-53 (Ohara): 94.93
CaF2: 94.99 (Fluorite)
Refractors using better glass are more expensive too.
If getting a Doublet or Triplet, you want to get a Field Flattener. These make the stars round all the way out to the edge of the
FOV (Field Of View). In most cases you want to get a Flattener from the same manufacturer that is matched to their scope.
The quality of your focuser matters too. You don't want a focuser that will sag or slip. Ideally you want one that can be motorized late on.
FYI: Unfortunately in the astronomy world
APO (apochromatic) has become a marketing term and different manufactures apply the term differently. There are no standards that a scope has to comply with to be called an
APO.
https://lenspire.zeiss.com/photo/en/art ... ifference/
You would not go wrong with the mid-tier scopes of any of these brands:
Stellavue
Sky Watcher
Willam Optics
Tele Vue
Takahashi
TS Optics