the question why some refractors deliver higher contrast views than the others,
is not just an object of the speculative philosophical reflection.
It is in fact about the reflected and scattered light suppression in the tube and in its parts.
The following documenting pics show where the problem emerges in the low cost astronomy OTAs,
in comparison to a premium spotting scope refractor,
1. The first source of scattered light is the unprotected rim of the achromat lenses, which shines really bright.
Its scattered light can be removed by a baffle in (or close to) the lens cell, at the cost of reducing the effective
2. The second source of scattered light are the unprotected surfaces in the
3. The third problem emerges in the focusers.
Focusers with the outer larger diameter focusing tubes make sense. But even then, putting a baffle into the focuser may help. An alternative are filter wheels, which also trap some of the scattered light.
4. The last source of scattered light are the walls in the zenith diagonal mirror house.
And finally, what matters as well, is the quality of blackening of the surfaces.
The popular knife edge rills will replace the reflections of light with the diffraction,
and so, you have a choice between the scattered light and the diffracted light, reducing the contrast.
A counter example is the Leica
So far I have tried out different solutions, the prisms seem to be less prone to scattering of light than the zenith diagonal mirrors.
And finally,
don't forget to mount an oversized lens hood on your refractor. Bigger is better.
I have the lens hoods for my astronomy binoculars as well.
Hoping this helps, and thank you for reading,
JG