At /12, it should be a doublet, and achromatic, but no matter. The higher the focal-ratio, the better whilst observing brighter, tighter objects, like the planets, and stars, single and double. It's good, that, at f/12, rather than a commercial instrument at f/10.
To improve contrast, to perhaps rival that as seen through apochromats, also to eliminate ghostings, blacken, and perhaps flock, the telescope, from fore to aft, beginning with the doublet and in making it "invisible"...
The spacers, if not a cemented affair, must be noted whilst separating the lens-elements, and replaced if needed. The order of an achromatic-doublet...
There should not be anything reflective within the lens-cell, the optical-tube, the inner areas of the focusser's housing, etc; only that of deepest, ultra-flat black...
Congratulations on the acquisition.