A tiny air gap (often maintained by three small tabs of plastic or foil spaced 120 degrees apart) is usually required between the crown and flint lenses.
Found it: Norman Remer – Making a Refractor Telescope (PDF, 12th New Edition) making a refractor telescope norman remer pdf 12 new
This keyword may indicate a search for a "new" take on this classic material. Among the ATM community, Remer’s book is recognized as the most modern and comprehensive guide available, serving as a definitive resource for building refractors. It is the "new" way compared to the outdated, scattered literature of the past. A tiny air gap (often maintained by three
: The book is listed on Open Library , where you may be able to borrow a digital copy if available. It is the "new" way compared to the
If you cannot find Remer's text, classic alternatives include Amateur Telescope Making by Albert G. Ingalls and Standard Handbook for Telescope Making by Neale E. Howard.
What (e.g., 4-inch f/15) are you planning to build?
Because light passes through a lens rather than reflecting off it, minor surface errors have a significantly reduced impact on the final wavefront. Furthermore, all four surfaces on a classic doublet are purely spherical. Spherical surfaces are naturally formed through the grinding process, making them far easier to polish and test than complex parabolas. Inside Chapter 12: The 7 Structural Prescriptions