Optical design
How lens telescopes work, what separates an achromat from an ED doublet from a triplet apo from a Petzval, and when a refractor is the right answer despite costing far more per millimetre of aperture.
Best suited to:Visual observingNebula imaging
A refractor forms an image by bending light through a lens at the front of the tube. Because glass bends different wavelengths by different amounts, a single lens cannot bring red, green and blue to the same focus — the entire history of refractor design is the story of correcting that. Two elements of different glass types cancel most of the error; adding extra-low-dispersion glass cancels more; a third element cancels almost all of it; and a four-element Petzval adds field flattening on top so a camera sees round stars to the corners.
Achromatic doublets use two elements of different glass. They are cheap, sealed and maintenance-free, and they show a violet halo around bright objects that gets worse as the focal ratio gets faster. Excellent as low-power rich-field instruments; poor for high-power planetary work and for imaging.
ED doublets replace one element with extra-low-dispersion glass. This removes most of the residual colour for a fraction of what a triplet costs, and it is the reason small ED refractors have become the standard first astrograph.
Apochromatic triplets add a third element and remove essentially all visible false colour. They are the sharpest, highest-contrast small telescopes available, and they are priced accordingly.
Petzval refractors add a fourth element group that flattens the field inside the telescope. They are astrographs first: no corrector to buy, no spacing to calculate, and often no eyepiece position at all.
If your goal is to see faint galaxies and globular clusters, a refractor is an expensive way to get there. The money that buys a 100 mm apochromat buys a 250 mm Dobsonian with change, and on faint deep-sky objects the Dobsonian is not close to being beaten.
Aperture is what deep-sky visual observing rewards, and refractors are the design where aperture costs the most.
Effectively none. The optics are sealed inside the tube, so dust and misalignment are rarely issues over years of use. Cooling is fast because there is little glass mass compared with a large mirror. The one real discipline is dew: a front objective is exposed to the sky and will fog before anything else on the telescope, so a dew shield is worth having and a heater is worth having in damp climates.
Apochromatic refractorOTA only
The classic do-everything four-inch apo: excellent visually, capable on nebulae with a flattener, and never the cheapest way to any single goal.
ED refractorOTA only
The most sensible entry into wide-field deep-sky imaging, provided you budget for the flattener it does not include.
Achromatic refractorOTA only
A cheap, light, genuinely fun rich-field refractor for sweeping the Milky Way, badly miscast as a planetary or imaging telescope.
Petzval refractorCamera onlyOTA only
A tiny, expensive, almost foolproof wide-field astrograph. You are paying for the fact that nothing about the optical train can go wrong.