Brand
A Newtonian and a reducer-corrector
A four-element reducer-corrector converts an f/10 Schmidt-Cassegrain to f/6.3, which widens the field and shortens the exposure needed for a given brightness. Four elements correct the field as well as reducing it — a two-element reducer does not.
The distance required between the reducer and the sensor. A reducer has almost no tolerance on that figure, and its absence is the single most important thing the page has to report.
A 150 mm Newtonian at f/5, published with both figures.
For anything in the imaging train, the spacing figure decides whether it works. Its absence is a finding, not an omission to be filled in.
150 mm mirror · 750 mm focal length · optics only
A 150/750 Newtonian mirror set — primary and secondary — sold on its own, with published surface accuracy and reflectivity figures rather than as a complete telescope.
Best for: Somebody rebuilding or building a Newtonian around a known mirror set
≈ £99 / €99
A four-element fully multi-coated reducer-corrector that threads onto the rear cell of a Schmidt-Cassegrain and converts f/10 to f/6.3.
Best for: A Schmidt-Cassegrain used for imaging, where f/10 makes exposures impractically long
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
