Collimation Calculator

Secondary Offset follows the standard rule from Suiter's "Star Testing Astronomical Telescopes," the secondary's minor axis divided by four times the focal ratio, so a physically larger secondary mirror produces a proportionally larger offset at the same focal ratio. This is because the offset exists to recenter the illuminated field on a mirror that itself has more surface area to miscenter -- a small secondary in a fast scope needs only a fraction of a millimeter of offset, while a large secondary in the same scope needs several times more.

Alignment Tolerance follows the quarter-wave depth-of-focus rule, which scales with the SQUARE of the focal ratio -- so dropping from f/8 to f/4 (half the focal ratio) cuts the tolerance to a quarter of its value, not half. This is why fast Newtonians (low f-numbers) have a reputation for being fussy to collimate: the same optical speed that makes them compact and wide-field also makes focus and alignment errors visible much sooner.

They're driven by the same underlying fact (fast scopes are less forgiving) but Collimation Difficulty is a simple three-tier label -- Easy at f/8 and slower, Moderate from f/5 to f/7, Critical at f/4 and faster -- while Alignment Tolerance is the actual continuous quarter-wave depth-of-focus figure in millimeters. Use the label for a quick read on how careful you'll need to be, and the millimeter figure when you're actually adjusting the secondary with a Cheshire or laser collimator.

Not directly -- Secondary Offset depends only on the secondary's own minor axis and the focal ratio, not the primary mirror's diameter. Primary Diameter does matter for the overall optical design (it sets the focal length together with focal ratio), but once focal ratio and secondary size are fixed, this calculator's offset formula does not change if you enter a larger or smaller primary.

550 nanometers (green light) is the wavelength human vision is most sensitive to, so it's the conventional reference wavelength for the quarter-wave (Rayleigh) criterion used across amateur telescope-making literature, even though starlight spans the whole visible spectrum. Using a single reference wavelength keeps the tolerance figure comparable across different telescopes and apertures.